A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube (or valve in British English) with three elements: the filament or cathode, the grid, and the plate or anode Lenovo S10-2 Battery 5200mAh .
The triode vacuum tube is the first electronic amplification device . The word is derived from the Greek ???????, tríodos, from tri-(three) and hodós (road, way), originally meaning the place where three roads meet Lenovo Y730 Battery 5200mAh .
The original three-element device was patented in 1907 by Lee De Forest who developed it from his original two-element 1906 Audion. The Audion did provide amplification Lenovo Y710 Battery 5200mAh .
However it was not until around 1912 that other researchers, while attempting to improve the service life of the audion, stumbled on the principle of the true vacuum tube. The name triode appeared later, when it became necessary to distinguish it from other generic kinds of vacuum tubes with more or fewer elements (eg diodes, tetrodes, pentodes etc.) Lenovo Y530 Battery 5200mAh .
The Audion tubes deliberately contained some gas at low pressure. The name triode is only applied to vacuum tubes which have been evacuated of as much gas as possible.
There was a parallel independent invention of the triode by Austrian Robert von Lieben Lenovo Y510 Battery 5200mAh .
The principle of its operation is that, as with a thermionic diode, the heated cathode (either directly or indirectly by means of a filament) causes a space charge of electrons that may be attracted to the positively charged plate (anode in UK parlance) and create a current Lenovo S10-3 Battery 5200mAh .
Applying a negative voltage (a 'bias') on the control grid will tend to repel some of the (also negatively charged) electrons back towards the cathode: the more negative the voltage on the grid, the smaller the current to the plate Lenovo G450 Battery 5200mAh.
If an AC signal is superimposed on the bias of the grid, an amplified version of the AC signal appears (inverted) across the external impedance in the plate(anode) circuit.
The triode is very similar in operation to the n-channel JFET Lenovo E30 Battery 5200mAh ;
it is normally on, and progressively switched off as the grid/gate is pulled increasingly negative of the source/cathode.
Although triodes are now largely obsolete in consumer electronics Lenovo E40 Battery 5200mAh ,
having been replaced by the transistor, triodes continue to be used in certain high-end andprofessional audio applications, as well as in microphone preamplifiers and electric guitar amplifiers.
Some guitarists routinely drive their amplifiers to the point of clipping (either grid-current limiting or valve cut-off), in order to produce a desired distortion tone (tube sound) Lenovo E50 Battery 5200mAh .
Many people prefer the sound of triodes in such an application, since the distortion of a tube amplifier, which has a "soft" saturation characteristic, can be more pleasing to the ear than that of a typical solid-state amplifier, which through negative feedback is linear up to the limits of its supply voltage and then clips abruptly Lenovo G460 Battery 5200mAh .
n triode datasheets, characteristics linking the anode current(Ia) to anode voltage (Va) and grid voltage (Vg) were usually given. From here, designer would choose the operating point of the particular triode Lenovo Y460 Battery 5200mAh .
In the example characteristic shown on the image, if an anode voltage Va of 200 V and a grid voltage bias of -1 volt are selected, a plate (anode) current of 2.25 mA will be present (using the yellow curve on the graph). Changing the grid voltage will change the plate current; by suitable choice of a plate load resistor, amplification is obtained Lenovo Y450 Battery 5200mAh .
In the class A triode amplifier, an anode resistor would be connected between the anode and the positive voltage source. For example, with Ra=10000 Ohms, voltage drop on it will be
if anode current of Ia=2.25 mA is chosen Lenovo N200 Battery 5200mAh .
Now, if the input voltage amplitude (at the grid) changes from -1.5 V to -0.5 V (difference of 1 V), anode current will change from 1.2 to 3.3 mA (see image). This will result in anode resistor voltage drop changes from 12 to 33 V (difference of 21 V) Lenovo T410 Battery 5200mAh .
Since grid voltage changes from -1.5 V to -0.5 V, and anode resistor voltage drop from 12 to 33 V, amplification of signal resulted. Amplification factor is 21 - output voltage amplitude divided by input voltage amplitude Lenovo T510 Battery 5200mAh .
A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour Lenovo SL410 Battery 5200mAh .
The grid nearest the cathode is the "control grid"; the voltage applied to it causes the anode current to vary. In normal operation, with a resistive load, this varying current will result in varying (AC) voltage measured at the anode Lenovo SL510 Battery 5200mAh.
With proper biasing, this voltage will be an amplified (but inverted) version of the AC voltage applied to the control grid, thus the tetrode can provide voltage gain.
The second grid, called "screen grid" or sometimes "shield grid", provides a screening effect, isolating the control grid from the anode, reducing the parasitic capacitance between the two Lenovo C100 Battery 5200mAh .
This helps to suppress unwanted oscillation, and to reduce an undesirable effect in triodes called the "Miller effect", where the gain of the tube causes a feedback effect which increases the apparent capacitance of the tube's grid, limiting the tube's high-frequency gainLenovo T400S Battery 5200mAh .
In normal operation the screen grid is connected to a positive voltage, and bypassed to the cathode with a capacitor. This shields the grid from theanode, reducing Miller capacitance between those two electrodes to a very low level and improving the tube's gain at high frequencies Lenovo Y430 Battery 5200mAh.
When the tetrode was introduced, a typical triode had an input capacitance of about 5 pF, but the screen grid reduced this capacitance to about 0.01 pF.
As the screen grid is positively charged, it collects electrons, which causes current to flow in the screen grid circuit Ibm ThinkPad G40 Battery 5200mAh .
This uses power and heats the screen grid; if the screen heats up enough it can melt and destroy the tube. There are two sources of electrons collected by the screen grid—in addition to the electrons emitted by the cathode, the screen grid can also collect secondary electronsejected from the anode by the impact of the energetic primary electrons Ibm ThinkPad X200T Battery 5200mAh .
Secondary emission can increase enough to decrease the anode current, since a single primary electron can eject more than one secondary electron. The reduction in anode current is because the external anode current (through the connection pin) is due to the cathode-to-anode current minus the secondary emission current Apple A1280 battery 5200mAh .
This can give the tetrode valve a distinctive negative resistance characteristic, sometimes called "tetrode kink". This is usually undesirable, although it can be exploited as in the dynatron oscillator. The secondary emission can be suppressed by adding a suppressor grid, making a pentode, or beam plates to make a beam tetrode/kinkless tetrode Apple A1281 battery 5200mAh .
The positive influence of the screen grid in the vicinity of the control grid allows a designer to shift the control grid operating voltage range entirely into the negative region (a triode of similar geometry would likely require positive grid drive to attain the same maximum anode current) Samsung R522 battery 5200mAh .
When any grid is driven positive relative to the cathode it can intercept electrons from the cathode, loading the drive circuitry. If the input signal causes the control grid to become positive (where current flow begins), nonlinearity is to be expected. (The control grid draws no current while negative—high impedance—but draws current while positive—low impedance.) Samsung P467 battery 5200mAh
With the control grid operating entirely in the negative region, and with the RF shielding afforded by the screen grid, tetrode input impedance is quite high even at high frequencies. Gain can be nearly flat from DC to full frequency. Linearity is good. Power gain in excess of 10,000 is possible Hp Pavilion dm3 Battery 5400mAh .
The triode vacuum tube also develops a "space charge" between the cathode and control grid, which reduces its gain, especially at low anode voltages. The screen grid of a tetrode neutralizes the space charge and increases the tube's gain Hp Pavilion dm4 Battery 5200mAh .
Power tetrodes are commonly used in radio transmitting equipment, because the need for neutralization is less than with triodes (see Radio transmitter design and Valve amplifier for more details). Screen current does represent loss Hp Pavilion cq62 Battery 5200mAh .
Some tube designers attempt to minimize screen current by placing each wire in the screen mesh directly behind a corresponding wire in the control grid mesh. Propagating electrons emerge from the control grid as a projected image of openings in the grid Hp Pavilion cq42 Battery 5200mAh .
By placing the screen in the shadow of the control grid, interception of electrons by the screen is minimized in normal operation. Screen current is negligible in many designs. Shadow grids are used in a variety of forms for a number of applications.
More than one screen grid can be used. For example the pentagrid converter has two Hp Pavilion cq72 Battery 5200mAh .
A tetrode can be converted to act as a triode by connecting the screen grid to the anode.
- Circuit design considerations
Under certain operating conditions, the tetrode exhibits negative resistance due to secondary emission of electrons from the anode (to the screen) Hp cq20 Battery 2600mAh .
The shape of the characteristic curve of a tetrode operated in this region led to the term "tetrode kink". In general, if the anode voltage exceeds the screen voltage, this region is avoided, and good performance can be expected. But this lower limit on total tube voltage drop prevents widespread adoption of tetrodes for consumer amplification applications Hp 2230 Battery 2600mAh .
Secondary emissions from a screen have the effect of pulling the screen upward, toward theanode voltage. This implies the need for both source and sink current capability in the ideal screen power supply. A bleeder resistor can usually be selected to prevent the screen voltage from getting out of control Dell Inspiron Mini 12 battery 7800mAh .
Arcs from the anode generally hit the screen. As such, special care is required in design of the socket wiring, to provide a direct discharge path for arc current. The undesirable nature of the tetrode kink led tube designers to add a third grid, called the suppressor grid Dell XPS M1730 battery 7800mAh ;
the resulting vacuum tube is called a pentode. More modern tubes have anodes treated to minimise secondary emission.
The negative resistance operating region of the tetrode is exploited in the dynatron oscillator, although this was practical only with earlier tubes with high secondary emission Dell XPS M2010 battery 7800mAh .
The tetrode tube was developed by Dr. Walter H. Schottky of Siemens & Halske GMBH in Germany during World War I. Thousands of variations of the tetrode design, as well as its later development the pentode, have been manufactured since then Dell Studio XPS 1340 battery 5200mAh/7800mAh ,
although vacuum tubes in low-power equipment have been almost totally superseded by solid-state semiconductor devices.
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube (thermionic valve), which was invented by the Dutchman Bernhard D.H. Tellegen in 1926 Dell Inspiron 9100 battery 7800mAh .
Pentodes (termed "triple-grid amplifiers" in some early literature ) are closely related to beam tetrodes, and an improvement over conventional tetrodes, which were themselves a development of triodes Dell Inspiron N4010 battery 5200mAh .
Types of pentodes
- Variable transconductance ("vari-mu", "remote-cutoff" or "super-control") tubes in general are those with a non-uniform grid wire spacing to allow them to handle a wide range of input signal levels without excessive cross-modulation distortion Dell Inspiron N3610 battery 5200mAh ,
- and so useful in Radio frequency stages where Automatic gain control is applied to the pentode. Examples include: 1T4, 6K7, 6BA6, and the EF83 (while perhaps the EF85/6BY7, and certainly the 6JH6, could be described as "semiremote-cutoff" pentodes) Dell Inspiron N5010 battery 5200mAh .
- Sharp-cutoff ("high slope" or ordinary) pentodes have the more ordinary uniform spacing of grid wires, and so mutual conductance decreases in an essentially uniform manner with increasing negative bias, and has a more abrupt cutoff. These pentodes are more suitable for audio amplifiers Dell Inspiron N7010 battery 5200mAh .
- Examples include: EF37A, EF86/6267, 1N5GT, 6AU6A, 6J7GT. Often in the European valve naming scheme for pentodes an even number indicated a sharp-cutoff device while odd indicated remote-cutoff; the EF37 was an exception to this general trend Dell Inspiron 1464 battery 5200mAh ,
- perhaps due to its history as an update to the EF36 ("The Mullard EF36, EF37 and EF37A" at the National Valve Museum).
- Power output pentodes, also referred to as "Beam Pentodes" , include the EL34/6CA7, 6K6GT and the EL84/6BQ5 Dell Inspiron 1564 battery 5200mAh .
- Sometimes the word "pentode" was avoided, in names such as "beam power amplifier" or "beam power tube", at a time when the pentode patent was an important issue.
Note: "triode-pentodes" are not a different type of pentode, but an envelope containing both a triode and a pentode, such as an ECF80 or ECL86 Dell Inspiron 1764 battery 5200mAh .
Advantages over the tetrode
A tetrode could supply sufficient power to a speaker or transmitter, and offered a larger amplification factor than the earlier triodeDell Inspiron 1320 battery 5200mAh .
However, the positively charged screen grid can collect the secondary electrons emitted from the anode, which can cause increased current toward the screen grid, and cause the anode current to decrease with increasing anode voltage over part of the Ia/Va characteristic Dell Inspiron 1320n battery 5200mAh .
A pentode, as introduced by Tellegen, has an additional electrode, or third grid, called the suppressor grid that solves the problem of secondary emission. The suppressor grid does this by being held at a low potential, usually either grounded or connected to the cathode Dell Vostro 1710 battery 5200mAh .
Secondary emission still occurs, but the electrons that come from it can no longer reach the screen grid as they have less energy than the primary electrons, and, hence, cannot pass the grounded suppressor grid. The electrons from the secondary emission are re-collected by the anode Dell Vostro 1720 battery 5200mAh .
Pentodes, therefore, can have higher current outputs and a wider output voltage swing; the anode/plate can even be at a lower voltage than the screen grid yet still amplify well .
Comparisons with the triode
- Pentodes (and tetrodes) tend to have a much lower feedback capacitance, due to the screening effect of the second grid Dell Studio XPS 1640 battery 5200mAh .
- Pentodes tend to have a higher noise,
- Triodes have a lower internal anode resistance, and hence higher damping factor when used in audio output circuits, compared with pentodes, whennegative feedback is absent. That also reduces the potential voltage amplification obtainable from a triode compared with a pentode of the same transconductance Dell Vostro A840 battery 5200mAh ,
- and usually means a more efficient output stage can be made using pentodes, with a lower power drive signal.
- Pentodes are almost unaffected by changes in supply voltage, and can thus operate with more poorly stabilised supplies than triodes Dell Vostro A860 battery 5200mAh .
- An examination of the characteristics shown here will show that the plate current hardly changes as plate voltage varies.
Pentode valves were first used in consumer-type radio receivers Dell Studio 1535 battery 7800mAh .
A well-known pentode type, the EF50, was designed before the start of the World War II, and was extensively used in radar sets and other military electronic equipment. The pentode contributed to the electronic preponderance of the Allies Dell Studio 1735 battery 7800mAh .
After World War II, pentodes were widely used in TV receivers, particularly the successor to the EF50, the EF80. Vacuum tubes were replaced by transistors during the 1960s. However, they continue to be used in certain applications Dell Vostro 1310 battery 5200mAh ,
including high-power radio transmitters and (because of their well-known valve sound) inhigh-end and professional audio applications, microphone preamplifiers and electric guitar amplifiers. Large stockpiles in countries of the former Soviet Union have provided a continuing supply of such devices Dell Latitude E6400 battery,10400mAh .
some designed for other purposes but adapted to audio use, such as the GU-50 transmitter tube.
Triode-strapped pentode circuits
A pentode (or, less commonly, a tetrode) can have its screen grid (grid 2) connected to the anode (plate) and the resulting "triode-strapped" (or "triode-connected") device has characteristics very similar to a triode (lower anode resistance Dell Latitude E6400 Battery,10400mAh ,
lower noise, more drive voltage required). This is sometimes provided as an option in audiophile pentode amplifier circuits, to give the sought-after "sonic qualities" of a triode power amplifier. There are situations where this arrangement is unsafe, for example when doing so (without a series resistor) could exceed the screen grid's power or voltage ratingDell Latitude E6400 Battery,10400mAh ,
but remains a valuable option due to the difficulty in obtaining good modern power triodes.
The pentagrid converter is a radio receiving valve (vacuum tube) with five grids used as the frequency mixer stage of a superheterodyne radio receiver Dell Latitude E6500 battery,10400mAh .
The pentagrid was part of a line of development of valves that were able to take an incoming RF signal and change its frequency to a fixed intermediate frequencywhich was then amplified, and detected in the remainder of the receiver circuitry. The device was generically referred to as a frequency changer or just mixer Dell Latitude E6500 battery,10400mAh .
The first devices designed to change frequency in the manner described above seem to have been developed by the French who simply put two grids into what would otherwise have been an ordinarytriode valve (the bi-grille) Dell Latitude E6500 battery,10400mAh .
Although technically a four electrode device, neither the term tetrode nor the tetrode valve as we know it today had yet appeared. Each grid was able to accept one of the incoming signals and the non-linearity of the device produced the sum and difference frequencies Dell Latitude E6500 battery,10400mAh .
The valve would have been very inefficient but, most importantly, the capacitive coupling between the two grids would have been very large. It would therefore have been quite impossible to prevent the signal from one grid coupling out of the other. At least one reference claims that the bi-grille was self oscillating, but this has not been confirmed HP Pavilion DV6-1210SA battery - 8800mAh .
When Edwin Armstrong invented the superheterodyne receiver in 1918, although the tetrode had been invented a couple of years earlier, he nevertheless employed only triodes in his design. Armstrong employed a pair of triodes as his mixer stage HP Pavilion DV7 battery - 7200mAh .
One operated in a conventional oscillator circuit, but he was able to employ the other as a mixer by coupling the oscillator signal into the mixer's cathode, and the received signal to the grid. The sum and difference frequencies were then available in the mixer's anode (or plate) circuit Ibm THINKPAD X200 Battery 8800mAh .
Once again, the problem of coupling between the circuits would be ever present.
Shortly after Armstrong invented the superhet, a triode mixer stage design was developed that not only mixed the incoming signal with the local oscillator, but the same valve doubled as the oscillator Ibm THINKPAD X200 Battery 8800mAh .
This was known as the autodyne mixer. Early examples had difficulty oscillating across the frequency range because the oscillator feedback was via the first intermediate frequency transformer primary tuning capacitor which was too small to give good feedback. Also keeping the oscillator signal out of the antenna circuit was difficult Ibm THINKPAD X200 Battery 8800mAh .
The invention of the tetrode demonstrated the idea of screening electrodes from each other by using additional earthed (grounded) grids (at least as far as the signal was concerned). In 1926, Philips invented a technique of adding yet another grid to combat the secondary emission that the tetrode suffered from SONY VAIO VGN-FW21E Battery .
All the ingredients for the pentagrid were now in place.
The development of the pentagrid or heptode valve was a novel development in the mixer story Asus M52N Battery .
The idea was to produce a single valve that not only mixed the oscillator signal and the received signal and produced its own oscillator signal at the same time but, importantly, did the mixing and the oscillating in different parts of the same valve Dell Latitude E6400 ATG Battery .
The invention of the device at first sight doesn't seem to be obscure, but it would appear that it was developed in both America and the United Kingdom, more or less at the same time. However, the UK device is different to its American counterpart SONY VAIO VGN-FW21J Battery . .
It is known that Donald G. Haines of RCA applied for a patent for the pentagrid on 28 March 1933 (subsequently granted on 29 March 1939) under US patent number 2,148,266. The pentagrid also featured in a UK patent (GB426802) granted on 10 April 1935 SONY VAIO VGN-FW21L Battery .
However, the Ferranti company of Great Britain entered the valve business with the first known UK produced pentagrid, the VHT4late in 1933 (though it must have been in development, and would certainly have existed as a prototype well before this time) SONY VAIO VGN-FW21M Battery .
The pentagrid proved to be a much better mixer. Since the oscillator circuit was more or less self contained, good feedback for reliable oscillation across the frequency range was easy to obtain. Some manufacturers that had adopted the autodyne mixer converted some, if not all, of their designs to pentagrid mixers SONY VAIO VGN-FW21Z Battery .
What was the goal to develop a reliable self oscillating mixer? The reasons were to differ from the UK to America. The UK radio manufacturers had to pay a royalty of £1 per valve holder to the British Valve Association to cover use of their members' patent rights SONY VAIO VGN-FW25T/B Battery .
Further, they dictated than not more than one electrode structure could be contained in a single envelope (which would have evaded the royalty - at least in part). The Americans appeared to be driven by the desire to produce a low cost 'every expense spared' design which was to lead to theAll American Five SONY VAIO VGN-FW26T/B Battery .
By making the mixer self oscillate, the necessity of providing a separate oscillator valve is avoided. The All American Five was to use a pentagrid converter from when it first appeared in 1934, right up until valves became obsolete when transistors took over SONY VAIO VGN-FW27/B Battery .
In the UK, the five grids operated thus. Grid 1 acted as the oscillator grid in conjunction with grid 2 which acted as its anode. Grid 4 accepted the incoming signal with the remaining two grids, 3 and 5 connected together (usually internally) which acted as screen grids to screen the anode, grid 4 and grid 2 from each other SONY VAIO VGN-FW27/W Battery .
Because grid 2 was a 'leaky' anode in that it allowed part of the modulated electron stream through, the oscillator was coupled into the mixing section of the valve. In fact, in some designs, grid 2 consisted of just the support rods, the actual grid wire itself being omitted SONY VAIO VGN-FW31ZJ Battery .
In America, the configuration was different. Grid 1 acted as the oscillator grid as before, but in this case, grids 2 and 4 were connected together (again usually internally). Grid 2 functioned as both a screen and the oscillator anode, in this case the grid wire had to be present to provide the screening SONY VAIO VGN-FW27T/H Battery .
Grid 3 accepted the incoming signal. Grid 4 screened this from the anode, and grid 5 was a suppressor grid to suppress secondary emission. This configuration limited the oscillator design to one where the oscillator 'anode' was operated from the HT+ (B+) rail SONY VAIO VGN-FW29/B Battery .
This was often accomplished by using a Hartley Oscillator circuit and taking the cathode to the tap on the coil.
It will be noted that the UK version would have had significant secondary emission and would also have had a tetrode kink SONY VAIO VGN-FW290JRB Battery .
This was exploited in providing the non linearity necessary to produce good sum and difference signals. The American devices although having no secondary emission due to the suppressor grid, nevertheless were able to get the required non linearity by biasing the oscillator such that the valve was overdriven SONY VAIO VGN-FW290JTB Battery .
The American version was also a little more sensitive because the grid that accepted the signal was closer to the cathode increasing the amplification factor.
The pentagrid converter in either guise operated extremely well, but it suffered from the limitation that a strong signal was able to 'pull' the oscillator frequency away from a weaker signal SONY VAIO VGN-FW290JTH Battery .
This was not considered a major problem in broadcast receivers where the signals were likely to be strong, but it became a problem when trying to receive weak signals that were close to strong signals. Some short wave radios managed quite satisfactorily with these devices SONY VAIO VGN-FW33G/E1 Battery .
Special high frequency versions appeared after World War II for the 100 MHz FM bands. Examples are the 6SB7Y (1946) and the 6BA7(1948). The pulling effect had a beneficial side effect in that it gave a degree of automatic tuning SONY VAIO VGN-FW33G/W Battery .
Another disadvantage was that in spite of the presence of the screen grids, the electron beam, modulated by the oscillator electrodes, still had to pass through the signal grid, and coupling of the oscillator into the signal circuit was inevitable SONY VAIO VGN-FW33GW Battery .
The American Federal Communication Commission (FCC) started requiring radio manufacturers to certify that their products avoided this interference under Part 15 of their rules. In the UK the Postmaster General (who was responsible for radio licensing), laid down a set of stringent rules concerning radio interference Dell XPS M1210 Battery) http://www.hdd-shop.co.uk .
It may come as a surprise that the Hexode was actually developed after the heptode or pentagrid. It was developed in Germany as a mixer but was designed from the start to be used with a separate triode oscillator Dell KM958 battery .
Thus the grid configuration was grid 1, signal input; grids 2 and 4 screen grids (connected together - again, usually internally) and grid 3 was the oscillator input. The device had no suppressor grid. A major advantage was that by using grid 1 as the signal input grid, the device was more sensitive to weak signals Dell Studio 1555 battery .
It was not long before they put the triode and hexode structures in the same glass envelope - by no means a new idea. The triode grid was usually internally connected to the hexode grid 3, but this practice was dropped in later designs when the mixer section operated as a straight IF amplifier in AM/FM sets when operating on FM Sony VGP-BPS13 battery ,
the mixing being carried out in a dedicated FM frequency changing section.
The UK manufacturers were initially unable to use this type of mixer because of the BVA prohibition on multiple structures (and indeed separate valves because of the levy) Sony VGP-BPS13/B battery .
Indeed one UK company, MOV, successfully enforced the cartel rules against the German Lissen company in 1934 when they attempted to market a radio in the UK which had the triode-hexode mixer.
Following pressure from the UK manufacturers, the BVA were compelled to relax the rules and the UK started to adopt triode-hexode mixers. The Mullard ECH35 was a popular choice Sony VGP-BPS13/S battery .
One company, Osram made an ingenious move. One of their popular pentagrid converter designs was the MX40, initially marketed in 1934. They put on sale in 1936, the X41 triode-hexode frequency changer. The clever bit was that the X41 was a direct plug in pin compatible replacement for the MX40 Sony VGP-BPS13A/B battery .
Thus a pentagrid radio could be easily converted to a triode-hexode without any other circuit modifications.
It is interesting to note, that America never really adopted the triode-hexode and it was seldom used, even though the 6K8 triode-hexode was available to manufacturers in 1938 Sony VGP-BPS13B/B battery .
In some designs, a suppressor grid was added to produce yet another heptode design. Mullard's ECH81 became popular with the move to miniature 9 pin valves.
Although not strictly a pentagrid (in that it does not have 5 grids), this device nevertheless operates on the pentagrid principle Sony VGP-BPL9 battery .
It resulted simply from the addition of a suppressor grid to the UK version of the pentagrid heptode. This was done mainly to reduce the power consumption for use in radio sets operated by dry batteries that were becoming increasingly popular Sony VGP-BPS13B/B battery .
In North America, the only octode manufactured was the 7A8. Introduced by Sylvania in 1939 (and used mostly by Philco), this valve was the product of adding a suppressor grid to type 7B8, which was the loctal version of type 6A7 Sony VGP-BPL15 battery .
Adding the suppressor allowed Sylvania to lower the current of the 6.3 volt heater from 300 milliamperes to 150 milliamperes while maintaining the same conversiontransconductance (550 microsiemens). This allowed Philco to use this valve in every line of radio throughout the 1940s Dell Inspiron E1505 battery .
One octode design worthy of mention was the Philips EK3 Octode. This was designated as a 'beam octode'. The novel part about the design was that grids 2 and 3 were constructed as beam forming plates. This was done in such a way that Philips claimed that the oscillator electron beam and the mixer electron beams were separated as much as possible and thus the pulling effect was minimisedHP Pavilion dv6000 Battery .
No information is available as to the degree of success.
The use of a pentode would seem an unlikely choice for a frequency converter. However, during the Great Depression HP Pavilion dv8000 battery ,
many American radio manufacturers used pentode types 6C6, 6D6, 77 and 78 in their lowest priced AC/DC receivers because they were cheaper than pentagrid type 6A7. In these circuits, the suppressor (grid 3) acted as the oscillator grid HP PAVILION DV6000 Battery, 8800mAh .
One UK company, Mazda, produced a triode-pentode frequency changer, the AC/TP. Designed for low cost AC radios, the device was deliberately designed to allow strong signals to pull the oscillator without the risk of radiating the oscillator signal from the aerial. The cathode was common to both sections of the valve HP PAVILION DV8000 Battery, 8800mAh .
The cathode was connected to a secondary coil on the oscillator coil and thus coupled the oscillator into the pentode mixer section, the signal being applied to grid 1 in the conventional manner. The AC/TP was one of the AC/ range of valves designed for low cost radios SONY VAIO VGN-FZ Battery .
They were considered durable for their time (even the AC/TP frequency changer, which was normally problematic). Any AC/ valves encountered today are likely to be brand new as service shops stocked up on spares which were seldom required HP Pavilion DV6000 battery - 8800mAh .
In order to distinguish between the two versions of the heptode or pentagrid, manufacturers data often describes them as 'heptode of the Hexode type' for a heptode without a screen grid, and a 'heptode of the octode type', where a screen grid is present HP Pavilion DV8000 battery - 7800mAh .
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals HP Pavilion DV6000 battery - 8800mAh .
Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components.
A device which performs the opposite function (converting DC to AC) is known as an inverter.
When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform) HP Pavilion DV8000 battery - 6600mAh ,
the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode SONY VAIO VGN-FZ18 Battery .
Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper(I) oxide or selenium rectifier stacks were used.
Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector" SONY VAIO VGN-FZ21E Battery .
Rectification may occasionally serve in roles other than to generate direct current per se. For example, in gas heating systems flame rectification is used to detect presence of flame. Two metal electrodes in the outer layer of the flame provide a current path, and rectification of an applied alternating voltage will happen in the plasma SONY VAIO VGN-FW31J Battery 10400mAh ,
but only while the flame is present to generate it.
In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked SONY VAIO VGN-FW31J Battery 10400mAh .
Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one-phase supply, or with three diodes in a three-phase supply SONY VAIO VGN-FW21E Battery .
A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient SONY VAIO VGN-NR11S/S Battery .
However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. (See semiconductors,diode). Four diodes arranged this way are called a diode bridge or bridge rectifier SONY VAIO VGN-NR11M/S Battery .
For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode) can form a full-wave rectifier. Twice as many windings are required on the transformer secondary to obtain the same output voltage compared to the bridge rectifier above SONY VAIO VGN-NR11Z/S Battery .
A very common vacuum tube rectifier configuration contained one cathode and twin anodes inside a single envelope; in this way, the two diodes required only one vacuum tube. The 5U4 and 5Y3 were popular examples of this configuration.
For three-phase AC, six diodes are used SONY VAIO VGN-NR11Z/T Battery .
Typically there are three pairs of diodes, each pair, though, is not the same kind of double diode that would be used for a full wave single-phase rectifier. Instead the pairs are in series (anode to cathode). Typically, commercially available double diodes have four terminals so the user can configure them as single-phase split supply use, for half a bridge, or for three-phase use Sony VAIO VGN-FZ21E Battery .
Most devices that generate alternating current (such devices are called alternators) generate three-phase AC. For example, an automobile alternator has six diodes inside it to function as a full-wave rectifier for battery charging applications Sony VAIO VGN-FW21E Battery .
The average and root-mean-square output voltages of an ideal single phase full wave rectifier can be calculated as:
An aspect of most rectification is a loss from the peak input voltage to the peak output voltage, caused by the built-in voltage drop across the diodes (around 0.7 V for ordinary silicon p-n-junction diodes and 0.3 V for Schottky diodes) Sony VAIO VGN-NR11S/S Battery .
Half-wave rectification and full-wave rectification using two separate secondaries will have a peak voltage loss of one diode drop. Bridge rectification will have a loss of two diode drops. This may represent significant power loss in very low voltage supplies Sony VAIO VGN-NR11Z/S Battery .
In addition, the diodes will not conduct below this voltage, so the circuit is only passing current through for a portion of each half-cycle, causing short segments of zero voltage to appear between each "hump".
Rectifier output smoothing
While half-wave and full-wave rectification suffice to deliver a form of DC output, neither produces constant-voltage DC Sony VAIO VGN-NR11M/S Battery .
In order to produce steady DC from a rectified AC supply, a smoothing circuit or filter is required. In its simplest form this can be just areservoir capacitor or smoothing capacitor, placed at the DC output of the rectifier. There will still remain an amount of AC ripplevoltage where the voltage is not completely smoothed Sony VAIO VGN-NR11Z/T Battery .
Sizing of the capacitor represents a tradeoff. For a given load, a larger capacitor will reduce ripple but will cost more and will create higher peak currents in the transformer secondary and in the supply feeding it. In extreme cases where many rectifiers are loaded onto a power distribution circuit SONY VAIO VGN-FZ180E Battery ,
it may prove difficult for the power distribution authority to maintain a correctly shaped sinusoidal voltage curve.
For a given tolerable ripple the required capacitor size is proportional to the load current and inversely proportional to the supply frequency and the number of output peaks of the rectifier per input cycle SONY VAIO VGN-FZ220E Battery .
The load current and the supply frequency are generally outside the control of the designer of the rectifier system but the number of peaks per input cycle can be affected by the choice of rectifier design.
A half-wave rectifier will only give one peak per cycle and for this and other reasons is only used in very small power supplies SONY VAIO VGN-FZ340E Battery .
A full wave rectifier achieves two peaks per cycle and this is the best that can be done with single-phase input. For three-phase inputs a three-phase bridge will give six peaks per cycle and even higher numbers of peaks can be achieved by using transformer networks placed before the rectifier to convert to a higher phase order SONY VAIO VGN-FZ430E Battery .
To further reduce this ripple, a capacitor-input filter can be used. This complements the reservoir capacitor with a choke(inductor) and a second filter capacitor, so that a steadier DC output can be obtained across the terminals of the filter capacitor. The choke presents a high impedance to the ripple current SONY VAIO VGN-FZ460E Battery .
A more usual alternative to a filter, and essential if the DC load is very demanding of a smooth supply voltage, is to follow the reservoir capacitor with a voltage regulator. The reservoir capacitor needs to be large enough to prevent the troughs of the ripple getting below the voltage the DC is being regulated to SONY VAIO VGN-FZ480E Battery .
The regulator serves both to remove the last of the ripple and to deal with variations in supply and load characteristics. It would be possible to use a smaller reservoir capacitor (these can be large on high-current power supplies) and then apply some filtering as well as the regulator, but this is not a common strategy SONY VAIO VGN-FZ4000 Battery .
The extreme of this approach is to dispense with the reservoir capacitor altogether and put the rectified waveform straight into a choke-input filter. The advantage of this circuit is that the current waveform is smoother and consequently the rectifier no longer has to deal with the current as a large current pulse SONY VAIO VGN-FZ31E Battery ,
but instead the current delivery is spread over the entire cycle. The downside is that the voltage output is much lower – approximately the average of an AC half-cycle rather than the peak.
The simple half wave rectifier can be built in two versions with the diode pointing in opposite directions SONY VAIO VGN-FZ31B Battery ,
one version connects the negative terminal of the output direct to the AC supply and the other connects the positive terminal of the output direct to the AC supply. By combining both of these with separate output smoothing it is possible to get an output voltage of nearly double the peak AC input voltage SONY VAIO VGN-FZ31J Battery .
This also provides a tap in the middle, which allows use of such a circuit as a split rail supply.
A variant of this is to use two capacitors in series for the output smoothing on a bridge rectifier then place a switch between the midpoint of those capacitors and one of the AC input terminals SONY VAIO VGN-FZ31M Battery .
With the switch open this circuit will act like a normal bridge rectifier with it closed it will act like a voltage doubling rectifier. In other words this makes it easy to derive a voltage of roughly 320V (+/- around 15%) DC from any mains supply in the world, this can then be fed into a relatively simple switched mode power supply SONY VAIO VGN-FZ31Z Battery .
Cascaded stages of diodes and capacitors can be added to make a voltage multiplier (Cockroft-Walton circuit). These circuits can provide a potential several times that of the peak value of the input AC, although limited in current output and regulation SONY VAIO VGN-FZ38M Battery .
Voltage multipliers are used to provide the high voltage for aCRT in a television receiver, or for powering high-voltage tubes such as image intensifiers or photo multipliers.
The primary application of rectifiers is to derive DC power from an AC supply SONY VGP-BPS8 Battery .
Virtually all electronic devices require DC, so rectifiers find uses inside the power supplies of virtually all electronic equipment.
Converting DC power from one voltage to another is much more complicated SONY VGP-BPS13/S Battery .
One method of DC-to-DC conversion first converts power to AC (using a device called an inverter), then use a transformer to change the voltage, and finally rectifies power back to DC.
Rectifiers also find a use in detection of amplitude modulated radio signals SONY VGP-BPS13A/B Battery .
The signal may be amplified before detection, but if un-amplified, a very low voltage drop diode must be used. When using a rectifier for demodulation the capacitor and load resistance must be carefully matched. Too low a capacitance will result in the high frequency carrier passing to the output and too high will result in the capacitor just charging and staying charged
Rectifiers are also used to supply polarised voltage for welding. In such circuits control of the output current is required and this is sometimes achieved by replacing some of the diodes in bridge rectifier with thyristors, whose voltage output can be regulated by means of phase fired controllers SONY VGP-BPS13A/S Battery .
Thyristors are used in various classes of railway rolling stock systems so that fine control of the traction motors can be achieved. Gate turn-off thyristors are used to produce alternating current from a DC supply, for example on the Eurostar Trains to power the three-phase traction motors SONY VGP-BPS13AS Battery .
Early power conversion systems were purely electro-mechanical in design, since electronic devices were not available to handle significant power Dell Inspiron 1320n Battery .
Mechanical rectification systems usually rely on some form of rotation or resonant vibration in order to move quickly enough to match the frequency of the input power source, and cannot operate beyond several thousand cycles per second Dell Studio 1450 Battery .
Due to the complexity of mechanical systems, they have traditionally needed a high level of maintenance to keep operating correctly. Moving parts will have friction, which requires lubrication and replacement due to wear. Opening mechanical contacts under load results in electrical arcs and sparks that heat and erode the contacts Dell Studio 1457 Battery .
To convert AC currents into DC current in electric locomotives, a synchronous rectifier may be used . It consists of a synchronous motor driving a set of heavy-duty electrical contacts Dell Latitude D610 Battery .
The motor spins in time with the AC frequency and periodically reverses the connections to the load just when the sinusoidal current goes through a zero-crossing. The contacts do not have to switch a large current, but they need to be able to carry a large current to supply the locomotive's DC traction motors Toshiba NB100 Battery .
In the past, the vibrators used in battery-to-high-voltage-DC power supplies often contained a second set of contacts that performed synchronous mechanical rectification of the stepped-up voltage Toshiba Satellite M65 battery .
Early power conversion systems were purely electro-mechanical in design, since electronic devices were not available to handle significant power Toshiba Satellite M60 battery .
Mechanical rectification systems usually rely on some form of rotation or resonant vibration in order to move quickly enough to match the frequency of the input power source, and cannot operate beyond several thousand cycles per second Dell Latitude D830 Battery .
Due to the complexity of mechanical systems, they have traditionally needed a high level of maintenance to keep operating correctly. Moving parts will have friction, which requires lubrication and replacement due to wear. Opening mechanical contacts under load results in electrical arcs and sparks that heat and erode the contacts Dell Latitude D620 Battery .
To convert AC currents into DC current in electric locomotives, a synchronous rectifier may be used . It consists of a synchronous motor driving a set of heavy-duty electrical contacts Dell Inspiron Mini 10 Battery .
The motor spins in time with the AC frequency and periodically reverses the connections to the load just when the sinusoidal current goes through a zero-crossing. The contacts do not have to switch a large current, but they need to be able to carry a large current to supply the locomotive's DC traction motors Sony VGN-FW11S Battery .
In the past, the vibrators used in battery-to-high-voltage-DC power supplies often contained a second set of contacts that performed synchronous mechanical rectification of the stepped-up voltage Sony VGN-FW11M Battery .
A rectifier used in high-voltage direct current power transmission systems and industrial processing between about 1909 to 1975 is a mercury arc rectifier ormercury arc valve. The device is enclosed in a bulbous glass vessel or large metal tub Sony VGN-FW139E/H battery .
One electrode, the cathode, is submerged in a pool of liquid mercury at the bottom of the vessel and one or more high purity graphite electrodes, called anodes, are suspended above the pool. There may be several auxiliary electrodes to aid in starting and maintaining the arc Dell Latitude E5400 Battery .
When an electric arc is established between the cathode pool and suspended anodes, a stream of electrons flows from the cathode to the anodes through the ionized mercury, but not the other way. [In principle, this is a higher-power counterpart to flame rectification, which uses the same one-way current transmission properties of the plasma naturally present in a flame] Dell Latitude E4200 Battery .
These devices can be used at power levels of hundreds of kilowatts, and may be built to handle one to six phases of AC current. Mercury arc rectifiers have been replaced by silicon semiconductor rectifiers and high power thyristor circuits, from the mid 1970s onward Dell Inspiron 300M Battery .
The most powerful mercury arc rectifiers ever built were installed in the Manitoba Hydro Nelson River Bipole HVDC project, with a combined rating of more than 1 GW and 450 kV Dell Studio 1737 battery .
Argon gas electron tube
The General Electric Tungar rectifier was an argon gas-filled electron tube device with a tungsten filament cathode and a carbon button anode. It was useful for battery chargers and similar applications from the 1920s until low-cost solid-state rectifiers (the metal rectifiers at first) supplanted it Dell Inspiron E1505 battery .
These were made up to a few hundred volts and a few amperes rating, and in some sizes strongly resembled an incandescent lamp with an additional electrode.
The 0Z4 was a gas-filled rectifier tube commonly used in vacuum tube car radios in the 1940s and 1950s Dell RM791 battery .
It was a conventional full wave rectifier tube with two anodes and one cathode, but was unique in that it had no filament (thus the "0" in its type number). The electrodes were shaped such that the reverse breakdown voltage was much higher than the forward breakdown voltage Toshiba Satellite P10 Battery .
Once the breakdown voltage was exceeded, the 0Z4 switched to a low-resistance state with a forward voltage drop of about 24 volts.
Vacuum tube (valve)
Since the discovery of the Edison effect or thermionic emission, various vacuum tube devices have been developed to rectify alternating currents Acer Aspire One battery 10400mAh .
Low-power devices are used as signal detectors, first used in radio by Fleming in 1904. Many vacuum-tube devices also used vacuum rectifiers in their power supplies, for example the All American Five radio receiver. Vacuum rectifiers were made for very high voltages Dell Precision M70 Battery ,
such as the high voltage power supply for the cathode ray tube of television receivers, and the kenotron used for power supply in X-ray equipment. However, vacuum rectifiers generally had low current capacity owing to the maximum current density that could be obtained by electrodes heated to temperatures compatible with long life SONY VGN -FZ210CE Battery .
Another limitation of the vacuum tube rectifier was that the heater power supply often required special arrangements to insulate it from the high voltages of the rectifier circuit.
Once common until replaced by more compact and less costly silicon solid-state rectifiers, these units used stacks of metal plates and took advantage of the semiconductor properties of selenium or copper oxide Toshiba Satellite L305 Battery .
While selenium rectifiers were lighter in weight and used less power than comparable vacuum tube rectifiers, they had the disadvantage of finite life expectancy, increasing resistance with age, and were only suitable to use at low frequencies Toshiba Satellite T4900 Battery .
Both selenium and copper oxide rectifiers have somewhat better tolerance of momentary voltage transients than silicon rectifiers.
Typically these rectifiers were made up of stacks of metal plates or washers, held together by a central bolt, with the number of stacks determined by voltage; each cell was rated for about 20 volts Toshiba PA3399U-2BRS battery .
An automotive battery charger rectifier might have only one cell: the high-voltage power supply for a vacuum tube might have dozens of stacked plates. Current density in an air-cooled selenium stack was about 600 mA per square inch of active area (about 90 mA per square centimeter) Sony VGP-BPS10 battery 10400mAh .
Silicon and germanium diodes
In the modern world, silicon diodes are the most widely used rectifiers for lower voltages and powers, and have largely replaced earlier germanium diodes Sony VGP-BPS9 battery 10400mAh .
For very high voltages and powers, the added need for controlability has in practice caused simple silicon diodes to be replaced by high-power thyristors (see below) and their newer gate-controlled cousins Sony VGP-BPS11 battery 10400mAh .
High power: thyristors (SCRs) and newer silicon-based voltage sourced converters
In high power applications, from 1975-2000, most mercury valve arc-rectifiers were replaced by stacks of very high power thyristors, a silicon device with an extra layer of semiconductor in comparison to a diode Sony VGP-BPS22 battery 10400mAh .
In medium power-transmission applications, more complex and sophisticated voltage sourced converter (VSC) silicon semiconductor rectifier systems, such asinsulated gate bipolar transistors (IGBT) and gate turn-off thyristors (GTO), have made made smaller high voltage DC power transmission systems economical Sony VGP-BPL8 battery 10400mAh .
All of these devices function as rectifiers.
In the future, such high-power silicon "self-commutating switches" such as IGBTs, and a variant thyristor related to the GTO called the integrated gate-commutated thyristor (IGCT) Sony VGP-BPS14 battery 10400mAh ,
may be scaled-up in power to the point that they will probably eventually replace the simple thyristor-based AC rectification systems now in use for all very high-power transmission DC applications
Usually the semiconductor of choice is silicon, but some chip manufacturers Sony VGP-BPS21 battery 10400mAh ,
most notably IBM and Intel, recently started using a chemical compound of silicon and germanium (SiGe) in MOSFET channels. Unfortunately, many semiconductors with better electrical properties than silicon, such as gallium arsenide, do not form good semiconductor-to-insulator interfaces, thus are not suitable for MOSFETs Toshiba Satellite A200 Battery .
Research continues on creating insulators with acceptable electrical characteristics on other semiconductor material.
In order to overcome power consumption increase due to gate current leakage, high-? dielectric replaces silicon dioxide for the gate insulator, while metal gates return by replacing polysilicon (see Intel announcement) Toshiba Satellite 1200 Battery .
The gate is separated from the channel by a thin insulating layer, traditionally of silicon dioxide and later of silicon oxynitride. Some companies have started to introduce a high-? dielectric + metal gate combination in the 45 nanometer node Toshiba Satellite M300 Battery .
When a voltage is applied between the gate and body terminals, the electric field generated penetrates through the oxide and creates an "inversion layer" or "channel" at the semiconductor-insulator interface. The inversion channel is of the same type, P-type or N-type, as the source and drain, thus it provides a channel through which current can pass SONY VGP-BPS13A/Q Battery .
Varying the voltage between the gate and body modulates theconductivity of this layer and allows to control the current flow between drain and source.
A variety of symbols are used for the MOSFET SONY VGP-BPS13A/Q Battery .
The basic design is generally a line for the channel with the source and drain leaving it at right angles and then bending back at right angles into the same direction as the channel. Sometimes three line segments are used for enhancement mode and a solid line for depletion mode SONY VGP-BPS13B/Q Battery .
Another line is drawn parallel to the channel for the gate.
The bulk connection, if shown, is shown connected to the back of the channel with an arrow indicating PMOS or NMOS SONY VGP-BPS13B/Q Battery .
Arrows always point from P to N, so an NMOS (N-channel in P-well or P-substrate) has the arrow pointing in (from the bulk to the channel). If the bulk is connected to the source (as is generally the case with discrete devices) it is sometimes angled to meet up with the source leaving the transistor SONY VGP-BPS13/Q Battery .
If the bulk is not shown (as is often the case in IC design as they are generally common bulk) an inversion symbol is sometimes used to indicate PMOS, alternatively an arrow on the source may be used in the same way as for bipolar transistors (out for nMOS, in for pMOS) SONY VGP-BPS13/Q Battery .
Comparison of enhancement-mode and depletion-mode MOSFET symbols, along with JFET symbols (drawn with source and drain ordered such that higher voltages appear higher on the page than lower voltages):
For the symbols in which the bulk, or body, terminal is shown, it is here shown internally connected to the source SONY VGP-BPS21A/B Battery .
This is a typical configuration, but by no means the only important configuration. In general, the MOSFET is a four-terminal device, and in integrated circuits many of the MOSFETs share a body connection, not necessarily connected to the source terminals of all the transistors SONY VGP-BPS21/S Battery .
A traditional metal–oxide–semiconductor (MOS) structure is obtained by growing a layer of silicon dioxide (SiO2) on top of a silicon substrate and depositing a layer of metal or polycrystalline silicon (the latter is commonly used) SONY VGP-BPS21B Battery .
As the silicon dioxide is a dielectric material, its structure is equivalent to a planarcapacitor, with one of the electrodes replaced by a semiconductor.
When a voltage is applied across a MOS structure, it modifies the distribution of charges in the semiconductor SONY VGP-BPS21A Battery .
If we consider a P-type semiconductor (withNA the density of acceptors, p the density of holes; p = NA in neutral bulk), a positive voltage, VGB, from gate to body (see figure) creates a depletion layer by forcing the positively charged holes away from the gate-insulator/semiconductor interface SONY VGP-BPS21 Battery ,
leaving exposed a carrier-free region of immobile, negatively charged acceptor ions (see doping (semiconductor)). If VGB is high enough, a high concentration of negative charge carriers forms in an inversion layerlocated in a thin layer next to the interface between the semiconductor and the insulator Sony VGP-BPS21A/B Battery .
Unlike the MOSFET, where the inversion layer electrons are supplied rapidly from the source/drain electrodes, in the MOS capacitor they are produced much more slowly by thermal generation through carrier generation and recombination centers in the depletion region Sony VGP-BPS21/S Battery .
Conventionally, the gate voltage at which the volume density of electrons in the inversion layer is the same as the volume density of holes in the body is called the threshold voltage.
This structure with p-type body is the basis of the N-type MOSFET, which requires the addition of an N-type source and drain regionsSony VGP-BPS21A Battery .
MOSFET structure and channel formation
A metal–oxide–semiconductor field-effect transistor (MOSFET) is based on the modulation of charge concentration by a MOS capacitance between a bodyelectrode and a gate electrode located above the body and insulated from all other device regions by a gate dielectric layer which in the case of a MOSFET is an oxide, such as silicon dioxide Sony VGP-BPS21 Battery .
If dielectrics other than an oxide such as silicon dioxide (often referred to as oxide) are employed the device may be referred to as a metal–insulator–semiconductor FET (MISFET). Compared to the MOS capacitor, the MOSFET includes two additional terminals (source and drain), each connected to individual highly doped regions that are separated by the body region Sony Vaio PCG-5G2L Battery .
These regions can be either p or n type, but they must both be of the same type, and of opposite type to the body region. The source and drain (unlike the body) are highly doped as signified by a '+' sign after the type of doping.
If the MOSFET is an n-channel or nMOS FET, then the source and drain are 'n+' regions and the body is a 'p' region Sony Vaio PCG-5G3L Battery .
As described above, with sufficient gate voltage, holes from the body are driven away from the gate, forming an inversion layer or n-channel at the interface between the p region and the oxide. This conducting channel extends between the source and the drain, and current is conducted through it when a voltage is applied between source and drain Sony Vaio PCG-5J1L Battery .
Increasing the voltage on the gate leads to a higher electron density in the inversion layer and therefore increases the current flow between the source and drain.
For gate voltages below the threshold value, the channel is lightly populated, and only a very small subthreshold leakage current can flow between the source and the drain Sony Vaio PCG-5K2L Battery .
If the MOSFET is a p-channel or pMOS FET, then the source and drain are 'p+' regions and the body is a 'n' region. When a negative gate-source voltage (positive source-gate) is applied, it creates a p-channel at the surface of the n region, analogous to the n-channel case, but with opposite polarities of charges and voltages Sony Vaio PCG-5J2L Battery .
When a voltage less negative than the threshold value (a negative voltage for p-channel) is applied between gate and source, the channel disappears and only a very small subthreshold current can flow between the source and the drain.
The source is so named because it is the source of the charge carriers (electrons for n-channel, holes for p-channel) that flow through the channel Sony Vaio PCG-5K1L Battery ;
similarly, the drain is where the charge carriers leave the channel.
The device may comprise a Silicon On Insulator (SOI) device in which a Buried OXide (BOX) is formed below a thin semiconductor layer Sony Vaio PCG-6S2L Battery .
If the channel region between the gate dielectric and a Buried Oxide (BOX) region is very thin, the very thin channel region is referred to as an Ultra Thin Channel (UTC) region with the source and drain regions formed on either side thereof in and/or above the thin semiconductor layer Sony Vaio PCG-6S3L Battery .
Alternatively, the device may comprise a SEMiconductor On Insulator (SEMOI) device in which semiconductors other than silicon are employed. Many alternative semiconductor materials may be employed.
When the source and drain regions are formed above the channel in whole or in part, they are referred to as Raised Source/Drain (RSD) regions Sony Vaio PCG-6V1L Battery .
Modes of operation
The operation of a MOSFET can be separated into three different modes, depending on the voltages at the terminals. In the following discussion, a simplified algebraic model is used that is accurate only for old technology. Modern MOSFET characteristics require computer models that have rather more complex behavior Sony Vaio PCG-6W1L Battery .
For an enhancement-mode, n-channel MOSFET, the three operational modes are:
- Cutoff, subthreshold, or weak-inversion mode
- When VGS < Vth:
- where Vth is the threshold voltage of the device Sony Vaio PCG-6W2L Battery .
- According to the basic threshold model, the transistor is turned off, and there is no conduction between drain and source. In reality, the Boltzmann distribution of electron energies allows some of the more energetic electrons at the source to enter the channel and flow to the drain Sony Vaio PCG-6W3L Battery ,
- resulting in a subthreshold current that is an exponential function of gate–source voltage. While the current between drain and source should ideally be zero when the transistor is being used as a turned-off switch, there is a weak-inversion current, sometimes called subthreshold leakage Sony Vaio PCG-7111L Battery .
- In weak inversion the current varies exponentially with gate-to-source bias VGS as given approximately by:
- where ID0 = current at VGS = Vth, the thermal voltage VT = kT / q and the slope factor n is given by
- n = 1 + CD / COX,
- with CD = capacitance of the depletion layer and COX = capacitance of the oxide layer Sony Vaio PCG-7112L Battery .
- In a long-channel device, there is no drain voltage dependence of the current once VDS > > VT, but as channel length is reduced drain-induced barrier lowering introduces drain voltage dependence that depends in a complex way upon the device geometry (for example, the channel doping, the junction doping and so on) Sony Vaio PCG-7113L Battery .
- Frequently, threshold voltage Vth for this mode is defined as the gate voltage at which a selected value of current ID0 occurs, for example, ID0 = 1 ?A, which may not be the same Vth-value used in the equations for the following modes.Some micropower analog circuits are designed to take advantage of subthreshold conduction Sony Vaio PCG-7133L Battery.
- By working in the weak-inversion region, the MOSFETs in these circuits deliver the highest possible transconductance-to-current ratio, namely: gm / ID = 1 / (nVT), almost that of a bipolar transistor.
- The subthreshold I–V curve depends exponentially upon threshold voltage Sony Vaio PCG-7Z2L Battery ,
- introducing a strong dependence on any manufacturing variation that affects threshold voltage; for example: variations in oxide thickness, junction depth, or body doping that change the degree of drain-induced barrier lowering Sony Vaio PCG-8Y1L Battery .
- The resulting sensitivity to fabricational variations complicates optimization for leakage and performance.
- Triode mode or linear region (also known as the ohmic mode )
- When VGS > Vth and VDS < ( VGS – Vth )
- The transistor is turned on, and a channel has been created which allows current to flow between the drain and the sourceSony Vaio PCG-8Y2L Battery .
- The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. The current from drain to source is modeled as:
- Sony Vaio PCG-8Z1L Battery
- where ?n is the charge-carrier effective mobility, W is the gate width, L is the gate length and Cox is the gate oxide capacitance per unit area. The transition from the exponential subthreshold region to the triode region is not as sharp as the equations suggest Sony Vaio PCG-8Z2L Battery .
- Saturation or active mode
- When VGS > Vth and VDS > ( VGS – Vth )
- The switch is turned on, and a channel has been created, which allows current to flow between the drain and source SONY VAIO VGN-FW31J Battery .
- Since the drain voltage is higher than the gate voltage, the electrons spread out, and conduction is not through a narrow channel but through a broader, two- or three-dimensional current distribution extending away from the interface and deeper in the substrate SONY VAIO VGN-FW31E Battery .
- The onset of this region is also known as pinch-off to indicate the lack of channel region near the drain. The drain current is now weakly dependent upon drain voltage and controlled primarily by the gate–source voltage, and modeled approximately as SONY VAIO VGN-FW31B Battery :
- The additional factor involving ?, the channel-length modulation parameter, models current dependence on drain voltage due to the Early effect, or channel length modulation. According to this equation, a key design parameter, the MOSFET transconductance is SONY VAIO VGN-FW31M Battery :
- where the combination Vov = VGS – Vth is called the overdrive voltage, and where VDSsat = VGS - Vth (which Sedra neglects) accounts for a small discontinuity in ID which would otherwise appear at the transition between the triode and saturation regions SONY VAIO VGN-FW31Z Battery .
- Another key design parameter is the MOSFET output resistance rout given by:
- rout is the inverse of gDS where . VDS is the expression in saturation region SONY VAIO VGN-FW38M Battery .
- If ? is taken as zero, an infinite output resistance of the device results that leads to unrealistic circuit predictions, particularly in analog circuits.
- As the channel length becomes very short, these equations become quite inaccurate SONY VAIO VGN-AW41JF Battery .
- New physical effects arise. For example, carrier transport in the active mode may become limited by velocity saturation. When velocity saturation dominates, the saturation drain current is more nearly linear than quadratic in VGS. At even shorter lengths, carriers transport with near zero scattering, known as quasi-ballistic transport SONY VAIO VGN-AW41JF/H Battery .
- In addition, the output current is affected by drain-induced barrier lowering of the threshold voltage.
The body effect describes the changes in the threshold voltage by the change in the source-bulk voltage, approximated by the following equation SONY VAIO VGN-AW41MF Battery :
where VTN is the threshold voltage with substrate bias present, and VTO is the zero-VSB value of threshold voltage, ? is the body effect parameter, and 2? is the surface potential parameter.
The body can be operated as a second gate, and is sometimes referred to as the "back gate" SONY VAIO VGN-AW41MF/H Battery ;
the body effect is sometimes called the "back-gate effect".
The primacy of MOSFETs
In 1959, Dawon Kahng and Martin M. (John) Atalla at Bell Labs invented the metal–oxide–semiconductor field-effect transistor (MOSFET) SONY VAIO VGN-FW30B Battery .
Operationally and structurally different from the bipolar junction transistor, the MOSFET was made by putting an insulating layer on the surface of the semiconductor and then placing a metallic gate electrode on that. It used crystalline silicon for the semiconductor and a thermally oxidized layer of silicon dioxide for the insulator SONY VAIO VGN-FW50B Battery .
The silicon MOSFET did not generate localized electron traps at the interface between the silicon and its native oxide layer, and thus was inherently free from the trapping and scattering of carriers that had impeded the performance of earlier field-effect transistorsSONY VAIO VGN-FW51B/W Battery .
Following the (expensive) development of clean rooms to reduce contamination to levels never before thought necessary, and of photolithography and the planar process to allow circuits to be made in very few steps, the Si–SiO2 system possessed such technical attractions as low cost of production (on a per circuit basis) and ease of integration SONY VAIO VGN-FW51MF Battery .
Largely because of these two factors, the MOSFET has become the most widely used type of transistor in integrated circuits.
The MOSFET is used in digital CMOS logic, which uses p- and n-channel MOSFETs as building blocks. Overheating is a major concern in integrated circuits since ever more transistors are packed into ever smaller chips SONY VAIO VGN-FW51MF/H Battery .
CMOS logic reduces power consumption because no current flows (ideally), and thus no power is consumed, except when the inputs to logic gates are being switched. CMOS accomplishes this current reduction by complementing every nMOSFET with a pMOSFET and connecting both gates and both drains together SONY VAIO VGN-FW51ZF Battery .
A high voltage on the gates will cause the nMOSFET to conduct and the pMOSFET not to conduct and a low voltage on the gates causes the reverse. During the switching time as the voltage goes from one state to another, both MOSFETs will conduct briefly. This arrangement greatly reduces power consumption and heat generation SONY VAIO VGN-FW51ZF/H Battery .
Digital and analog CMOS applications are described below.
The growth of digital technologies like the microprocessor has provided the motivation to advance MOSFET technology faster than any other type of silicon-based transistorSONY VAIO VGN-FW52JB Battery .
A big advantage of MOSFETs for digital switching is that the oxide layer between the gate and the channel prevents DC current from flowing through the gate, further reducing power consumption and giving a very large input impedance. SONY VAIO VGN-FW54FB Battery
The insulating oxide between the gate and channel effectively isolates a MOSFET in one logic stage from earlier and later stages, which allows a single MOSFET output to drive a considerable number of MOSFET inputs. Bipolar transistor-based logic (such as TTL) does not have such a high fanout capacity SONY VAIO VGN-FW70DB Battery .
This isolation also makes it easier for the designers to ignore to some extent loading effects between logic stages independently. That extent is defined by the operating frequency: as frequencies increase, the input impedance of the MOSFETs decreases SONY VAIO VGN-FW71DB/W Battery .
The MOSFET's advantages in digital circuits do not translate into supremacy in all analog circuits. The two types of circuit draw upon different features of transistor behavior SONY VAIO VGN-FW72JGB Battery .
Digital circuits switch, spending most of their time outside the switching region, while analog circuits depend on MOSFET behavior held precisely in the switching region of operation. The bipolar junction transistor (BJT) has traditionally been the analog designer's transistor of choice SONY VAIO VGN-FW73JGB Battery ,
due largely to its higher transconductance and its higher output impedance (drain-voltage independence) in the switching region.
Nevertheless, MOSFETs are widely used in many types of analog circuits because of certain advantages SONY VAIO VGN-FW74FB Battery .
The characteristics and performance of many analog circuits can be designed by changing the sizes (length and width) of the MOSFETs used. By comparison, in most bipolar transistors the size of the device does not significantly affect the performance SONY VAIO VGN-FW81HS Battery .
MOSFETs' ideal characteristics regarding gate current (zero) and drain-source offset voltage (zero) also make them nearly ideal switch elements, and also make switched capacitor analog circuits practical. In their linear region, MOSFETs can be used as precision resistors, which can have a much higher controlled resistance than BJTs SONY VAIO VGN-FW81NS Battery .
In high power circuits, MOSFETs sometimes have the advantage of not suffering from thermal runaway as BJTs do. Also, they can be formed into capacitors and gyrator circuits which allow op-amps made from them to appear as inductors, thereby allowing all of the normal analog devices SONY VAIO VGN-FW81S Battery ,
except for diodes (which can be made smaller than a MOSFET anyway), to be built entirely out of MOSFETs. This allows for complete analog circuits to be made on a silicon chip in a much smaller space.
Some ICs combine analog and digital MOSFET circuitry on a single mixed-signal integrated circuit SONY VAIO VGN-FW82DS Battery,
making the needed board space even smaller. This creates a need to isolate the analog circuits from the digital circuits on a chip level, leading to the use of isolation rings and Silicon-On-Insulator (SOI). The main advantage of BJTs versus MOSFETs in the analog design process is the ability of BJTs to handle a larger current in a smaller space SONY VAIO VGN-FW82JS Battery .
Fabrication processes exist that incorporate BJTs and MOSFETs into a single device. Mixed-transistor devices are called Bi-FETs (Bipolar-FETs) if they contain just one BJT-FET and BiCMOS (bipolar-CMOS) if they contain complementary BJT-FETs. Such devices have the advantages of both insulated gates and higher current density SONY VAIO VGN-FW82XS Battery .
Over the past decades, the MOSFET has continually been scaled down in size; typical MOSFET channel lengths were once several micrometres, but modern integrated circuits are incorporating MOSFETs with channel lengths of tens of nanometers SONY VAIO VGN-FW83DS Battery .
Intel began production of a process featuring a 32 nm feature size (with the channel being even shorter) in late 2009. The semiconductor industry maintains a "roadmap", the ITRS, which sets the pace for MOSFET development SONY VAIO VGN-FW83JS Battery .
Historically, the difficulties with decreasing the size of the MOSFET have been associated with the semiconductor device fabrication process, the need to use very low voltages, and with poorer electrical performance necessitating circuit redesign and innovation (small MOSFETs exhibit higher leakage currents, and lower output resistance, discussed below) SONY VAIO VGN-FW83XS Battery .
Reasons for MOSFET scaling
Smaller MOSFETs are desirable for several reasons. The main reason to make transistors smaller is to pack more and more devices in a given chip area SONY VAIO VGN-FW90NS Battery .
This results in a chip with the same functionality in a smaller area, or chips with more functionality in the same area. Since fabrication costs for a semiconductor wafer are relatively fixed, the cost per integrated circuits is mainly related to the number of chips that can be produced per wafer Dell LATITUDE D800 battery .
Hence, smaller ICs allow more chips per wafer, reducing the price per chip. In fact, over the past 30 years the number of transistors per chip has been doubled every 2–3 years once a new technology node is introduced. For example the number of MOSFETs in a microprocessor fabricated in a 45 nm technology is twice as many as in a 65 nmchip Dell Inspiron 6400 AC Adapter .
This doubling of the transistor count was first observed by Gordon Moore in 1965 and is commonly referred to as Moore's law .