Field Effect Transistors and MOSFETs

Now on to FETs and MOSFETs. FET stands for Field Effect Transistor, and MOSFET means Metal Oxide Semiconductor Field Effect Transistor. This topic is something of a can of worms, not because of some deficiency in the devices, but because of the huge array of different types. The basic FET types are

N-Channel Enhancement Mode MOSFETs
P-Channel Enhancement Mode MOSFETs
N-Channel Depletion Mode MOSFETs
P-Channel Depletion Mode MOSFETs

There are a couple of major sub-classes of MOSFET - lateral and vertical. Lateral MOSFETs are particularly suited to audio applications, as they are far more linear than their vertical brethren, although their gain is generally lower. Vertical MOSFETs are ideally suited to switching applications, and this includes Pulse Width Modulated (PWM) amplifiers.

The terms ‘lateral’ and ‘vertical’ refer to internal fabrication methods, so many others you may come across (such as HEXFETs ^®) are essentially variations of the vertical process. This is still not all the possibilities, because there are additional sub-classes as well, particularly with switching MOSFETs. However, for the purpose of a general article on their characteristics and how they work, I will concentrate on the most commonly used versions. This narrows the field, and we are left with both polarities of junction FETs, and both polarities of enhancement mode MOSFETs. With these, we cover the major proportion of current designs, so even ‘though I will be leaving out a lot, the stuff I leave out is not all that common (he says hopefully).

FETs are “unipolar” devices, in that they use only one polarity of carrier, in contrast to bipolar transistors, which use both majority and minority charge carriers (electrons or “holes”, depending on the polarity). FETs are far more resistant to the effects of temperature, X-Rays and cosmic radiation - any of these can cause the production of minority carriers in bipolar transistors).

I shall concentrate only on three terminal FETS, and the terminals are

Source - The electron “source” (for N-channel devices), and is the equivalent of the cathode of a valve or the emitter of a transistor
Gate - Control terminal - (more or less) equivalent to the grid of a valve or base of a transistor
Drain - The terminal from which current is “drained” - equivalent to the plate of a valve or collector of a transistor

There is no simple equivalent circuit for FETs (as there is for transistors), but this is of no consequence. The gate is the controlling element, and affects the electron flow not by amplifying a current (as in the transistor), but by the application of a voltage. The input impedance of junction FETs is very high at all usable frequencies, but MOSFETs are different. They have an almost infinite input resistance, but appreciable capacitance between the gate and the rest of the device. This can make MOSFETs hard to drive, because the capacitive loading makes most amplifier devices unhappy.The junction FET is common in the inputs of high performance opamps, and offers extremely high input impedance. Indeed this is the case for discrete FETs as well, and a simple voltage amplifier using a junction FET and a power MOSFET are both shown in Figure 3.1. Both devices are N-Channel, and note that the arrow points in a different direction for each. The arrows point in the opposite direction for a P-Channel device, and all polarities are reversed. Vdd is +20V.

Junction FETs are depletion mode devices, and (like all depletion mode FETs and MOSFETs) can be biased in exactly the same way as a valve. Depletion mode means that without a negative bias signal on the controlling element (the gate), there will be current flow between the drain (equivalent to plate or collector) and source (equivalent to cathode or emitter).

An enhancement mode device remains turned off until a threshold voltage is reached, after which the device conducts, passing more current as the voltage increases. Although there are MOSFETs made for low power operation, the majority (in audio, anyway) are power devices. These are almost exclusively enhancement mode, and can be capable of very high current.

In Figure 3.1, the power MOSFET is an enhancement mode device, and the junction FET is depletion mode. These are the most commonly used in audio. Enhancement mode power MOSFETs are also used in switching power supplies, and are far better than bipolar transistors in this role. They are faster, so switching losses are not as great (therefore the MOSFETs run cooler), and they are more rugged, and able to withstand abuses that would kill a bipolar transistor almost instantly.

This ruggedness (coupled with the complete freedom from second breakdown effects), means that MOSFETs are very popular as output devices for high power professional amplifiers. In this area, the MOSFET is second to none, and they are firmly entrenched as the device of choice for high power.

This is not to say that this is the only place MOSFETs are used. There are many fine audiophile power amps (and even preamps) that use power MOSFETs, and there are many claims that they are sonically superior to bipolar transistors (again, a debate that I will not discuss here).

Somewhat like valves, FETs and MOSFETs are very device dependent, and it is not normally possible to just substitute one device for a different type. Also like valves, the gain that can be expected from a voltage amplifier circuit is device dependent, and the manufacturer’s data sheet (or testing) is the only way that one can be sure of obtaining the gain required in a given circuit.

FET Power Amplifiers - MOSFET Output Power Amplifier Some (especially very high power) amps get around this by using a low current (but higher voltage) secondary power supply for the drive circuit, and the main high current supply...
FET Current Amplifier Again, I have shown both a junction FET and a MOSFET in Figure 3.3, both common-drain or source-follower circuits. The MOSFET requires a positive voltage, and this must be greater...
Touch Activated Light The gate of the MOSFET draws no current so the voltage on the gate will be half the supply voltage or 6 volts when the resistance across the touch contacts...
Microphone Transistors PreAmplifiers Below exist two choices for preamplifiers of microphone that use transistor, in asymmetrical of input connection. The first choice, are with diode zener, the other choice, use voltages that give...
Two Transistor Wien Bridge Oscillator A high gain transistor like the MPSA-18 for the first transistor will allow a much larger value for R, up to 1 Megohm. The circuit should draw between 18 and...
FET Power Amplifiers - Single Ended MOSFET Class-A Amplifier In exactly the same way as a power valve can be used in single-ended Class-A, so too can a MOSFET. The distortion from a circuit such as that shown will...

Related posts brought to you by Yet Another Related Posts Plugin.

del.icio.us Digg it Furl iFeedReaders ma.gnolia Maple.nu Netscape reddit Spurl StumbleUpon Wink Yahoo MyWeb

This entry was posted on Thursday, January 10th, 2008 at 2:45 am and is filed under Audio power amplfier, Audio preamp circuits, Electronics And Projects, Schematics. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.