FET Power Amplifiers - MOSFET Output Power Amplifier
Although there are a few, all MOSFET power amplifiers are uncommon. Most use a combination of bipolar transistors (for the input and gain stages), and MOSFETs for the output devices. This seems to be the most popular circuit arrangement, so I will concentrate on this. Figure below shows a fairly typical arrangement (in simplified form), and the operation of this is almost identical to that of an amplifier using bipolar transistors in the output. Note that emitter followers are needed to be able to provide the low impedance drive that MOSFETs need, although in some circuits they are not used. Instead, the Class-A driver stage (Q3) is operated at a higher than normal current to allow it to drive the MOSFETs properly.
One problem with this arrangement is that the drain to source voltage represents a circuit loss, so the power supply voltage needs to be typically +/-6V higher than the required peak output voltage to the load. Although this is not a major problem, it does increase dissipation in the output stage, and the loss increases with lower impedance loads.
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 for the MOSFETs. In an amp using +/-50V at 20 Amp main supplies, the secondary supply might be +/-60V, but capable of perhaps 1A maximum.
As with the bipolar amp (did you notice how similar they are?), I have not included components for stability. These are typically the same as for a standard bipolar transistor amp, but will usually include “stopper” resistors in series with the gates of the MOSFETs, and sometimes additional capacitance to prevent parasitic oscillation - the need for these varies from one device type to the next.
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