ABSTRACT
Mosfet amplifier presents a tantalising prospect for simple circuitry. The 'standard' mosfet amplifier circuit differs from the equally standard bipolar-style circuit mainly in possessing a sort of push-pull/current-mirror configuration in the voltage amplifier stage, probably intended to provide better charge/discharge of the mosfet input capacitances. This stage is sometimes called the pre-driver, but it is less confusing to call the first full-voltage-swing stage 'the voltage amplifier stage'. For a better amplifier, two routes were examined. The first is to reduce the cost of a mosfet power amp by using two N-channel devices in a form of quasi-complementary output stage. The second is to increase linearity and improve quiescent stability by using bipolar drivers with local feedback around each output FET. Distortion performance is not easy to specify completely, ideally requiring a spectrum analysis of every combination of level, frequency and load impedance.
