FET output stages
The characteristics of power FETS
An FET is essentially a voltage-controlled device. So are BJTs, despite the conventional wisdom that persists in regarding them as current-controlled. They are not, even if BJT base currents are non-negligible.
The power FETs normally used are enhancement devices – in other words, with no voltage between gate and source they remain off. In contrast, the junction FETs found in small-signal circuitry are depletion devices, requiring the gate to be taken negative of the source (for the most common N-channel devices) to reduce the drain current to usable proportions. (Please note that the standard information on FET operation is in many textbooks and will not be repeated here.)
Power FETs have ...