FET Output Stages
The Characteristics of Power FETs
A field-effect transistor (FET) is essentially a voltage-controlled device. So are bipolar junction transistors (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 ...