4.1 Introduction

MOSFETs can be used to create both common source voltage amplifiers and common drain voltage followers (i.e., source followers). Both circuits offer the potential for very high input impedance due to the extremely low gate current MOSFETs provide. As with JFET amplifiers, at higher frequencies input capacitance dominates and reduces the input impedance. Not all bias prototypes lend themselves to all possible AC circuits. For example, zero bias for a DE-MOSFET is not suitable for followers or swamped amplifiers as it lacks a source resistor. The same is true for voltage divider biasing used with both DE- and E-MOSFETs. These biasing schemes are suitable for non-swamped amplifiers, though.

In general, MOSFET amplifiers tend to have good high frequency performance, offer low noise and exhibit low distortion with modestly sized input signals. Compared to BJTs, their voltage gain magnitude is lower.

A key parameter in determining gain is the device’s transconductance, 𝑔𝑚 . Transconductance varies widely depending on the kind of MOSFET used. A small signal DE-MOSFET may exhibit a transconductance of just a few millisiemens. In contrast, a high power E-MOSFET may exhibit a transconductance of over 100 siemens.