MOSFET Amplifiers

Small-Signal Model

As MOSFETs have no gate current, their small signal model is much simpler than that of a BJT.

Between the gate and the source is an open circuit, but the voltage between the two sets the dependant current source . The MOSFET also has infinite input impedance.

Common-Source Amplifier

Similar to a BJT common emitter amplifier, can construct a MOSFET common source amp:

Using the small signal model of the MOSFET, this amplifier looks like this:

Drain current , so:

Note that transconductance in a MOSFET is:

This is much lower than transconductance in a BJT, hence the gain is much lower/

Bypass Capacitors

Adding a bypass capacitor to the amplifier increases the gain, while keeping the DC Q-point stable. Remember that capacitors act as short circuits in AC, and open circuits in DC.

The gain of the amplifier with a bypass capacitor is much higher.

Input and Output Impedance

  • The input impedance of a MOSFET is infinite, as no current flows between gate and source.
  • The overall input impedance of a common source MOSFET amp is , as the two gate bias resisisors will act as impedances to input signals
  • The output impedance of the bypassed amplifier above is just , as that's the only impedance in the model.
    • If , like in a common drain/source follower, then this becomes
  • MOSFETs have higher input impedances for this reason, so MOSFET amplifiers are used over BJTs where high impedance is required.