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.
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/
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.
- 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.