transistors

Consider a MOSFET transistor amplifier circuit operating in its active region to provide linear amplification.

The total instantaneous gate-to-source voltage, which combines the DC voltage and the input signal, results in an instantaneous drain current comprising three distinct components.

The first component represents the DC bias current, while the second is directly proportional to the input signal. The third component, proportional to the input signal's square, contributes to nonlinear distortion.

This nonlinear distortion can be mitigated by keeping the input signal's magnitude small.

Under the small-signal condition, the last term in the instantaneous drain current expression can be neglected.

The ratio of the output signal drain current to the gate-source signal voltage is defined as the MOSFET transconductance, which is determined by the slope of the characteristic curve at the bias point.

The signal component of the drain voltage can be represented using the transconductance.

As a result, the voltage gain equals the negative product of the transconductance and the load resistance. The negative sign indicates a 180° phase shift between the amplified and input signals.