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&#39;s square, contributes to nonlinear distortion.
This nonlinear distortion can be mitigated by keeping the input signal&#39;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&#176; phase shift between the amplified and input signals.

small-signal analysis of mosfet amplifiers

In small-signal analysis, a MOSFET transistor amplifier acts as a linear amplifier when operating in its saturation region. The gate-to-source voltage (VGS) of the MOSFET is the sum of the DC biasing voltage and the small time-varying input signal. This combination sets up the operating point and modulates the drain current (ID) that flows from the drain to the source. When a small AC signal is superimposed on the DC bias voltage at the gate, the instantaneous drain current comprises three parts:
&#8226; The DC bias current (ID0) is the quiescent current with no signal applied.
&#8226; A component that varies linearly with the input signal is considered for amplification purposes.
&#8226; A nonlinear component is a function of the input signal&#39;s square and leads to distortion in the amplified output.
For small signals, where the amplitude of the AC component of VGS is sufficiently small, the nonlinear component of the drain current (which includes terms proportional to the square of the input signal) becomes negligible. By ignoring these nonlinear terms, the MOSFET&#39;s behavior can be linearly approximated, greatly simplifying analysis.
The MOSFET&#39;s transconductance (gm), represents the rate of change of drain current in response to changes in VGS , evaluated at the bias point is defined by:
<img alt="Equation 1" src="/files/ftp_upload/15169/15169_Equation_1.svg" style="text-align: center; height: 44px;" />
Where VGS0 is the DC bias voltage.
In the linear region of operation, the small-signal output current (id) is given by:
<img alt="Equation 2" src="/files/ftp_upload/15169/15169_Equation_2.svg" style="text-align: center; height: 20px;" />
Where vgs is the small-signal component of VGS, the voltage gain (Av) of the amplifier is given by:
<img alt="Equation 3" src="/files/ftp_upload/15169/15169_Equation_3.svg" style="text-align: center; height: 18px;" />
Where RL is the load resistance connected to the drain, the negative sign in the voltage gain equation indicates that the output signal is inverted relative to the input signal.

Small-Signal Analysis of MOSFET Amplifiers

Consider a MOSFET transistor amplifier circuit operating in its active region to provide linear amplification.
The total instantaneous gate-to-source ...