Name: mosfet amplifier gain and signal distortion
Uploaded: 2025-05-22T09:06:15.000Z
Duration: 1 min 23 s
Channel: JoVE

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

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'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's behavior can be linearly approximated, greatly simplifying analysis.
The MOSFET'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="Transconductance equation: gm = &#8706;ID/&#8706;VGS | VGS=VGS0; formula illustration for semiconductor study." src="/files/ftp_upload/15169/15169_Equation_1.svg" style="text-align: center; height: 44px;" data-altupdatedat="1754904545000" data-alt="Transconductance formula \(g_m\), derivative equation; useful in semiconductor device analysis." />
Where VGS0 is the DC bias voltage.
In the linear region of operation, the small-signal output current (id) is given by:
<img alt="MOSFET operation formula, i&#60;sub&#62;d&#60;/sub&#62; = g&#60;sub&#62;m&#60;/sub&#62;V&#60;sub&#62;gs&#60;/sub&#62;, semiconductor analysis." src="/files/ftp_upload/15169/15169_Equation_2.svg" style="text-align: center; height: 20px;" data-altupdatedat="1754904545000" data-alt="MOSFET current equation \( i_d = g_m V_{gs} \), electrical engineering formula." />
Where vgs is the small-signal component of VGS, the voltage gain (Av) of the amplifier is given by:
<img alt="Operational amplifier voltage gain formula \( A_v = -g_m R_L \) equation." src="/files/ftp_upload/15169/15169_Equation_3.svg" style="text-align: center; height: 18px;" data-altupdatedat="1754904545000" data-alt="Amplifier gain equation, Av=-gmRL, formula." />
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.

In small-signal analysis, a MOSFET transistor amplifier acts as a linear amplifier when operating in its saturation region. The gate-to-source voltage ...

small-signal-analysis-of-mosfet-amplifiers

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mosfet amplifier gain and signal distortion

MOSFET amplifiers can be treated as linear circuits when they operate in the saturation region. The gate-to-source voltage, or VGS, is the sum of a DC bias voltage and a small time-varying input signal. That bias sets the operating point and controls the drain current that flows from drain to source.

When a small AC signal is added at the gate, the drain current has three parts. One part is the DC bias current, ID0, which is the quiescent current with no signal applied. A second part changes linearly with the input signal and is used for amplification. A third part depends on the square of the input signal and can cause distortion in the output.

For small signals, the AC part of VGS is small enough that the nonlinear current term becomes negligible. That lets the MOSFET be modeled with a linear approximation, which makes analysis much simpler. The transconductance, gm, describes how much the drain current changes when VGS changes at the bias point.

In this linear small-signal model, the output current id depends on the small-signal voltage vgs. The voltage gain, Av, is found from gm and the load resistance RL connected to the drain. The negative sign in the gain equation shows that the output signal is inverted relative to the input signal.

MOSFET Amplifier Gain and Signal Distortion

mosfet-amplifier-gain-and-signal-distortion

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MOSFET amplifiers can be treated as linear circuits when they operate in the saturation region. The gate-to-source voltage, or VGS, is the sum of a DC bias voltage and a small time-varying input signal. That bias sets the operating point and controls the drain current that flows from drain to source. When a small AC signal is added at the gate, the drain current has three parts. One part is the DC bias current, ID0, which is the quiescent current with no signal applied. A second part changes...

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Learn how MOSFET amplifiers use DC bias, small AC signals, transconductance, and load resistance to set gain and create inverted output.

MOSFET Amplifier Gain and Signal Distortion | JoVE K12

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MOSFET Amplifier Gain and Signal Distortion

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