12.19 : MOSFET Amplifiers

The MOSFET, when operating in its active region, functions as a voltage-controlled current source. In this region, the gate-to-source voltage controls the drain current. This principle underlies the operation of the transconductance MOSFET amplifier. The output current is directed through a load resistor to convert this amplifier into a voltage amplifier. The output voltage is then obtained by subtracting the voltage drop across the load resistance from the supply voltage. This process results in an inverted output voltage that is shifted by the supply voltage.

The voltage-transfer characteristic plot of the amplifier illustrates the relationship between the output drain voltage and the input gate voltage. This plot is crucial for understanding the amplifier's behavior. It highlights the amplifier's active region, where the slope of the curve is steep, indicating maximum gain. However, this region is also nonlinear in terms of drain voltage.

A DC voltage bias is applied to the gate-to-source junction to achieve nearly linear amplification, positioning the MOSFET at a quiescent point (Q-point) within the active region. This biasing ensures that the MOSFET operates in a region where its behavior is approximately linear. When a small, time-varying signal is superimposed onto this DC bias voltage, it causes the MOSFET to operate around the Q-point. The MOSFET, as a result, functions within a short, nearly linear segment of its characteristic curve, resulting in an amplified output drain voltage.

In practical applications, this setup allows the MOSFET amplifier to amplify small AC signals effectively. The amplification occurs because the small input signal modulates the gate voltage around the Q-point, causing proportional variations in the drain current. These variations are translated into a larger output voltage across the load resistor, as a result achieving amplification.