Operation of MOSFET amplifier

Introduction

• Fig. 7.1.1 shows n-channel, enhancement mode MOSFET common-source circuit with a time-varying (a.c.) voltage source in series with the d.c. source. We assume the time-varying input signal is sinusoidal.

• Fig. 7.1.1 shows the MOSFET characteristics, d.c. load line, and Q-point, where the d.c. load line and Q-point are functions of vGS, VDD RD and the MOSFET parameters.

• For the output voltage to be a linear function of the input voltage, the MOSFET is biased in the saturation region.

• The Fig. 7.1.1 (b) shows sinusoidal variations in the gate-to-source voltage, drain current and drain-to-source voltage, as a result of the sinusoidal source v .

• The total gate-to-source voltage is the sum of VGSQ and vi.

• As Vi increases, the instantaneous value of VGS increases, and the bias point moves up the load line. A larger value of VGS means a larger drain current and a smaller value of VDS .

Review Question

1. Explain the operation of CS EMOSFET amplifier with the help of dc load line.