Semiconductor Devices — MOSFET Amplifier Topologies

MOSFET Transistor

NMOS / PMOS — Cutoff → Triode → SaturationI_d = ½·k'·(W/L)·V_ov²

Calculated

RegionCUTOFF

Id0.00 mA

Vgs0.00 V

Vds0.00 V

Vov0.00 V

gm0.000 mS

Av0.00 V/V

Zin0.0 kΩ

Zout0.0 Ω

Pdiss0.00 mW

Operating Regions

cutoff

Vgs < Vth. No inversion layer. Transistor is OFF. Only tiny subthreshold leakage.

triode

Vgs > Vth and Vds < Vov. Channel is continuous. Acts as voltage-controlled resistor.

saturation

Vgs > Vth and Vds > Vov. Channel pinches off at drain. Current independent of Vds. Used for amplification.

Topology

Voltage gain: high (Av = -gm·Rd). Current gain: high. Input Z: very high (Rg). Output Z: Rd. Phase: 180°. Most common amplifier.

Curriculum

WAECNECOIGCSESATJUPEBA-Level

Teacher notes

•The MOSFET is the workhorse of modern electronics — billions exist in every smartphone.
•Common-Source: High voltage gain (Av = -gm·Rd), 180° phase shift. Most common amplifier topology.
•Common-Drain (Source Follower): Gain ≈ 1, very high Zin, low Zout. Used as voltage buffer.
•Common-Gate: Low Zin (1/gm), high gain, no Miller effect. Used in RF and high-frequency circuits.
•Cutoff: Vgs < Vth — transistor is OFF. Triode: Vds < Vov — acts as voltage-controlled resistor. Saturation: Vds > Vov — used for amplification.

Practice Problems

common source

Design a common-source amplifier with the following specifications.

Find Vgs for Id = 1mA and the resulting voltage gain Av.

Ans: Vgs = 2.11V, Av = -4.0 V/V

common source

A common-source NMOS amplifier operates in saturation.

Find Id, Vds, gm, and Av.

Ans: Id = 1.69mA, Vds = 3.45V, gm = 2.6mS, Av = -13.0 V/V

common drain

A source follower (common-drain) has the following parameters.

Find the voltage gain Av and output impedance Zout.

Ans: Av = 0.8 V/V, Zout = 500Ω