MOSFET as Amplifier Introduction

A MOSFET can work as more than an on-off switch. When it is biased in a controlled operating region, a small change in gate-source voltage changes drain current, and a load converts that current change into a larger voltage change.

Learning Objectives

By the end of this lesson, you should be able to explain why a MOSFET can amplify a signal, describe bias point and transconductance, recognize the common-source topology, identify clipping and capacitance limits, and know when a MOSFET input stage is useful.

The Amplifying Action

The gate is insulated, so almost no DC current enters it. The input voltage VGS controls drain current ID. Around a bias point, the small-signal relationship is approximately:

$$
g_m=\frac{\Delta I_D}{\Delta V_{GS}}
$$

where gm is transconductance in siemens. If a small gate voltage change causes a useful drain current change, the MOSFET can amplify.

Bias Point

An amplifier needs a quiet operating point before the signal is applied. This is the bias point or Q point. If the bias is too close to cutoff, the negative half of the signal clips. If it is too close to the supply rail, the positive half clips.

flowchart LR IN["Small input voltage"] --> G["Gate-source change"] --> ID["Drain current change"] --> RD["Drain resistor"] --> OUT["Larger output voltage"]

Common-Source Amplifier

The common-source stage uses the gate as input, the source as the common node, and the drain as output. The output is inverted. A simple small-signal gain estimate is:

$$
A_v \approx -g_m R_D
$$

If gm = 5 mS and RD = 4.7 kOhm:

$$
A_v \approx -0.005 \times 4700 = -23.5
$$

Real gain is lower because of load resistance, device output resistance, source degeneration, and capacitances.

Source Degeneration

Adding a resistor in the source improves stability and linearity. It reduces gain but makes the circuit less sensitive to device variation.

$$
A_v \approx -\frac{g_m R_D}{1+g_m R_S}
$$

This is often a better engineering tradeoff than chasing maximum gain.

Practical Limits

MOSFET amplifier design must account for input capacitance, Miller effect, supply headroom, thermal drift, and linear-mode power dissipation:

$$
P = V_{DS} I_D
$$

MOSFET input stages are useful for high-impedance sensors, sample-and-hold buffers, low-input-current measurements, audio preamps, and CMOS integrated circuits. For precision low-noise work, an op-amp or dedicated instrumentation amplifier is usually easier.

Common Mistakes

Trying to amplify without setting a DC bias point.
Confusing switching saturation with analog saturation.
Forgetting the output is inverted in a common-source stage.
Ignoring gate capacitance with high-value sensor sources.
Expecting identical bias from two random MOSFETs without feedback.

Summary

A MOSFET amplifier converts small gate-voltage changes into drain-current changes. Biasing sets the usable operating point, gm sets the current sensitivity, and the drain load converts current into voltage. Real designs must leave headroom and control capacitance, temperature, and power dissipation.

Mind Map

mindmap root((MOSFET Amplifier)) Core concept VGS controls ID Bias sets Q point Drain load makes voltage Formulas gm equals delta ID over delta VGS Av approx negative gm RD Source resistor lowers gain P equals VDS ID Applications Sensor buffers Audio preamps CMOS stages Current sources Design rules Leave headroom Stabilize bias Manage capacitance Check heat Practical checks Clipping both rails Bias over temperature Input source impedance Output load Common mistakes No DC bias Wrong region terms Ignoring Miller effect Expecting exact gain