Terminology » RF Components » Amplifier Classes: A, B, AB, and C - A Comparison

Amplifier Classes: A, B, AB, and C - A Comparison

amplifier class a class b class ab class c

This article compares different amplifier classes: Class A, Class B, Class AB, and Class C, highlighting their key characteristics and performance.

Amplifier Class-A Operation

In Class-A operation, the amplifier operates in the active region for the entire input cycle (2π).
Stage efficiency = P_L(max) / P_S x 100% where P_S is the DC input power.
Load Power = P_L = (V_L)² / R_L = (V_PP)² / (8 * R_L)

Amplifier Class-B Operation

In Class-B operation, current flows for only half of the input cycle (π).
The Q-point is located at the cutoff point on the DC and AC load lines. V_CEQ = V_CC / 2
AC load power of a Class-B push-pull amplifier: P_L = (V_PP)² / (8 * R_L)

Amplifier Class-AB Operation

In Class-AB operation, the amplifier operates between half and the full input cycle (between π and 2π).

Amplifier Class-C Operation

In Class-C operation, the amplifier operates for less than half of the input cycle (less than π).
It’s primarily a tuned amplifier.
AC load power for a Class-C amplifier: P_L = (V_PP)² / (8 * R_L)
- Where V_PP is the peak-to-peak load voltage and R_L is the load resistance.

Comparison Table: Class A, B, AB, and C

Amplifier Class Type	Q point	I_q	Conduction Angle	Maximum Efficiency
Class A	0.5	0.5	2π	50%
Class B	0	0	π	78%
Class AB	0-0.5	0-0.5	π-2π	50-78%
Class C	<0	0	0-π	Approaches 100%

To operate a transistor (FET) in a particular class, the gate and drain DC voltages must be carefully biased to a specific operating point, called the Quiescent point or Q-point. Class A offers the least distortion, while Class C offers the maximum distortion of the output signal.