Power Amplifiers Class AB

A Class AB amplifier is a hybrid operating mode that combines the efficiency advantage of Class B with the linearity of Class A by introducing a small quiescent bias current. It is the most widely used power amplifier topology in audio electronics, where low distortion and reasonable efficiency must coexist. Understanding how this small bias current eliminates crossover distortion is critical for GATE and analog electronics courses.

Core Concept: Why Class AB Exists

The diagram below compares Class B crossover distortion with Class AB operation and shows the diode biasing circuit used to establish the required quiescent current.

Class B amplifiers are efficient but suffer from crossover distortion because both transistors are biased at exactly cut-off. As the signal transitions through zero volts, neither transistor conducts for the brief interval when the input is within plus or minus one V_BE of zero. The resulting dead zone introduces odd harmonic distortion components into the output. Class AB solves this by biasing both transistors slightly into their active region simultaneously, so that there is always a small continuous collector current even with no input signal applied.

The quiescent current I_Q is typically set to a small fraction of the peak output current, often between 5 percent and 10 percent of I_peak. With both transistors conducting a small current at rest, the transition through zero volts is handled smoothly because one transistor is already conducting and simply increases its current while the other decreases from its quiescent level to zero, rather than having to overcome the full V_BE threshold from complete cut-off.

The conduction angle in Class AB is therefore between 180 degrees and 360 degrees, placing it between Class B and Class A. The precise angle depends on the quiescent bias level. A higher quiescent bias pushes the operation toward Class A, with lower distortion but lower efficiency. A lower quiescent bias approaches Class B behavior.

Biasing Techniques for Class AB

The simplest and most common method to bias a Class AB push-pull stage is to use two diodes in the signal path between the bases of the NPN and PNP transistors. These diodes are typically placed between a resistor divider connected to the supply rails. Each diode produces approximately 0.6 V drop, totaling about 1.2 V between the two bases. Since each transistor needs V_BE approximately 0.6 V to begin conducting, this 1.2 V bias puts both transistors just at the edge of conduction simultaneously.

A more flexible technique is the V_BE multiplier, also called the rubber diode or transistor Q3 circuit. A small transistor with a voltage divider on its base-emitter terminal is connected between the two output transistor bases. By adjusting the resistor ratio, the voltage developed across the V_BE multiplier can be set to any desired value between V_BE and several times V_BE, giving precise control over quiescent current. This circuit is standard in discrete and IC audio amplifier designs.

Thermal stability is a critical concern. Because V_BE decreases with temperature at about minus 2 mV per degree Celsius, quiescent current increases as the transistors heat up during operation. If the bias diodes or V_BE multiplier are thermally coupled to the output transistors, they track the temperature change and automatically reduce the bias voltage, compensating for the rise in quiescent current. Failure to implement proper thermal tracking causes thermal runaway, where rising temperature causes increasing I_Q, which causes more power dissipation, which further raises temperature in a destructive positive feedback loop.

Efficiency and Practical Performance

The efficiency of a Class AB amplifier lies between Class A (maximum 25 percent for resistive load, 50 percent for transformer coupled) and Class B (maximum 78.54 percent). The exact value depends on the quiescent current setting. For small quiescent bias, efficiency is close to Class B. As I_Q increases toward the Class A boundary, efficiency drops toward that of Class A.

In typical audio amplifier implementations with moderate quiescent bias, practical efficiency ranges from 50 to 70 percent at rated power output. The low quiescent current in Class AB means that idle power dissipation is low compared to Class A, which is a major practical advantage in battery-operated equipment and high power audio systems.

Solved Numerical Example

For a Class AB amplifier, the efficiency analysis at maximum output closely resembles the Class B calculation, since the quiescent current contribution to DC power is small at full power. At low signal levels the quiescent current dominates, and efficiency drops significantly.

Given:
V_CC = 12 V (dual supply: +12V and -12V)
R_L = 8 ohm
Quiescent current I_Q = 50 mA (each transistor)
Peak output voltage V_m = 10 V at full output

Why this formula applies:
At full output power, Class AB efficiency approximates Class B.
At idle, extra power is dissipated due to quiescent current.

Formula:
P_out = V_m^2 / (2 * R_L)
P_quiescent = 2 * V_CC * I_Q  (total idle dissipation)
P_DC_full = (2 * V_CC * V_m) / (pi * R_L) + P_quiescent (approximate)
eta_approx = P_out / P_DC_full

Substitution:
P_out = (10)^2 / (2 * 8) = 100 / 16 = 6.25 W
P_quiescent = 2 * 12 * 0.05 = 1.2 W
P_DC_full = (2 * 12 * 10) / (pi * 8) + 1.2 = 240/25.13 + 1.2 = 9.55 + 1.2 = 10.75 W
eta_approx = 6.25 / 10.75 = 58.1%

Idle dissipation only:
P_idle = 2 * V_CC * I_Q = 2 * 12 * 0.05 = 1.2 W

Final Answer:
Full power output = 6.25 W
Approximate efficiency at full output = 58.1%
Idle power dissipation = 1.2 W
(Compare: ideal Class B eta at V_m=10, V_CC=12 = (pi/4)*(10/12) = 65.4%)

Exam Tip: Class AB efficiency is always less than Class B (78.5%) due to quiescent current. In GATE problems, if a small bias current is mentioned, treat efficiency as slightly below Class B maximum. Diode biasing sets 2*VBE between bases to bias both transistors just at conduction threshold simultaneously.

Quick Revision

• Class AB = Class B with small quiescent bias current I_Q. Conduction angle between 180 and 360 degrees.
• Crossover distortion is eliminated because both transistors are biased slightly into active region at all times.
• Efficiency is between Class A (25-50%) and Class B (78.5%). Closer to Class B for small I_Q.
• Diode biasing: two diodes provide ~1.2V between bases of NPN and PNP, matching their VBE requirements.
• VBE multiplier (rubber diode): transistor-based bias circuit allowing precise adjustment of quiescent voltage.
• Thermal tracking is mandatory. Bias diodes must be thermally coupled to output transistors to prevent thermal runaway.
• Class AB is the standard topology for audio power amplifiers due to the combination of low distortion and acceptable efficiency.