Class AB Push-Pull Power Amplifier

Design a complementary push-pull Class AB power amplifier with Vbe multiplier bias. Class AB eliminates crossover distortion by keeping both transistors slightly on at idle, while maintaining most of Class B's efficiency. This is the most common audio amplifier topology.

Power Requirements

Supply voltage (Vcc): V
Load impedance: Ω
Low frequency (-3dB): Hz

Bias Settings

Quiescent current (Iq): mA

Transistor Selection

Transistor pair:
β (hFE):
Ic max: A
Pd max (each): W
θjc: °C/W

Thermal Environment

Ambient temperature: °C
Max junction temp: °C
Heatsink (θsa) each: °C/W

Hover over labels for explanations.

Schematic

Component Values

Cin:
Cout:
Rb:
Rbias (divider):
R1 (Vbe mult):
R2 (Vbe mult):
Q3 (Vbe mult):

Bias Circuit

Bias voltage (Vbias):
Multiplier ratio:
Bias current:

Operating Point

Quiescent current (Iq):
Output DC bias (no signal):
Max output swing:
Peak collector current:

Input Requirements

Input voltage (peak):
Input voltage (RMS):
Input impedance:

Power Analysis

Max output power:
Output voltage (RMS):
Output current (RMS):
DC input power (full signal):
Efficiency at full power:

Thermal Analysis

Class AB adds idle dissipation from quiescent current to the Class B dissipation curve. Higher Iq reduces distortion but increases heat at all power levels.

Thermal limit (each):
Dissipation (idle):
Dissipation (full power):
Dissipation (worst case):
Junction temp (worst):

Power vs Output

Output Waveforms

Class AB eliminates crossover distortion - note the smooth transition through zero.

Design Notes

About the Vbe multiplier

The Vbe multiplier (also called "rubber diode" or "bias spreader") is a transistor circuit that creates an adjustable voltage drop between the output transistor bases.

How it works

The transistor Q3 maintains Vbe ≈ 0.7V across R2. The total voltage across the circuit is:

Vbias = Vbe × (1 + R1/R2)

By choosing R1 and R2, we can set any voltage from ~0.7V (R1=0) to several volts.

Thermal tracking

The key advantage over fixed diodes: if Q3 is thermally coupled to the output transistors (mounted on the same heatsink), its Vbe decreases as temperature rises (~-2mV/°C). This reduces the bias voltage when hot, preventing thermal runaway.

Setting quiescent current

For a given Vbias, the quiescent current depends on the output transistors' Vbe curves. In practice, R1 is often made adjustable (trimmer pot) to set the exact Iq.

Thermal runaway explained

Thermal runaway is a destructive positive feedback loop:

  1. Transistor heats up from power dissipation
  2. Vbe decreases (~-2mV/°C for silicon)
  3. With fixed bias voltage, collector current increases
  4. More current = more power dissipation = more heat
  5. Cycle repeats until transistor fails

Prevention

  • Vbe multiplier on heatsink: Bias tracks temperature (this design)
  • Emitter resistors: Re provides local negative feedback
  • Adequate heatsinking: Keep junction temperature rise low
  • Conservative Iq: Don't over-bias the output stage
Class A vs AB vs B comparison
Class A Class AB Class B
Conduction 360° 180°-360° 180°
Max efficiency 25% 50-78% 78.5%
Idle current High Low-Medium Zero
Crossover distortion None None (if biased correctly) Yes
Use case Hi-fi, low power Most audio amps RF, switching