Class AB Amplifier

The TIP31/TIP32 class AB amplifier on a 20 V supply, driving an 8 Ω speaker. Step or play through one cycle of a sine wave and watch where the current goes. Voltages update at each test point.

+20 V R3 330ΩR4 330Ω R7 1kR8 680ΩVR1 1k Q4 2N3904(bias) Q2 TIP31Q3 TIP32 R50.47ΩR60.47Ω C3 100µFC22200µF 8ΩAudio inC1 10µF Rb 1.2kQ1 2N3904RE1 27ΩRE2 22ΩCE 470µF(bypass)+Rf 7.5k +++ 15.4 V 10.8 V 9.6 V 10.2 V 0.0 V 1.3 V 0.65 V
Output current (speaker loop) Driver and bias current Input and feedback current Thicker, faster dashes = more current (log scale)

Signal

60° 100%
1 A0.50 0°180°360° TIP31 collector TIP32 collector

What's happening

Currents right now

Approximate values from a simple model of the circuit (transistor gain about 70, idle current 30 mA), at full volume about 3 W into 8 Ω. CE carries only the signal part of Q1's current; the steady 14 mA flows through RE2.

Project status Work in progress

Where it stands

  • Prototype built on a breadboard with a 20 V bench supply. Resting voltages came within about 0.1 V of the predictions.
  • With a 1 kHz test tone below 1 V, the scope shows a clean, unclipped sine wave at the speaker.

Still being tested

  • Output bias. The original two-diode bias drove too much idle current and overheated a TIP32. It is being replaced by the adjustable VBE multiplier shown here.
  • Voltage gain. Measured about 3.3, against about 8 predicted. Splitting the emitter resistor (27 Ω + 22 Ω, with the 22 Ω bypassed by 470 µF) should raise it to about 5.5. Not yet measured.
  • Idle current, crossover and clipping checks on the scope.

Next steps

  • Bluetooth input from a phone, using a small receiver board powered from the amp's supply.
  • Stereo: a second identical channel sharing the supply, with a dual volume control.
  • Powered speaker units of about 5 W, with laser-cut and 3D-printed enclosures, as a student design project.