Amplifier Circuit Diagrams

4 Transistor Class AB Audio Amplifier

Description: This is a very simple and easy-to-build class AB audio amplifier using four transistors. Each output device in class AB operation conducts more than half of the input signal cycle. Class AB designs can achieve up to 78% efficiency while reducing cross-over distortion. The circuit shown here is suitable for small radio receivers, audio players, intercoms, telephones, and other similar devices.

The transistor Q1 and its associated components form a pre-amplifier stage. The audio input is connected to Q1’s base via resistor R1 and capacitor C1. Q1 is biassed from the collector to the base by resistor R3, and C3 is an AC bypass capacitor for the collector resistor R4. For circuits like this, collector-to-base biassing is a good method of biassing because it provides enough negative feedback, prevents thermal runway, and stabilizes the operating point. The second stage is the push-pull pair’s driver stage.

This is done by Q2 and its associated components. This stage is also collector to base biassed, and its input is coupled to the preamplifier stage’s output via capacitor C2. Q2’s collector current is limited by resistor R8. The third stage consists of the class AB push-pull section, which includes transistors Q3 and Q4. The bias voltage for the push-pull stage is provided by diodes D1 and D2. The amplifier’s output is coupled to the loudspeaker via capacitor C4. The capacitors C5 and C6 are power supply filter capacitors.

Circuit diagram of 4 Transistor Class AB Audio Amplifier

Class AB Audio Amplifier

4 Transistor Class AB amplifier


Below are a few important notes for Class AB audio amplifie, please check them:

  • The circuit can be put together on a Vero board.
  • K1 can function as an 8 ohm/5W speaker.
  • C6 must be grounded near Q1, and C5 must be grounded near the ground of the loudspeaker. This reduces background noise.
  • Power the circuit with 5V DC.

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