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Quasi complementary-symmetry class AB amplifier

Short answer

Quasi complementary symmetry uses two NPN power transistors: an NPN Darlington for the upper half and a PNP driver with NPN power transistor (Sziklai pair) for the lower half, acting as a high-power PNP. Diodes provide bias for class AB operation, reducing crossover distortion.

Draw the circuit diagram of Quasi complementary-symmetry class AB amplifier using diodes.

A quasi-complementary class AB amplifier is used when a matched pair of NPN and PNP power transistors is not available. Both output (power) transistors are NPN. The upper half is an NPN Darlington pair, and the lower half is a PNP driver feeding an NPN power transistor (a feedback or Sziklai pair, which acts like a high-power PNP).

Circuit diagram

          +VCC ------------------+------------------+
            |                    |                  |
          [R1]                   C                  C
            |                    |                  |
            +------------------- B  Q1 (NPN)        |
            |                    E ---------------- B  Q2 (NPN power)
           D1                                       E
            |                                       |
 Vi o--||---+                                     [Re]
       Cc   |                                       |
           D2                    +------------------+-----+----o Vo
            |                    |                  |     |
           D3                    E                  C    [RL]
            |                    |                  |     |
            +------------------- B  Q3 (PNP)        |    GND
            |                    C ---------------- B  Q4 (NPN power)
          [R2]                                      E
            |                                       |
          -VCC -------------------------------------+
  • Q1-Q2: NPN Darlington (upper half). Q1 base is at the top of the diode string; Q2 emitter goes through Re to the output.
  • Q3-Q4: PNP driver Q3 (emitter to the output) whose collector drives the base of the NPN power transistor Q4 (collector to the output, emitter to −VCC). This pair is the lower half.
  • R1, R2 and the diodes form the bias chain between the two driver bases; the input is coupled in through Cc. Exact wiring and the number of diodes vary between textbooks.

Working

  • The diodes (or a VBE multiplier) develop enough voltage to forward bias the base-emitter junctions in the signal path. There are three junctions between the two driver bases (Q1, Q2 and Q3), so three diodes are used (some texts show two diodes plus a small resistor). This sets a small quiescent current, which removes crossover distortion (class AB).
  • Positive half-cycle: Q1-Q2 conduct and source current into RL from +VCC.
  • Negative half-cycle: Q3-Q4 conduct and sink current from RL to −VCC. The combination Q3-Q4 acts as a single PNP transistor with current gain of about β3 β4.
  • Small emitter resistors Re give thermal stability and some current limiting.

✓ Answer checked by Claude Opus 5.5 (high effort)

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