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Complementary-symmetry class AB with Darlington pair

Short answer

The Darlington complementary-symmetry class AB uses NPN and PNP Darlington pairs. A diode string provides 4VBE bias to eliminate crossover distortion, and small emitter resistors give thermal stability.

Draw the circuit diagram of Complementary-Symmetry class-AB amplifier using Darlington pair transistors.

A complementary-symmetry class AB amplifier uses an NPN and a PNP power transistor in push-pull, with the bases biased slightly above cut-off to remove crossover distortion. When high current gain is needed, each power transistor is replaced by a Darlington pair, so that the driver can be small.

Circuit diagram

  +VCC ---+------------------+------------------+
          |                  |                  |
        [R1]                 C                  C
          |                  |                  |
          +--------------B  Q3 (NPN)            |
          |                  E --------------B  Q1 (NPN)
         D1                                     E
         D2                                     |
 Vi o-||--+                                    [Re]
         D3                                     |
         D4                                     +--------o Vo
          |                                     |        |
          |                                    [Re]     [RL]
          |                                     |        |
          |                                     E       GND
          |                  E --------------B  Q2 (PNP)
          +--------------B  Q4 (PNP)            C
          |                  C                  |
        [R2]                 |                  |
          |                  |                  |
  -VCC ---+------------------+------------------+

(Q3–Q1 form the upper NPN Darlington and Q4–Q2 the lower PNP Darlington. In each pair the emitter of the first transistor drives the base of the second, and the collectors are joined to the supply.)

Working

  • The diode string D1–D4 (or a VBE multiplier), carrying current through R1 and R2, provides about 4 × 0.7 V of bias: one diode drop for each of the four base-emitter junctions between the two input bases. This keeps each Darlington just conducting, giving class AB operation without crossover distortion.
  • For a positive half-cycle the upper NPN Darlington conducts and sources current to RL; for a negative half-cycle the lower PNP Darlington sinks current from RL.
  • The total current gain of each pair is β ≈ β1 β2, so the input source needs to supply only a small base current.
  • Small emitter resistors (Re) give thermal stability and short-circuit protection.

✓ Answer checked by Claude Opus 5.5 (high effort)

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