Class B push-pull amplifier biasing In a class B (or class AB) push-pull audio power stage, transistors are biased slightly above cutoff primarily to avoid which problem?

Introduction / Context:Class B push-pull amplifiers split the waveform between complementary devices so that each handles approximately half of the cycle. Without proper biasing, the handoff near the zero-crossing is non-linear, causing audible distortion. Biasing slightly above cutoff moves operation toward class AB to smooth the transition.

Given Data / Assumptions:

• Complementary emitter followers or source followers in a push-pull topology.
• Goal is reduced distortion with reasonable efficiency.
• Small standing quiescent current is acceptable.

Concept / Approach:Transistor conduction begins only after a threshold (base–emitter or gate–source) is exceeded. Near zero crossings, neither device may conduct in pure class B, creating a dead zone that clips the waveform. A small forward bias makes both devices slightly on around zero, eliminating the dead zone and minimizing crossover distortion while keeping efficiency high compared to class A.

Step-by-Step Solution:Identify the non-linear region around zero due to device thresholds.Introduce bias (e.g., diode-connected junctions, VBE multipliers) to establish a small quiescent current.Ensure the transfer characteristic is continuous through zero.Result: significantly reduced crossover distortion with modest efficiency penalty.

Verification / Alternative check:Oscilloscope measurement of an undriven output reveals a slight warm bias current. A low-level sine test shows the notch distortion vanishing once proper bias is applied.

Why Other Options Are Wrong:Unusually high efficiency: biasing above cutoff slightly reduces efficiency, it does not create a new problem.Negative feedback: independent of class; feedback may help but is not the bias goal.Low input impedance: dictated by stage topology, not this bias choice.

Common Pitfalls:Over-biasing can push the stage toward class A and waste power; under-biasing leaves residual crossover notch.

Final Answer:crossover distortion