Introduction / Context:
Class B push-pull power amplifiers are ubiquitous in audio and RF power stages because they significantly improve efficiency compared to class A. Understanding their ideal efficiency limit is a core topic in analog electronics and power amplifier design.
Given Data / Assumptions:
- Ideal class B push-pull stage (complementary devices or transformer-coupled), no crossover distortion and sinusoidal output.
- No quiescent current (each device conducts over 180° of the cycle).
- Resistive load, ideal supplies, and devices with zero saturation voltage.
Concept / Approach:
Efficiency η is defined as P_out / P_in. For an ideal class B amplifier delivering a sinusoid to a resistive load, the average input power from the supply is related to the average of the rectified output current waveforms, while the output power equals V_rms^2 / R_L. The well-known ideal limit is η_max = π/4 ≈ 0.785, i.e., 78.5%, which is significantly higher than 50%.
Step-by-Step Solution:
For a sinusoidal output v_o(t) = V_m sin(ωt) across R_L, the rms value is V_m/√2.Output power: P_out = (V_m^2/2)/R_L.Average supply current per half cycle leads to total input power P_in = (2/π) * (V_m^2/R_L).Efficiency: η = P_out/P_in = [(V_m^2/2R_L)] / [(2/π)(V_m^2/R_L)] = π/4 ≈ 0.785.
Verification / Alternative check:
Textbook derivations for class B consistently yield η_max = 78.5% under ideal conditions; practical values are lower due to device drops and crossover effects.
Why Other Options Are Wrong:
'True': contradicts the standard 78.5% ideal limit.Other qualifiers (transformer coupling, low power, regulated supply) do not change the fundamental ideal limit; they affect practical realizations, not the theoretical maximum.
Common Pitfalls:
Confusing class A (25–50% ideal) with class B; mixing up conduction angle effects on efficiency.
Final Answer:
False
Discussion & Comments