Side-by-side comparison
| Parameter | Class A | Class B Amplifier |
|---|---|---|
| Conduction angle | 360° (full cycle) | 180° (half cycle each transistor in push-pull) |
| Quiescent current I_Q | High — transistor biased at midpoint continuously | Near zero — transistors biased at cutoff |
| Maximum efficiency (η) | 25% (resistive load), 50% (transformer-coupled) | 78.5% (theoretical maximum) |
| Distortion type | Low harmonic distortion; linear operation | Crossover distortion at 0 V transition |
| THD (typical) | < 0.1% (well-biased) | 5–10% (without correction) |
| Heat dissipation | High even at idle (P = V_CC × I_Q) | Low at idle; rises with signal |
| Biasing requirement | Fixed DC bias at Q-point midway on load line | V_BE = 0 (or near 0) — transistors at cutoff |
| Output stage transistors | Single transistor (or complementary pair biased ON) | Complementary NPN+PNP pair, each active 180° |
| Typical application | Low-power precision audio, small signal stages | High-power audio, RF power amplifiers (not audio) |
| Example circuit | BC547 single-ended preamp; LM1875 biased class A | TDA2030 push-pull; discrete 2N3055+MJ2955 pair |
Key differences
Class A's 25% maximum efficiency (resistive load) means three-quarters of supply power is wasted as heat — a 10 W output class A amp dissipates at least 30 W in the transistor even with no music playing. Class B's 78.5% theoretical efficiency makes it attractive for battery-powered and high-power applications, but crossover distortion — where neither transistor is active during the zero-crossing — introduces THD of 5–10% without correction. This distortion is audible in speakers; the 2N3055/MJ2955 push-pull pair used in textbook class B circuits sounds noticeably harsh on voice. For the same 10 W output, class B dissipates only about 3 W in the transistors. There is no configuration where class A efficiency equals class B, and no class B without crossover distortion unless you add a forward bias — which turns it into class AB.
When to use Class A
Use class A when linearity and low distortion matter more than efficiency — typically in small-signal preamp stages, headphone amplifiers, and precision measurement amplifiers. A single BC547 in a class A common-emitter stage with a 4.7 kΩ collector load delivers < 0.1% THD at 1 kHz.
When to use Class B Amplifier
Use class B when you need to deliver high power efficiently and can accept correction for crossover distortion through signal processing or a small forward bias. A 2N3055/MJ2955 complementary push-pull pair driving an 8 Ω speaker from ±30 V supplies delivers 50 W at 78% efficiency — impossible with class A at that power level.
Recommendation
For any audio output stage above 1 W, choose class B or class AB — efficiency wins. For input stages and low-power precision amplifiers, choose class A — the distortion figure is decisive. No real design uses pure class B; but understanding it is essential because class AB is simply class B with a small corrective bias added.
Exam tip: Examiners ask for derivations of maximum efficiency for both classes — prove η_max = 25% for class A (P_out / P_DC = V_m²/2R_L ÷ V_CC²/R_L, where V_m = V_CC/2) and η_max = π/4 ≈ 78.5% for class B using the push-pull analysis.
Interview tip: Interviewers expect you to explain crossover distortion in class B physically — not just that it exists, but that both transistors are simultaneously off during zero crossing because V_BE < 0.6 V for each, creating a dead zone that causes audible distortion in the output waveform.