Classifying Output Stages: how long is the transistor switched on?
One parameter — conduction angle — separates Class A from Class C, and decides everything else.
An output stage has to deliver real power to a speaker or an antenna, and here efficiency stops being an academic concern: wasted power becomes heat you must remove. The classification scheme is beautifully simple. Ask one question — for what fraction of the input cycle does the transistor conduct? — and the answer determines the class, the efficiency, and the distortion.
Class Conduction angle Max efficiency Distortion
─────────────────────────────────────────────────────────
A 360° 25% / 50% lowest
AB 180°–360° ~50–70% low
B 180° 78.5% moderate
C < 180° > 90% severe
(Class A: 25% with a resistive load, 50% transformer-coupled)
🎯 Where each class is actually used
Class ASmall-signal preamplifiers, high-end audio, and any stage where distortion matters far more than heat. The transistor is biased in the middle of its load line and never switches off.
Class BPush-pull power stages, where one transistor handles each half-cycle. Theoretically excellent, but suffers crossover distortion at the handover.
Class ABThe practical answer: a small forward bias keeps both devices slightly on at the crossover, eliminating that distortion for a modest efficiency cost. Standard in audio power amplifiers.
Class CRadio transmitters only. The output is a series of pulses, hopelessly distorted — but a tuned LC tank at the output rings sinusoidally and reconstructs a clean carrier. Useless for audio, ideal for RF.
Class C would destroy an audio signal, yet it's the standard for RF transmitters. The reason is that a tuned circuit at the output acts as a very narrow filter. Feed it short pulses at the resonant frequency, and it rings out a pure sine wave — discarding the harmonics. You can only do this when the signal is a single frequency — which is true for a carrier, and false for music.
Worked numerical 1 — comparing power waste
Each class delivers 20 W to a load. Compare the DC power drawn and the heat dissipated.
Efficiency η = P_out/P_dc, so P_dc = P_out/η
Heat wasted = P_dc − P_out
Class A (η = 25%):
P_dc = 20/0.25 = 80 W
Heat = 80 − 20 = 60 W ← needs a big heatsink
Class AB (η ≈ 60%):
P_dc = 20/0.60 = 33.3 W
Heat = 13.3 W
Class B (η = 78.5%):
P_dc = 20/0.785 = 25.5 W
Heat = 5.5 W
Class C (η = 90%):
P_dc = 20/0.90 = 22.2 W
Heat = 2.2 W
Class A wastes 27× more heat than Class C for the same
output. In a 100 W amplifier that difference is the
distinction between a small heatsink and a fan-cooled
chassis.
Worked numerical 2 — conduction angle from bias
A transistor with V_BE(cutoff) = 0.6 V is biased at 0.6 V DC with a 1 V peak sine input. Find the conduction angle and classify the stage.
The device conducts when v_BE > 0.6 V:
0.6 + 1·sin θ > 0.6
sin θ > 0 → conducts for 0° to 180°
Conduction angle = 180° → CLASS B
Now bias at 0.7 V instead (100 mV above cutoff):
0.7 + 1·sin θ > 0.6
sin θ > −0.1
θ from −5.7° to 185.7°
Conduction angle = 191.4° → CLASS AB ✔
And bias at 0.5 V (below cutoff):
0.5 + 1·sin θ > 0.6
sin θ > 0.1
θ from 5.7° to 174.3°
Conduction angle = 168.6° → CLASS C
A 200 mV change in bias walks the stage through three
classes. That is how sensitive the classification is, and
why bias stability matters so much in power stages.
💡 Exam angle: a reliable 4-mark question — "classify amplifiers based on conduction angle". Give the table (360°/180°/<180° with 25%/78.5%/>90%), then add one application each. The mark that separates answers is explaining why Class C is usable in RF (tuned output filters the harmonics) but not in audio.
Syllabus points
Class A, B, AB, C overview
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