Side-by-side comparison
| Parameter | Class B | Class AB Amplifier |
|---|---|---|
| Quiescent bias | V_BE ≈ 0 V; transistors at cutoff at idle | Small forward bias (10–50 mA I_Q) to avoid crossover |
| Conduction angle | Exactly 180° per transistor | Slightly more than 180° per transistor |
| Crossover distortion | Significant (~5–10% THD without correction) | Negligible (< 0.1% THD in well-designed stage) |
| Efficiency (η) | Up to 78.5% (theoretical) | Slightly lower (~70–75% in practice) |
| Idle power dissipation | Near zero | Small but non-zero (I_Q × V_CC) |
| Bias network | No bias — transistors at V_BE = 0 | 1N4148 diodes or V_BE multiplier (TIP31 + R1, R2) |
| Thermal stability | No thermal concern at idle | I_Q can increase with temperature; needs thermal tracking |
| Audio quality | Poor at low levels due to dead zone | Good across full output range |
| Common devices | Theoretical; rarely used as-is | LM386, TDA2030, TDA2050, discrete 2N3055+MJ2955 |
| Use case | RF power amps (tuned output filters remove distortion) | Audio output stages: 5 W to 100 W range |
Key differences
Class B and class AB differ by a single design decision: whether to apply a small forward bias to both output transistors. In class B, both transistors are at cutoff (V_BE ≈ 0) at idle — when the input crosses zero, there is a dead band lasting until V_BE reaches 0.6 V, causing crossover distortion visible on an oscilloscope as a kink in the output waveform. In class AB, a 1N4148 diode string (or a V_BE multiplier) pre-biases each transistor with 10–50 mA — the dead band disappears. TDA2030 uses a class AB output stage to deliver 18 W into 4 Ω at < 0.1% THD. Efficiency drops from the theoretical 78.5% to about 70–75%, but the audio quality improvement is far more important for any listening application. Thermal tracking is a new concern in class AB: the bias diodes must be thermally coupled to the output transistors to prevent I_Q from running away with temperature.
When to use Class B
Use class B when the output is filtered (as in RF tuned amplifiers) or when maximum efficiency at high power is the only metric. An RF power transistor in a CB radio transmitter operates class B because the tank circuit eliminates harmonic distortion that would be unacceptable in audio.
When to use Class AB Amplifier
Use class AB for any audio output stage that must deliver low distortion across the full signal range. A TDA2030 class AB amplifier with 18 V supply drives a 4 Ω speaker to 18 W at < 0.08% THD — the two forward-biased diodes in the bias network are all it takes to eliminate crossover distortion.
Recommendation
For audio applications, always choose class AB — crossover distortion in class B is audible even at moderate volume and degrades listening quality significantly. Class B is only appropriate when a tuned filter removes harmonics, as in RF power stages. Every real audio amplifier IC you will encounter in the lab or in practice is class AB.
Exam tip: Examiners ask you to sketch the output waveform of class B with and without crossover distortion correction, and to show what the V_BE multiplier circuit looks like — know the transistor-plus-resistor V_BE multiplier that replaces the diode string in high-power designs.
Interview tip: Interviewers expect you to explain thermal runaway risk in class AB — as transistor temperature rises, V_BE drops, increasing I_Q, further raising temperature; the bias diodes must be thermally bonded to the heatsink to track and compensate this drift.