Design a complementary push-pull Class B power amplifier. Each transistor conducts for half the cycle, achieving high efficiency (~78.5% max) at the cost of crossover distortion. This topology is common in audio power amplifiers (often biased slightly into Class AB).
| Supply voltage (Vcc): | V |
| Load impedance: | Ω |
| Low frequency (-3dB): | Hz |
| Bootstrap circuit: |
| Transistor pair: | |
| β (hFE): | |
| Ic max: | A |
| Pd max (each): | W |
| θjc: | °C/W |
| Ambient temperature: | °C |
| Max junction temp: | °C |
| Heatsink (θsa) each: | °C/W |
Hover over labels for explanations.
| Cin: | |
| Cout: | |
| Cboot: | |
| Rb: |
| Quiescent current (Iq): | |
| Output DC bias (no signal): | |
| Max output swing: | |
| Peak collector current: |
This is an emitter follower configuration (voltage gain ~1). The input signal must be approximately equal to the desired output voltage. A preamp or driver stage is typically needed.
| Input voltage (peak): | |
| Input voltage (RMS): | |
| Input impedance: |
| Max output power: | |
| Output voltage (RMS): | |
| Output current (RMS): | |
| DC input power (full signal): | |
| Efficiency at full power: |
Class B dissipates maximum power at about 63% of full output, not at idle or full power. This is the critical design point for thermal management.
| Thermal limit (each): | |
| Dissipation (idle): | |
| Dissipation (full power): | |
| Dissipation (worst case): | |
| Junction temp (worst): |
Shows output power, DC input power, and transistor dissipation vs output level. Note how dissipation peaks at ~63% output - this is the worst-case thermal condition.
In pure Class B, each transistor conducts for exactly half the cycle (180°). When the input signal is positive, Q1 (NPN) conducts and sources current to the load. When the input is negative, Q2 (PNP) conducts and sinks current from the load.
The main disadvantage of Class B is crossover distortion. Near zero crossing, neither transistor is conducting because the input must exceed ~0.7V (Vbe) to turn on each transistor. This creates a "dead zone" in the output waveform.
In practice, most power amplifiers use Class AB - adding a small bias current (via diodes or a Vbe multiplier) so both transistors are slightly on at idle. This eliminates crossover distortion while maintaining most of Class B's efficiency.
Unlike Class A where dissipation is maximum at idle, Class B dissipation:
Maximum dissipation per transistor: Pd_max = Vcc² / (π² × Rload)
η = (π/4) × (Vout_peak / Vcc) ≈ 78.5% at maximum output
| Class A | Class B (this design) | |
|---|---|---|
| Conduction angle | 360° (full cycle) | 180° (half cycle each) |
| Max efficiency | 25% (resistive load) | 78.5% |
| Quiescent current | High (always conducting) | Zero (ideally) |
| Worst-case heat | At idle (no signal) | At ~63% output |
| Distortion | Very low | Crossover distortion |
| Transistor count | 1 (single-ended) | 2 (complementary pair) |
| Typical use | Hi-fi, low power | Higher power, PA systems |
The bootstrap capacitor (Cboot) is a key component in this design that solves a problem specific to emitter-follower output stages.
When the output swings high (toward Vcc), the NPN transistor's base needs to be driven even higher than the output. But if Rb is connected to a fixed Vcc rail, the available base drive decreases as output rises - limiting the positive swing.
The bootstrap capacitor connects from the output back to the junction of Rb and the input. As the output rises, Cboot "lifts" the bottom of Rb along with it. This keeps the voltage across Rb constant, maintaining base drive current even at high output levels.
With bootstrapping, the output can swing nearly rail-to-rail (limited only by Vce_sat), maximizing output power capability.