Class B Amplifier
Understanding the Class B Amplifier
The Class B Amplifier is a topology designed strictly to maximize power efficiency by eliminating idle power consumption. In this configuration, the transistor's gate or base is biased exactly at the threshold of conduction (pinch-off). When no RF signal is present, absolutely zero DC current flows through the device, meaning it generates zero heat at idle.
When an AC radio frequency signal is applied, the transistor only turns on during the positive half of the sine wave. During the negative half, the transistor is driven deeper into cutoff and remains entirely off. Therefore, the transistor conducts current for exactly 180 degrees (a conduction angle of π radians). Because the transistor is off half the time, it acts as a severe half-wave rectifier, horribly distorting the original sine wave and generating massive amounts of harmonic energy. To use Class B in a linear system, this distortion must be resolved.
The Push-Pull Architecture
A single-ended Class B amplifier is virtually useless for linear RF communication. To fix the distortion, engineers use a Push-Pull configuration. The incoming RF signal is split 180-degrees out of phase (using a balun) and fed into two identical Class B transistors. Transistor 1 amplifies the positive half of the wave, while Transistor 2 amplifies the negative half. The two halves are then recombined by an output balun. This brilliantly restores the full 360-degree sine wave, cancels out all even-order harmonics (like the 2nd and 4th), and delivers high efficiency (theoretically 78.5%).
Quiescent Current: Idq = 0 Amps
Maximum Theoretical Efficiency:
η = π / 4 = 0.785 (78.5%)
Push-Pull Power: Ptotal = Ptransistor_1 + Ptransistor_2
Comparison
| Metric | Single-Ended Class A | Push-Pull Class B | Advantage |
|---|---|---|---|
| Max Efficiency | 50% | 78.5% | Massively reduced heat and power draw. |
| Idle Current | Maximum (Imax/2) | Zero | No battery drain when not transmitting. |
| Even Harmonics | Present | Cancelled out | Requires less output filtering. |
| Component Count | 1 Transistor | 2 Transistors + Baluns | Class B is larger and more complex. |
Frequently Asked Questions
What is 'Crossover Distortion' in a Class B amplifier?
In reality, transistors do not turn on instantly at exactly 0.000 volts; they require a small forward voltage (e.g., 0.7V for a BJT). In a push-pull Class B setup, there is a tiny 'dead zone' where the wave crosses zero and neither transistor is fully on. This creates a nasty kink in the waveform called crossover distortion, generating massive high-order intermodulation products.
How is crossover distortion fixed?
Engineers fix it by slightly increasing the bias voltage just above the cutoff threshold, providing a tiny trickle current so the transistors are 'awake' before the zero-crossing. However, by doing this, the amplifier is no longer Class B; it has officially become a Class AB amplifier.
Are true Class B amplifiers used in RF?
True, strict Class B (zero idle current) is very rare in high-frequency microwave design because RF transistors behave poorly right at the threshold edge. They are mostly used in low-frequency audio amplifiers and legacy AM broadcast transmitters. Modern RF engineering relies almost exclusively on Class AB or highly-switched Class E/F topologies.