How do I design a push-pull amplifier for improved even harmonic suppression?
Push-Pull PA Design
In a push-pull configuration, the input balun splits the signal into two paths with 180° phase difference. Transistor A amplifies the positive half-cycle; transistor B amplifies the negative half-cycle. The output balun recombines the amplified halves into a single output signal. The complete waveform is reconstructed at the output with combined power from both transistors.
| Parameter | LNA | Driver | Power Amplifier |
|---|---|---|---|
| Noise Figure | 0.3-2.0 dB | 3-8 dB | 5-15 dB (not specified) |
| Gain | 10-25 dB | 10-20 dB | 8-15 dB |
| P1dB | -10 to +10 dBm | +15 to +25 dBm | +30 to +50 dBm |
| OIP3 | +5 to +25 dBm | +25 to +40 dBm | +40 to +55 dBm |
| DC Power | 10-100 mW | 0.5-5 W | 5-500 W |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
How much harmonic suppression?
Ideal push-pull: infinite suppression of all even harmonics. Practical: 20-40 dB suppression of the 2nd harmonic, limited by amplitude and phase imbalance between the two transistors and the balun. Achieving better than 30 dB requires transistor matching within 0.5 dB gain and 5° phase.
What about odd harmonics?
Odd harmonics are combined in-phase at the output and are not suppressed. A separate lowpass filter may be needed to meet harmonic emission specifications. However, the 3rd harmonic is typically 15-25 dB below the fundamental for Class AB operation.
Is push-pull always better?
Push-pull adds the complexity of two baluns and requires matched transistors. For narrowband applications where a simple harmonic filter is adequate, single-ended designs are simpler and cheaper. Push-pull is most advantageous for broadband applications where wideband harmonic filtering is difficult.