How do I calculate the load impedance required for maximum output power from a transistor?
Loadline Analysis
The loadline represents the trajectory of the drain voltage and drain current as the transistor amplifies the RF signal. For maximum power, the loadline must span the full available voltage swing (from Vknee to 2Vdd - Vknee) and the full current swing (from 0 to Imax). The slope of this loadline is 1/Ropt, which determines the required load resistance.
| 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
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
Why is the optimum impedance so low for high power?
Ropt decreases with increasing power because higher power requires more current swing, and the load resistance must be lower to allow the higher current at the available voltage swing. A 100W PA at 28V has Ropt ≈ 3 Ω, requiring a 17:1 impedance transformation to 50 Ω.
Does frequency affect Ropt?
The resistive part of Ropt is frequency-independent (determined by DC operating point). However, the reactive part (needed to resonate out Cds) is strongly frequency-dependent. The complete optimum impedance Zopt = Ropt + jXopt changes with frequency.
What about Class F and harmonic tuning?
Class F requires specific harmonic impedances (short at 2f0, open at 3f0) in addition to the fundamental Ropt. The harmonic impedances shape the drain waveform to reduce overlap between voltage and current, improving efficiency beyond the Class AB theoretical limit.