What is the sequential power amplifier concept and how does it improve efficiency at backoff?
Sequential PA Efficiency
The Doherty and sequential PA architectures have become the dominant PA topology for 4G/5G base stations because they dramatically improve the average efficiency under modulated signal conditions.
| Parameter | Class A | Class AB | Class F/Doherty |
|---|---|---|---|
| Max Efficiency | 50% | 50-78% | 70-90% |
| Linearity | Excellent | Good | Moderate (needs DPD) |
| P1dB Backoff | 0-3 dB | 3-6 dB | 6-10 dB |
| Complexity | Low | Low | High |
| Common Use | Test, small signal | General PA | Base station, broadcast |
Compression Behavior
When evaluating the sequential power amplifier concept and how does it improve efficiency at backoff?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
Efficiency Trade-offs
When evaluating the sequential power amplifier concept and how does it improve efficiency at backoff?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
Thermal Budget
When evaluating the sequential power amplifier concept and how does it improve efficiency at backoff?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
- 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
Linearization Methods
When evaluating the sequential power amplifier concept and how does it improve efficiency at backoff?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
Frequently Asked Questions
How widely is Doherty used?
The Doherty PA is the dominant architecture for wireless infrastructure: 4G/5G macro base stations: virtually all modern base station PAs use Doherty (2-way or 3-way) with GaN transistors. Efficiency: 40-55% at average power. Small cells: Doherty is used for higher-power small cells (greater than 5 W). Lower-power small cells may use Class-AB for simplicity. Satellite: Doherty TWTAs and SSPAs are being adopted for satellite communication downlinks. Military: Doherty GaN PAs are replacing linear Class-AB PAs in tactical radios and base stations.
What are the design challenges?
Doherty PA design challenges: impedance inverter design (the quarter-wave line between main and peaking amplifiers must be designed for the correct impedance at the operating frequency; at GHz frequencies: the line is short (a few mm) and sensitive to layout parasitics), peaking amplifier biasing (the Class-C peaking amplifier must turn on at the correct input power level; the turn-on point determines the backoff level at which the efficiency enhancement begins), bandwidth (the quarter-wave impedance inverter limits the bandwidth to approximately 10-20% for a standard Doherty; wideband Doherty variants can achieve 30-50% bandwidth), and DPD interaction (the Doherty PA's AM-AM and AM-PM curves have kinks at the transition point where the peaking amplifier activates; the DPD must correct these non-smooth nonlinearities).
What about envelope tracking?
Envelope tracking (ET) is an alternative efficiency enhancement technique. Instead of an impedance-modulated architecture (Doherty): ET modulates the PA's supply voltage to follow the signal envelope. The PA always operates near compression (peak efficiency) regardless of the signal level because the supply voltage is reduced for low-power portions of the signal. ET efficiency: similar to Doherty (40-55% at average power) but with: wider bandwidth (not limited by impedance inverter), better linearity (the PA always operates at the same compression level), and greater complexity (requires a high-speed, high-efficiency envelope modulator). ET is used in: 5G mobile devices (Qualcomm QET4101, Murata ET modules) where efficiency is critical for battery life.