Doherty Amplifier
Understanding Doherty Amplifiers
The Doherty amplifier solves the fundamental efficiency problem of amplifying high-PAPR signals without the complexity of envelope tracking. By using load modulation (the peaking amplifier changes the impedance seen by the main amplifier), the main amplifier stays near optimal efficiency even when the output power is well below its maximum.
How Doherty Works
- At low power (below 6 dB backoff): Only the main amplifier is active. It sees a high impedance (2x Z_opt) from the quarter-wave transformer, staying near compression.
- At medium power: The peaking amplifier begins to turn on, lowering the load impedance seen by the main amplifier.
- At peak power: Both amplifiers are fully active, each driving Z_opt, producing maximum output power.
Doherty Efficiency
- Class-AB single PA at 6 dB backoff: ~15-20% PAE.
- Doherty at 6 dB backoff: ~40-50% PAE.
- Asymmetric Doherty: Extends efficiency range to 9-10 dB backoff.
Frequently Asked Questions
What is a Doherty amplifier?
A Doherty amplifier uses a main and peaking amplifier with load modulation to maintain high efficiency over a wide power range. It is 2-3x more efficient than a single PA for high-PAPR signals like LTE/5G OFDM. It dominates cellular base station PA design.
How does Doherty compare to envelope tracking?
Doherty uses load modulation (passive technique). ET modulates the supply voltage (active technique). Doherty is simpler and dominant for base stations. ET is more common in handsets. Both achieve similar efficiency improvements. They can be combined (ET-Doherty).
What is an asymmetric Doherty?
An asymmetric Doherty uses a larger peaking amplifier than main amplifier. This extends the high-efficiency power range from 6 dB to 9-10 dB backoff, better matching the PAPR of modern OFDM signals. The trade-off is lower peak power.