Carrier Amplifier
Understanding Carrier Amplifier
Doherty Configuration and Load Modulation
The Doherty power amplifier is a high-efficiency architecture that splits the input signal between two paths: the carrier (or main) amplifier and the peaking (or auxiliary) amplifier. The carrier amplifier is biased in Class-AB or Class-B mode, meaning it is active even at low signal levels. When the input signal is small, only the carrier amplifier is operating, delivering power to the load with high efficiency.
As the input signal increases toward the peak power level, the carrier amplifier enters voltage saturation, which would normally limit its efficiency. To prevent this, the peaking amplifier (biased in Class-C) turns on. Through a quarter-wave impedance inverter, the peaking amplifier modulates the load impedance seen by the carrier amplifier. This load modulation allows the carrier amplifier to continue delivering current while remaining in saturation, maintaining peak efficiency up to peak output power.
Key Mathematical Relations
Technical Specifications Comparison
| Parameter | Carrier (Main) Amplifier | Peaking (Auxiliary) Amplifier |
|---|---|---|
| Bias Class | Class-AB or Class-B | Class-C |
| Activation Threshold | Active across the entire power range | Turns on at the power back-off point (typically -6 dB) |
| Impedance Trend with Power | Decreases from 2\cdot Z_0 to Z_0 during load modulation | Decreases from open-circuit (infinite) to Z_0 |
| Primary Role | Maintains high efficiency during power back-off | Supplies peak current and modulates carrier load |
Frequently Asked Questions
Why is the carrier amplifier biased in Class-AB while the peaking amplifier is in Class-C?
Class-AB bias provides linear gain at low input levels, ensuring the amplifier can handle low-amplitude signals. The peaking amplifier is biased in Class-C so that it remains completely off at low power levels (preventing unnecessary power consumption) and only turns on when the input signal is large enough to overcome the Class-C bias threshold.
What is load modulation in a Doherty amplifier?
Load modulation is the dynamic adjustment of the load impedance seen by the carrier amplifier as the peaking amplifier turns on. By injecting current into the combining node, the peaking amplifier changes the effective impedance at that node. Through a quarter-wave transmission line, this change decreases the impedance seen by the carrier amplifier, allowing it to output more power while remaining in voltage saturation.
How does the carrier amplifier achieve two efficiency peaks in a Doherty design?
The first efficiency peak occurs at the power back-off point (typically 6 dB below peak power) when the carrier amplifier first enters saturation. The second peak occurs at peak power, where both the carrier and peaking amplifiers are saturated, and the load modulation has reduced the carrier's load impedance to its nominal value.