Doherty Power Amplifier Biasing for Backoff Efficiency and Peak Power
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Solution Overview
Problem
Traditional BJT-based Doherty power amplifiers face a trade-off between peak output power and power backoff efficiency due to limitations in biasing the peaking amplifier, leading to reduced P1 dB power and inefficient load modulation, which is exacerbated by the need for additional power detectors and envelope shaping circuits that are not suitable for modern RF bandwidths.
Innovation Solution
An amplifier structure with a carrier and peaking amplifier configured as common-emitter stages, coupled with power adaptive biasing circuitry that senses direct current base voltages to dynamically adjust the biasing, allowing the peaking amplifier to operate in deep class C or class AB, enhancing load modulation and efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the peaking amplifier is biased in class C to improve power backoff efficiency, then power backoff efficiency is improved, but peak output power is reduced due to insufficient pPA gain and weak load modulation
Solution Approach 1:
The patent implements dynamic biasing control where the peaking amplifier's bias point is not fixed but dynamically adjusted based on operating conditions. The bias circuit responds to the carrier amplifier's output power level, automatically adjusting the pPA bias between class C (at low power for efficiency) and class AB/class A (at high power for sufficient gain and load modulation), thereby resolving the contradiction between power backoff efficiency and peak output power
Solution Approach 2:
The patent employs a feedback mechanism where the bias circuit monitors the carrier amplifier's operation and adjusts the peaking amplifier's bias accordingly. This feedback loop ensures that the pPA operates at the optimal bias point for the current operating condition, enabling the system to achieve both high power backoff efficiency and adequate peak output power through adaptive bias control
2Power
If the bias is raised to move the peaking amplifier into class B or class AB to improve peak output power, then peak output power is improved, but power backoff efficiency is reduced
Solution Approach 1:
The bias circuit dynamically adjusts the peaking amplifier's operating class based on power demand. At low power levels, the pPA operates in class C for maximum efficiency. As power demand increases and the carrier amplifier approaches its limits, the bias circuit automatically raises the pPA bias to transition it into class AB or class A, providing the necessary gain and load modulation capability without permanently sacrificing power backoff efficiency
3Ease of operation
If traditional power detectors with long RC time constants are used to control bias, then bias control is achieved, but the system is not suitable for modern RF bandwidths of 100 MHz or higher due to slow response
Solution Approach 1:
The patent replaces the traditional passive RC time-constant-based power detection mechanism with an active impedance sensing approach. Instead of relying on slow capacitor charging/discharging, the system uses the carrier amplifier's existing regulated base voltage as a power indicator and employs active circuitry to generate the bias control signal, dramatically reducing the response time to accommodate modern 100 MHz and higher RF bandwidths while maintaining effective bias control
Data Source
AI summary
Disclosed is an amplifier having a carrier amplifier configured as a common-emitter carrier power stage and a peaking amplifier configured as a common-emitter peaking power stage. Further included is power adaptive biasing circuitry coupled between the carrier amplifier and the peaking amplifier, wherein the power adaptive biasing circuitry is configured to sense direct current base voltages of the common-emitter carrier power stage and to generate control currents that debias the common-emitter carrier power stage in response to the current base voltages of the common-emitter carrier power stage.