RF Amplifier Cascode Bias Control for Doherty Back-Off Efficiency
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Solution Overview
Problem
Existing RF amplifiers face challenges in achieving high efficiency, particularly at power back-off levels, due to inherent inefficiencies in traditional Doherty amplifiers and the complexity of envelope tracking systems.
Innovation Solution
The implementation of a Doherty amplifier system with a cascode configuration for both carrier and peaking amplifiers, coupled with an envelope tracking bias circuit that provides a bias signal to the output transistor of the peaking amplifier, optimizing efficiency and linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If traditional Doherty amplifier configuration is used, then device complexity is reduced, but power added efficiency at power back-off levels deteriorates
Solution Approach 1:
The amplifier is divided into a carrier amplifier and a peaking amplifier, with the peaking amplifier further segmented into a driver stage and a cascode stage. This segmentation allows each stage to be optimized for specific operating conditions, improving overall efficiency at power back-off levels while managing complexity through functional division.
Solution Approach 2:
An envelope tracking bias circuit is implemented to dynamically adjust the bias voltage applied to the output transistor of the peaking amplifier based on the signal envelope. This dynamic biasing optimizes the operating point of the amplifier stages according to instantaneous power levels, significantly improving power added efficiency at back-off conditions.
2Use of energy by moving object
If envelope tracking is implemented, then power added efficiency is improved, but load current and capacitance increase
Solution Approach 1:
The envelope tracking bias circuit applies bias adjustment locally and selectively only to the output transistor of the peaking amplifier, rather than to the entire amplifier system or carrier amplifier. This localized approach achieves efficiency improvement while minimizing the overall load current and capacitance requirements of the envelope tracking system.
Solution Approach 2:
Instead of applying envelope tracking to all amplifier stages, the invention applies it partially only to the output transistor of the peaking amplifier. This partial action is sufficient to achieve significant efficiency improvements at power back-off levels while keeping the envelope tracking circuit complexity and resource requirements manageable.
3Reliability
If cascode configuration with envelope tracking bias is used, then power added efficiency and linearity are improved, but device complexity increases
Solution Approach 1:
The peaking amplifier is segmented into a driver stage and a cascode stage, with the cascode stage specifically optimized for linearity and efficiency. This segmentation allows the driver stage to handle amplitude modulation while the cascode stage maintains signal integrity and linearity, achieving improved performance while organizing complexity into manageable functional blocks.
Solution Approach 2:
The envelope tracking bias circuit provides feedback based on the signal envelope to dynamically adjust the bias voltage of the output transistor. This feedback mechanism optimizes the operating point in real-time, improving linearity and efficiency while the systematic implementation keeps the feedback circuitry manageable in complexity.
Data Source
AI summary
In some embodiments, an amplifier system can include an amplifier circuit having first and second amplifiers configured to amplify respective first and second portions of an input signal. Each of the first and second amplifiers can include a cascode stage with input and output transistors arranged in a cascode configuration. The amplifier system can further include an envelope tracking bias circuit coupled to the amplifier circuit and configured to provide a bias signal to the output transistor of the cascode stage of at least one of the first and second amplifiers. The amplifier system can further include a supply circuit configured to provide a non-envelope tracking supply voltage to the output transistor of the cascode stage of the at least one of the first and second amplifiers.