Cascode Doherty Amplifier Biasing for Efficiency and Linearity
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Solution Overview
Problem
Doherty amplifiers face efficiency and linearity tradeoffs due to inherent AMAM/AMPM discontinuities and require envelope tracking systems with high load currents and capacitances, which can be inefficient and complex.
Innovation Solution
A Doherty amplifier system with a cascode configuration and an envelope tracking bias circuit providing a bias signal to the peaking amplifier's output transistor, reducing load current and capacitance, and optimizing efficiency and linearity through envelope control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If envelope tracking is implemented with high load currents and capacitances to achieve high efficiency, then power added efficiency is improved, but device complexity and system complexity increase
Solution Approach 1:
The amplifier is divided into separate carrier and peaking amplifiers, each with dedicated cascode stages. The envelope tracking bias circuit selectively applies bias signals to specific transistors (input or output) in the peaking amplifier, segmenting the complexity management to achieve efficiency without requiring full-system complexity
Solution Approach 2:
The bias circuit dynamically selects whether to provide envelope tracking bias signals to the input transistor, output transistor, or both transistors of the peaking amplifier based on operating conditions. This dynamic adaptation allows the system to maintain high efficiency across different power levels while avoiding fixed complex circuitry
2Loss of energy
If Doherty amplifier configuration is used to improve efficiency, then power added efficiency is improved, but linearity deteriorates due to AMAM/AMPM discontinuities
Solution Approach 1:
The bias voltage applied to the peaking amplifier transistors is dynamically changed based on the envelope signal. By adjusting the bias parameter of the input transistor, output transistor, or both, the amplifier maintains optimal linearity and efficiency characteristics across different operating points, resolving the AMAM/AMPM discontinuity issue
Solution Approach 2:
The envelope tracking mechanism uses feedback from the RF signal envelope to dynamically adjust the bias conditions of the peaking amplifier. This feedback control ensures that the amplifier operates in the optimal region for both linearity and efficiency, compensating for the inherent discontinuities in Doherty configuration
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.