Bias Circuit for Doherty Amplifier Thermal Tracking
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Solution Overview
Problem
Conventional bias circuits, such as current mirrors, are not suitable for Doherty amplifiers that require Class C bias levels, as they are designed for Class AB operation and fail to provide efficient thermal tracking and process compensation.
Innovation Solution
A bias circuit employing a transistor connected between the output and ground, acting as a non-linear load for quick discharge of load capacitance, with a current mirror arrangement using MOS transistors to generate bias voltage, allowing for fast thermal tracking and low video impedance, suitable for Doherty and Time Division amplifiers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a conventional current mirror is used for biasing, then Class AB bias level is provided, but Class C bias level cannot be achieved
Solution Approach 1:
The bias circuit is segmented into two functional parts: a current mirror for generating the base bias voltage and an additional transistor for subtracting a fixed voltage. This segmentation allows the circuit to provide Class C bias level while maintaining the simplicity of the conventional current mirror structure.
2Reliability
If thermal compensation is implemented using current mirror, then process compensation is achieved, but thermal tracking speed is insufficient
Solution Approach 1:
The bias circuit performs preliminary thermal compensation by providing a stable bias voltage that anticipates thermal drift. The additional transistor pre-adjusts the bias level to account for expected thermal effects, enabling faster response to temperature changes in Doherty amplifiers.
3Reliability
If bias circuit is integrated on the same die, then thermal tracking is improved, but circuit size increases
Solution Approach 1:
The bias circuit merges the current mirror functionality with an additional transistor in a compact integrated structure on the same die. This merging achieves thermal tracking by placing the bias circuit close to the power transistor while maintaining a reduced overall area through efficient layout and shared components.
4Loss of energy
If Class C bias is required for Doherty amplifier, then efficiency in back-off is improved, but conventional bias circuits become unsuitable
Solution Approach 1:
The bias circuit changes the bias parameter from Class AB to Class C by subtracting a fixed voltage from the current mirror output. This parameter change enables the Doherty amplifier to operate efficiently in back-off conditions while the additional transistor is designed to minimize complexity and maintain compatibility with standard fabrication processes.
Data Source
AI summary
Proposed is a bias circuit for a transistor in a C class amplifier. The bias circuit comprises: a class AB amplifier bias voltage generating means adapted to generate a bias voltage at an output terminal; and a transistor connected between the output terminal and a first reference voltage, the control terminal of the transistor being connected to a second reference voltage via a switch. Closure of the switch connects the second reference voltage to the control terminal of the transistor to cause a shift in the bias voltage generated by the class AB amplifier bias voltage generating means to achieve a predetermined class C bias voltage at the output terminal.