Power Amplifier Bias Circuit Using FET Feedback at Low Battery Voltage
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Solution Overview
Problem
Existing power amplification modules in mobile communication devices require a minimum battery voltage of 2.8 V, which is higher than the desired 2.5 V for improved talking and communication times, and suffer from gain fluctuations due to temperature changes caused by differing temperature characteristics of resistors and FETs in bias circuits.
Innovation Solution
A power amplification module configuration using a first and second bipolar transistor, a current source, a control voltage generation circuit, and FETs, where the FETs' control voltage is adjusted to stabilize the bias current and gain, allowing operation with a lower battery voltage of 2.5 V or 2.0 V while maintaining improved temperature characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If an emitter follower type bias circuit using bipolar transistors is used, then the bias circuit can be driven at a higher battery voltage, but the minimum battery voltage required is about 2.8 V which prevents achieving the desired 2.5 V operating voltage
Solution Approach 1:
The patent changes the operating parameters of the bias circuit by using a FET instead of a bipolar transistor, which allows the circuit to operate at lower voltages. The FET's gate-source voltage requirement is lower than the base-emitter voltage of bipolar transistors, enabling operation at 2.5 V or lower while maintaining reliable bias current supply to the power amplifier.
2Use of energy by moving object
If a FET is used in the bias circuit to enable low-voltage operation, then the battery voltage can be reduced to 2.5 V, but the gain of the power amplifier fluctuates with temperature changes due to different temperature characteristics between the FET and bipolar transistor
Solution Approach 1:
The patent introduces a feedback mechanism where the control voltage generation circuit monitors the bias current and adjusts the control voltage applied to the FET's gate accordingly. This feedback loop compensates for temperature-induced variations in the FET's characteristics, maintaining stable bias current and thereby stabilizing the power amplifier's gain across different temperatures.
Solution Approach 2:
The control voltage generation circuit acts as an intermediary between the FET and the power amplifier. It generates a control voltage that compensates for temperature effects on the FET, thereby mediating the temperature characteristic mismatch between the FET and the bipolar transistor in the power amplifier, and maintaining stable gain.
3Device complexity
If the control voltage to the FET is not adjusted, then the circuit is simpler, but the bias current and gain become unstable due to temperature changes
Solution Approach 1:
The control voltage generation circuit implements feedback by sensing the bias current and dynamically adjusting the FET's gate voltage to compensate for temperature variations. This feedback mechanism ensures stable bias current despite temperature changes, while adding minimal complexity to the overall circuit structure.
Data Source
AI summary
A power amplification module includes a first transistor which amplifies and outputs a radio frequency signal input to its base; a current source which outputs a control current; a second transistor connected to an output of the current source, a first current from the control current input to its collector, a control voltage generation circuit connected to the output and which generates a control voltage according to a second current from the control current; a first FET, the drain being supplied with a supply voltage, the source being connected to the base of the first transistor, and the gate being supplied with the control voltage; and a second FET, the drain being supplied with the supply voltage, the source being connected to the base of the second transistor, and the gate being supplied with the control voltage.