Power Amplifier Bias Follower for Stable RF Output Power
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Solution Overview
Problem
In power amplifier circuits for mobile communication devices, an increase in RF signals leads to a rise in base current of the common-base stage transistor, resulting in a decrease in base voltage and subsequently reducing the maximum output current and power of the common-emitter stage transistor, thereby limiting the maximum output power of the entire circuit.
Innovation Solution
The power amplifier circuit incorporates a third transistor acting as an emitter or source follower to supply a bias voltage to the base of the common-base stage transistor, ensuring that the base voltage remains stable even with increased RF signals, thereby preventing a reduction in maximum output power. This is achieved through a configuration where the bias voltage is supplied from the emitter or source of the third transistor, which is connected to the collector or drain of the second transistor, and a voltage corresponding to the power supply voltage is applied to the base or gate of the third transistor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the RF signal is increased to amplify the output power, then the base current of the common-base stage transistor is increased, but the base voltage decreases and the maximum output power is reduced
Solution Approach 1:
A third transistor is introduced as an intermediary component between the power supply and the common-base stage transistor. This third transistor acts as a buffer that supplies the base voltage to the common-base stage transistor, isolating it from the effects of increased base current. The third transistor's emitter supplies a stable bias voltage to the base of the second transistor (common-base stage), preventing the base voltage from decreasing when RF signal power is increased.
2Productivity
If the base current of the common-base stage transistor is increased to handle higher RF signals, then the signal amplification capability is improved, but the base voltage drops and limits the maximum output current
Solution Approach 1:
The third transistor is configured to automatically supply the appropriate bias voltage to the base of the common-base stage transistor based on its own base voltage reference. The third transistor's base receives a reference voltage (third voltage corresponding to the first voltage), and its emitter automatically provides the corresponding bias voltage (second voltage corresponding to the first voltage) to maintain stable operation of the common-base stage transistor, regardless of variations in base current demand.
Data Source
AI summary
A power amplifier circuit includes a first transistor, wherein a radio frequency signal is inputted to a base or gate of the first transistor; a second transistor having an emitter connected to a collector or drain of the first transistor, wherein a first voltage is supplied to a collector of the second transistor, and a first amplified signal obtained by amplifying the radio frequency signal is outputted from the collector of the second transistor; and a third transistor configured to supply a bias voltage to a base of the second transistor. A second voltage is supplied to a collector or drain of the third transistor, a third voltage corresponding to the first voltage is supplied to a base or gate of the third transistor, and the bias voltage, which corresponds to the third voltage, is supplied from an emitter or source of the third transistor.