Power Amplifier Bias Network for Linearity and Impedance Matching
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Solution Overview
Problem
Power amplifier circuits in mobile communication devices face linearity issues due to thermal positive feedback in bipolar transistors, leading to decreased power gain and impedance mismatch between the amplifier and preceding circuits.
Innovation Solution
A power amplifier circuit design that includes a bias circuit with a voltage generation circuit, transistors, and a resistance element to control bias currents and voltages, and an impedance circuit to manage impedance matching, operating in different modes to optimize linearity and power gain based on signal power levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a ballast resistor is disposed between the base of the bipolar transistor and a base bias voltage supply terminal to suppress thermal positive feedback, then thermal runaway is suppressed, but voltage drop across the ballast resistor decreases base voltage when base current increases, leading to decreased power gain and deteriorated linearity
Solution Approach 1:
A capacitor is introduced as an intermediary component connected between the base of the bipolar transistor and the base bias voltage supply terminal. This capacitor mediates the interaction between the ballast resistor and the base voltage, allowing AC signal components to bypass the ballast resistor while the DC bias current still flows through it. This resolves the contradiction by maintaining thermal stability through the ballast resistor while preserving linearity by preventing voltage drop for AC signals.
2Ease of operation
If a capacitance element is connected between the signal input terminal and the base bias voltage supply terminal to improve linearity, then base voltage reduction is suppressed, but the capacitance element affects impedance matching between the amplifier and the preceding circuit
Solution Approach 1:
The capacitor connected between the base and base bias voltage supply terminal acts as an intermediary that selectively affects different signal components. It provides a low-impedance path for AC signals to maintain linearity while having minimal impact on DC bias conditions and impedance matching characteristics. This resolves the contradiction by being an intermediary element that benefits linearity without significantly complicating impedance matching.
3Use of energy by moving object
If the power amplifier operates in different power modes, then power consumption is optimized, but maintaining consistent linearity and impedance matching across different power levels becomes challenging
Solution Approach 1:
The bias circuit is designed to dynamically adjust bias conditions based on operating power levels. The ballast resistor and capacitor combination provides adaptive behavior where the effective impedance seen by the base changes with operating conditions. This dynamic adaptation maintains consistent linearity and impedance matching characteristics across different power modes while optimizing power consumption, resolving the contradiction through dynamic circuit behavior.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design improves linearity of power gain and impedance matching, reducing thermal runaway and maintaining proportional collector current to base current, thus enhancing the performance of power amplifier circuits in mobile communication devices.
Implementation Method 1
a voltage drop across the ballast resistor suppresses the increase in the base current
Implementation Method 2
a capacitance element between a signal input terminal and a base bias voltage supply terminal
Data Source
AI summary
A power amplifier circuit includes an amplifier transistor, a bias circuit that supplies a bias current or voltage to the amplifier transistor, and a resistance element connected between a base of the amplifier transistor and the bias circuit. The bias circuit includes a voltage generation circuit, a first transistor having a base to which a first direct-current voltage is supplied and an emitter from which the bias current or voltage is supplied, a second transistor having a base to which a second direct-current voltage is supplied and an emitter connected to the emitter of the first transistor, a signal supply circuit that supplies an input signal to the base of the second transistor, and an impedance circuit disposed between the base of the first transistor and the base of the second transistor.