Power Amplifier Bias Circuit for Gain Compression Suppression
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Solution Overview
Problem
Existing power amplifier circuits face challenges in maintaining transistor gain during large-signal operations due to gain compression and increased distortion, primarily caused by the voltage drop across resistors in the bias circuits, which affects the base voltage of transistors.
Innovation Solution
The implementation of an impedance circuit that applies a portion of the radio-frequency signal to the output section of the second bias circuit, thereby increasing the voltage applied to the resistor and maintaining the base voltage of the second transistor, is introduced. This is achieved by connecting the impedance circuit between the emitter of the second transistor and the capacitor, ensuring the impedance circuit is open for DC and conducting for AC, using configurations such as capacitors or series combinations of capacitors and inductors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a resistor is used in the bias circuit to apply bias to the base of the second transistor, then the bias can be provided, but the voltage drop across the resistor increases during large-signal operation, causing gain compression and distortion
Solution Approach 1:
An impedance circuit is introduced as an intermediary component between the bias circuit output and the second transistor base. This impedance circuit includes a capacitor connected in series with the base, which blocks DC while allowing AC signals to pass. The capacitor acts as a mediator that separates the DC bias provision function from the AC signal transmission function, preventing the bias resistor from causing gain compression during large-signal operation while maintaining proper transistor biasing.
2Productivity
If the current amplitude of the AC signal inputted to the emitter of the second transistor increases, then the signal amplification increases, but the base current increases causing increased voltage drop across the bias resistor, making it difficult to keep the transistor in the on state
Solution Approach 1:
The circuit is segmented into separate functional paths: a DC bias provision path through the resistor and an AC signal transmission path through the capacitor. This segmentation allows the bias resistor to provide stable DC bias without being affected by AC signal current variations, while the capacitor ensures AC signals are transmitted to the transistor base without being blocked by the resistor's voltage drop.
3Ease of operation
If a capacitor is connected in series with the base of the second transistor to block DC, then AC signals can be transmitted, but the impedance circuit complexity increases
Solution Approach 1:
The capacitor connected in series with the transistor base serves multiple functions simultaneously: it blocks DC from the bias circuit from reaching the transistor base (which would cause unwanted voltage variations), transmits AC signal components to the transistor base for amplification, and isolates the bias circuit from signal variations. This multi-functionality achieves complex objectives with a single simple component.
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
This configuration effectively suppresses the reduction in base voltage during large-signal operations, maintains transistor gain, improves input and output gain characteristics, and reduces output signal distortion.
Implementation Method 1
a base configured to receive the first bias via a first resistor and receive a radio-frequency input signal via a first capacitor
Implementation Method 2
a collector electrically connected to a first power supply potential via a third inductor, and an emitter configured to receive the radio-frequency signal
Data Source
AI summary
A power amplifier circuit includes first and second bias circuits configured to provide first and second biases, respectively, a first transistor having an emitter connected to a reference potential, a base configured to receive the first bias via a first resistor and receive a radio-frequency input signal via a first capacitor, and a collector configured to output an amplified radio-frequency signal, a second transistor having a base connected to the reference potential via a second capacitor and configured to receive the second bias via a second resistor, an emitter configured to receive the radio-frequency signal, and a collector connected to a power supply potential via a third inductor and configured to output a radio-frequency output signal, and an impedance circuit having a first end connected to an output section of the second bias circuit and configured to apply an alternating-current signal to a path extending from the second bias circuit.