Bias Circuit Layout for Power Amplifier Gain Linearity
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Power amplifier circuits in mobile communication devices face challenges in maintaining linearity of power gain due to thermal positive feedback in bipolar transistors, which can lead to decreased power gain and impedance mismatching, especially at high input power levels.
Innovation Solution
The power amplifier circuit incorporates a bias circuit with a series-connected resistance element and a signal supply circuit, including transistors and capacitors, to manage bias voltages and currents, preventing thermal runaway and improving impedance matching between the amplifier and preceding circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a ballast resistor is connected between the base of a bipolar transistor and the base bias voltage supply terminal to suppress thermal positive feedback, then thermal stability is improved, but voltage drop across the resistor decreases base voltage when base current increases, causing power gain to decrease and linearity to deteriorate
Solution Approach 1:
The patent introduces a capacitor as an intermediary element 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 bias voltage supply, allowing AC signal components to bypass the resistor while maintaining DC bias stability. The capacitor thus resolves the contradiction by enabling thermal stability through the resistor while preserving linearity by providing an alternative path for signal currents.
2Reliability
If a capacitance element is connected between the signal input terminal and the base bias voltage supply terminal to improve linearity, then power gain linearity is improved, but the capacitance element may affect impedance matching between the amplifier and the preceding circuit
Solution Approach 1:
The patent applies local quality by differentiating the functional requirements at different parts of the circuit. The capacitor connected to the base bias voltage supply terminal provides local improvement in linearity by stabilizing the base voltage, while the overall circuit design maintains impedance matching through proper selection of component values and configuration. This localized approach allows linearity improvement without compromising overall system adaptability.
3Power
If the base current increases with an increase in the power level of an input signal, then the amplifier can handle higher power levels, but voltage drop increases across the ballast resistor, causing a decrease in base voltage and power gain
Solution Approach 1:
The capacitor acts as an intermediary that separates the DC bias function from the AC signal function. It allows the ballast resistor to maintain its thermal stability function for DC bias while providing a low-impedance path for AC signal currents, thereby preventing the resistor from causing excessive voltage drop during high power operation. This enables the amplifier to handle higher power levels while maintaining stable power gain.
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 enhances the linearity of power gain and reduces gain compression, maintaining better power-added efficiency and impedance matching, even at high input power levels, compared to traditional designs.
Implementation Method 1
a capacitance element between a signal input terminal and a base bias voltage supply terminal
Implementation Method 2
a resistance element (hereinafter also referred to as a ballast resistor) is connected between a base of a bipolar transistor and a base bias voltage supply terminal. In this configuration, a voltage drop across the ballast resistor suppresses an increase in base current
Data Source
AI summary
A power amplifier circuit includes an amplifier transistor having a base, a collector, a bias circuit, and a first resistance element connected between the 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 disposed between the base of the amplifier transistor and 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.