RF Power Amplifier Bias Circuit Noise Isolation
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Solution Overview
Problem
Conventional RF power amplifiers suffer from noise interference due to the leakage of radio frequency signals from the amplification circuit to the bias circuit, which affects the reception of radio frequency signals, particularly due to unnecessary frequency components generated by the difference and higher frequency components between transmission and receiving frequencies.
Innovation Solution
Incorporating an attenuation filter in the bias circuit to suppress the difference frequency and higher frequency components, using a configuration that includes transistors and resistors with specific filter positions to attenuate these frequencies before they are transferred back to the amplification circuit, thereby reducing output noise in the receiving frequency band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a bias circuit supplies bias current to the amplification circuit using a conventional configuration, then the amplification circuit can operate with proper biasing, but radio frequency signals leak from the amplification circuit to the bias circuit causing noise interference
Solution Approach 1:
The patent introduces an emitter follower circuit as an intermediary buffer between the bias circuit and the amplification circuit. This intermediary stage isolates the bias circuit from direct connection to the RF signal path, preventing RF signal leakage while maintaining proper DC bias current supply. The emitter follower acts as a buffer that passes DC bias current while blocking RF frequencies from propagating to the bias circuit.
2Device complexity
If the bias circuit is directly connected to the amplification circuit, then the circuit configuration is simple, but unnecessary frequency components are transferred from the bias circuit to the amplification circuit causing output noise
Solution Approach 1:
The emitter follower serves as a mediating stage that maintains circuit simplicity while eliminating noise transfer. It provides galvanic isolation between the bias circuit and amplification circuit, preventing the transfer of unwanted frequency components (difference frequency fL and frequency fH) while requiring only minimal additional components (one transistor and one capacitor).
Solution Approach 2:
The patent converts the potential harm of direct coupling into a benefit by using the emitter follower's inherent frequency selectivity. The emitter follower naturally passes DC and low-frequency signals while blocking high-frequency RF signals, thus converting what would be a direct noise path into a frequency-selective isolation mechanism that benefits overall system performance.
3Adaptability or versatility
If radio frequency signals are transmitted at frequency ft and received at frequency fr, then the amplification circuit can handle both transmission and reception, but difference frequency components and higher frequency components interfere with signal reception
Solution Approach 1:
The emitter follower intermediary circuit provides frequency-based isolation that enables dual transmission and reception operations. It allows the system to operate at transmission frequency ft while blocking the generation and propagation of interfering frequency components (difference frequency fL=|fr-ft| and frequency fH=2ft-fr) that would otherwise interfere with reception at frequency fr.
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 proposed solution effectively reduces output noise in the receiving frequency band by attenuating the unwanted frequency components, improving the overall performance of the RF power amplifier.
Implementation Method 1
The bias circuit includes a filter for attenuating at least either one of a difference frequency fL between a receiving frequency fr and a transmission frequency ft of the radio frequency signal, and a component of a frequency fH which is lower than the transmission frequency ft by the difference frequency fL
Data Source
AI summary
A bias current to be supplied to an amplification circuit 60 is drawn out of a collector of a transistor Q11 of a bias circuit 10. The drawn-out bias current is input to a base of a transistor Q13 via an attenuation filter F2 and is output from an emitter of the transistor Q13 in the state where the voltage thereof is reduced by a level corresponding to Vbe. The attenuation filter F2 is conducted in a DC manner, and attenuates a component of a frequency fH(=2ft−fr) defined by a transmission frequency ft and a receiving frequency fr of a radio frequency signal. The bias current output from the emitter of the transistor Q13 is supplied to the amplification circuit 60 via an attenuation filter F1. The attenuation filter F1 is conducted in a DC manner, and attenuates a component of a frequency fL(=|fr−ft|).