Bias Circuit Memory Effect Reduction via Low Impedance
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Solution Overview
Problem
Real amplifiers exhibit distortion due to memory effects caused by feedback loops with substantial time constants, such as Automatic Gain Control (AGC) circuits and bias circuits, which affect the amplitude of the output based on input at multiple prior points in time.
Innovation Solution
A circuit with a master-slave source follower configuration, a filter circuit, and a signal ground circuit is used to provide a bias voltage to a biased transistor, ensuring that the reference transistor has a transconductance identical to the biased transistor, and the filter circuit presents different impedances at different frequencies to reduce memory effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If feedback loops with substantial time constants (such as AGC circuits and bias circuits) are used in amplifiers, then the amplifier can maintain stable operation and adapt to varying signal conditions, but memory effects are introduced that cause distortion in the output
Solution Approach 1:
The bias circuit is segmented into a master bias circuit and a slave bias circuit. The master bias circuit generates a master bias voltage that is coupled to the gate of the biased transistor, while the slave bias circuit generates a slave bias voltage that is coupled to the source of the biased transistor. This segmentation allows the biasing function to be divided into separate stages, reducing the time constant of the overall feedback loop and minimizing memory effects while maintaining stable operation.
Solution Approach 2:
A coupling capacitor is introduced as an intermediary element between the master bias circuit and the slave bias circuit. The coupling capacitor blocks DC components while allowing AC components to pass, thereby reducing the effective time constant of the feedback loop. This intermediary element enables the bias circuit to respond more quickly to signal variations without introducing significant memory effects, thus reducing distortion while maintaining stability.
2Ease of manufacture
If a traditional bias circuit is used, then the circuit is simple to implement, but return currents are generated that contribute to memory effects and distortion
Solution Approach 1:
The return current path is extracted and separated from the main signal path by introducing a dedicated coupling capacitor in the bias circuit. This capacitor blocks the return currents from flowing back into the amplifier core, thereby eliminating their harmful effects on distortion while maintaining the overall simplicity of the circuit implementation. The slave bias circuit configuration also helps to isolate return currents from affecting the main signal path.
Data Source
AI summary
A circuit includes a bias circuit for a biased transistor. The bias circuit includes a master-slave source follower circuit, a reference transistor, and a bias circuit voltage output coupled to the biased transistor and configured to provide a bias voltage. The reference transistor has a transconductance substantially identical to a transconductance of the biased transistor. A signal ground circuit may be coupled between the biased transistor and one or more components of the bias circuit that do not generate significant return currents to a power supply ground. A method includes generating a current in a reference transistor according to a first voltage generated using a master source follower circuit, generating a second voltage substantially identical to the first voltage using a slave source follower circuit, and providing the second voltage to a biased transistor. The reference transistor has a transconductance substantially identical to a transconductance of the biased transistor.