Class C Amplifier Common-Mode Feedback for Wideband Stability
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Solution Overview
Problem
Pseudo-differential class C amplifiers face uncontrollable common-mode current draw, leading to instability in output common-mode, which degrades differential bandwidth and settling time, and existing feedback circuits are inadequate to address this issue.
Innovation Solution
A class C amplifier with a common-mode feedback system comprising a resistor network, feedback amplifiers, and transistors that generate control voltages to provide feedback currents based on sensed common-mode currents, allowing for adjustable thresholds to manage common-mode current effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a common-mode feedback loop is employed to stabilize the output common-mode, then the common-mode stability is improved, but the device complexity increases
Solution Approach 1:
The feedback system is segmented into two distinct circuits: a first common-mode feedback circuit that provides feedback current to the resistor network when common-mode current is below a threshold, and a second common-mode feedback circuit that provides feedback current to the input terminals when common-mode current exceeds the threshold. This segmentation allows each circuit to be optimized for its specific operating range, improving overall stability while managing complexity through functional division.
Solution Approach 2:
The feedback system dynamically switches between two different feedback paths based on the common-mode current level. The dual-feedback mechanism with threshold-based activation allows the system to adapt its feedback strategy in real-time, providing optimal common-mode stability across varying operating conditions without requiring a single complex feedback circuit.
2Device complexity
If conventional feedback circuits are used, then the circuit structure is simple, but the differential bandwidth degradation occurs
Solution Approach 1:
The patent implements a sophisticated feedback mechanism where the second amplifier senses the common-mode current and generates a control voltage that drives the feedback circuits. This feedback loop dynamically adjusts the feedback current based on the actual common-mode current level, preventing bandwidth degradation by actively compensating for variations rather than using a fixed simple structure.
Solution Approach 2:
The system changes the feedback parameter (feedback current magnitude and path) based on the common-mode current threshold. When common-mode current is below the threshold, one feedback path is active; when above, the other path activates. This parameter switching maintains differential bandwidth by adapting the feedback strength to operating conditions, avoiding the bandwidth degradation associated with fixed feedback structures.
3Loss of time
If common-mode feedback is implemented to maintain settling time, then the settling time is preserved, but the device complexity increases
Solution Approach 1:
The settling time performance is maintained by segmenting the feedback response into two stages: a first feedback circuit for low common-mode current conditions and a second feedback circuit for high common-mode current conditions. Each segment is optimized to maintain settling time in its respective operating range, achieving overall settling time preservation without requiring a single overly complex circuit.
Solution Approach 2:
The dynamic switching between feedback circuits based on common-mode current threshold ensures that the appropriate feedback strength is applied at each moment, preserving settling time across all operating conditions. The system transitions between feedback modes dynamically, maintaining rapid settling performance without the need for a continuously complex feedback structure.
Data Source
Figure 1
Figure 2~3
AI summary
In a class C amplifier (100), a common-mode current is applied to a common-gate amplifier (100) and the common-mode current is sensed. In response to the sensed common-mode current, a control voltage is generated. A first feedback current (FBI) generated in response to the control voltage is applied to a differential ground of the common-gate amplifier (110) if the common- mode current is less than a predetermined threshold. Additionally, a second feedback current (FB2) generated in response to the control voltage is applied to input terminals (IN) of the common-gate amplifier (110) if the common-mode current is greater than the predetermined threshold.