VGA Gain-Control Stage With Feedback-Stabilized Bias Current
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Solution Overview
Problem
Current-steering-based variable gain amplifiers (VGAs) face issues with variations in gain-control signals causing changes in output common-mode voltage, which can affect the biasing of subsequent stages and potentially drive transistors outside their expected operation mode.
Innovation Solution
A gain-control stage comprising two differential amplifier units, each receiving a reference voltage and a gain-control voltage, generates differential gain-control signals to control the gain of an external variable-gain amplifying unit, using controllable current sources and active-load units to stabilize the biasing current and maintain transistors within a linear operation region.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If current-steering-based VGA is used to amplify voltage amplitude with gain control, then gain adjustment capability is improved, but output common-mode voltage varies causing biasing issues in subsequent stages
Solution Approach 1:
A common-mode feedback circuit is introduced as an intermediary between the VGA and subsequent stages. This feedback circuit detects the output common-mode voltage and generates a correction signal that is fed back to the VGA's gain control input, thereby stabilizing the common-mode voltage and preventing biasing issues in DC-coupled subsequent stages while maintaining gain adjustment capability.
2Adaptability or versatility
If gain-control signal varies to adjust VGA gain, then gain control flexibility is improved, but transistors may be driven outside expected operation mode
Solution Approach 1:
A feedback mechanism is implemented where the output common-mode voltage is continuously monitored and fed back to the gain control stage. This feedback loop automatically adjusts the gain control signal to maintain the transistors within their expected operation mode, preventing saturation or cutoff conditions even when large gain adjustments are required, thus ensuring reliable transistor operation throughout the full gain range.
Data Source
AI summary
The invention relates to a gain-control stage (100) for generating gain-control signals (Vc+, Vc−) for controlling an external variable-gain amplifying unit (101). The gain-control stage comprises a first (102) and a second differential amplifier unit (112) that receive, at a respective input interface (104,114) a reference voltage signal (VRef) and a variable gain-control voltage signal (VGC). The second differential amplifier unit is configured to provide, via a second output interface (120), a control voltage signal (V1) to a controllable first current source (106) of the first differential amplifier unit (102). The first differential amplifier unit (102) is configured to provide, via a first output interface (110), the first and the second gain-control signal (VC+, VC−) in dependence on the variable gain-control voltage signal (VGC), the reference voltage signal (VRef) and a first biasing current (IB1) that depends on the control voltage signal.