Variable Gain Amplifier Degeneration Circuit for Broad-Band Linearity
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Solution Overview
Problem
Existing variable gain amplifier circuits face challenges in maintaining signal linearity across a broad frequency band, particularly due to the limitations of gain control mechanisms that affect the output signal's frequency response.
Innovation Solution
The proposed variable gain amplifier circuit incorporates a degeneration circuit with MOS transistors connected in series between the emitters of bipolar transistors, along with a series resistor circuit and bias resistors, to adjust resistance values and control voltage gains, ensuring that gate potentials fluctuate in association with input voltage signals across a broad frequency range, thereby enhancing linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a gain control transistor is used to control the gain of the amplifier circuit, then the gain can be variably controlled, but it becomes difficult to maintain linearity of the output signal in a broad frequency band
Solution Approach 1:
The amplifier circuit is divided into multiple independent amplification transistor units (first amplification transistor unit and second amplification transistor unit), each with its own current source and load elements. This segmentation allows each unit to operate independently, improving signal linearity by eliminating the distortion introduced by gain control transistors while maintaining variable gain control through the differential pair configuration.
2Device complexity
If a single transistor is used for gain control, then the circuit structure is simple, but the output signal linearity across broad frequency cannot be maintained
Solution Approach 1:
The amplification transistor units serve multiple functions: they provide signal amplification, enable variable gain control through the differential configuration, and maintain signal linearity across broad frequencies. The current sources and load elements work together to achieve both gain control and linearity maintenance without requiring separate gain control transistors, thus avoiding the trade-off between circuit simplicity and signal quality.
Data Source
AI summary
A variable gain amplifier circuit includes first and second input terminals, first and second output terminals, first and second transistors respectively having bases electrically connected to the first and second input terminals and having collectors electrically connected to the first and second output terminals, and a degeneration circuit connected between emitters of the first and second transistors. The degeneration circuit has first and second MOS transistors each having two current terminals connected in series between the emitters of the first and second transistors, series resistor circuits, first and second current sources, two resistive elements connected between the first and second current sources and gates of the first and second MOS transistors, and two resistive elements connected between the first and second current sources and two nodes of the series resistor circuits.