Variable-Gain Differential Amplifier With Cross Point Compensation
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Solution Overview
Problem
Conventional differential amplifiers with variable gain suffer from output cross point shifts when gain is adjusted, leading to distortion in output signals.
Innovation Solution
A differential circuit design with an amplifying stage and a control stage, where the control stage compensates for reduced currents in the amplifying stage to maintain a constant cross point during gain variation, using a pair of load resistors, current sources, and power supply, with specific transistor configurations and control signals to adjust currents and prevent cross point shifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the gain of the differential amplifier is varied by adjusting the current in the amplifying stage, then the gain is improved, but the cross point of the output shifts causing distortions
Solution Approach 1:
The differential amplifier is divided into two independent stages: an amplifying stage (first to third units) for gain control and a control stage (fourth and fifth units) for cross point compensation. This segmentation allows each stage to perform its specific function independently, resolving the contradiction between gain variation and cross point stability.
Solution Approach 2:
The control stage acts as an intermediary that compensates for the cross point shifts caused by the amplifying stage. By introducing this intermediate compensation mechanism, the system can vary gain while maintaining cross point stability through the corrective action of the control stage.
2Ease of operation
If conventional differential amplifiers vary the current to adjust gain, then the gain adjustment is achieved, but the cross point varies synchronous with gain change causing output distortions
Solution Approach 1:
The control stage receives feedback about the cross point shifts caused by the amplifying stage and automatically compensates for them. This feedback mechanism ensures that gain can be adjusted easily while the cross point remains stable, maintaining output signal integrity.
Solution Approach 2:
The control stage is designed to preemptively counteract the cross point shifts that would otherwise occur when the amplifying stage adjusts its current for gain control. By applying compensatory current adjustments in advance, the system prevents cross point drift before it can cause output distortions.
Data Source
AI summary
A differential circuit with a function of a variable gain without shifting the output cross point is disclosed. The differential circuit includes an amplifying stage and a control stage. The amplifying stage includes three units each having a pair of transistors, a pair of load resistors, and a pair of current sources. The second and third units each put between the first unit and the load resistor to bypass the current. The control stage includes two units and two current sources to compensate the current bypassed by the second or third unit to keep the DC output level substantially in constant.