Variable-Gain Amplifier Circuit With Current-Controlled Bias Stability
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing variable-gain amplifiers face challenges in achieving precise gain control due to the dependence of resistance values on transistor on-state resistances, leading to inaccuracies and difficulties in transitioning between low and high gains, and performance is affected by biasing current variations based on gain selection.
Innovation Solution
A circuit arrangement with a symmetrical differential structure using MOSFETs and bipolar transistors, where input and output biasing currents are controlled through reference currents, allowing for constant biasing and precise gain control via digital-to-analog converters, with compensation loops to maintain accurate collector and diode currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If variable resistance components are used on the input terminal and feedback branch to implement variable gain, then the gain can be adjusted, but the gain precision deteriorates due to dependence on transistor on-state resistances
Solution Approach 1:
The patent changes the control parameter from resistance values to current values. By using current sources to control the gain through transistor current ratios rather than resistance ratios, the gain precision is improved because current ratios are less sensitive to transistor parameter variations and on-state resistances.
Solution Approach 2:
The patent replaces the mechanical/resistive control system with an electronic current control system. Instead of using variable resistors and relying on resistance ratios, the invention uses current sources and current mirrors to control gain, substituting a more precise electronic mechanism for the less precise resistive mechanism.
2Ease of operation
If MOSFETs are used as selection components to impose gain values, then the gain can be controlled, but the precision deteriorates because the on-state resistances are comparable with variable resistors
Solution Approach 1:
The patent extracts the resistance elements from the gain control mechanism and replaces them with current sources. By removing the dependence on transistor on-state resistances and variable resistors, the precision problem is eliminated while maintaining ease of operation through current-controlled gain.
Solution Approach 2:
The patent introduces current sources as intermediary elements between the control signal and the gain determination. These current sources act as mediators that translate control voltages into precise current ratios, eliminating the direct dependence on transistor on-state resistances for gain control.
3Adaptability or versatility
If biasing current is made dependent on gain selection to achieve variable gain, then the gain can be varied, but the circuit performance deteriorates due to biasing current variations
Solution Approach 1:
The patent segments the biasing current into two independent parts: a constant biasing current that maintains circuit performance, and a control current that adjusts the gain. This segmentation allows gain variability without compromising circuit performance consistency, as the constant biasing current ensures stable operation while the control current provides gain adjustment.
Solution Approach 2:
The patent makes the biasing current dynamic by separating it into a static constant component and a dynamic control component. The constant part ensures reliable circuit performance, while the dynamic part enables gain adjustment, allowing the system to adapt to different gain requirements without sacrificing performance consistency.
4Measurement precision
If differential pairs with cross-coupled transistors are used to generate current for gain control, then high gain precision can be achieved, but the circuit complexity increases
Solution Approach 1:
The patent makes the differential pair circuit serve multiple functions: it generates the control current for gain adjustment, provides current mirroring for precision, and maintains constant biasing for performance stability. By making the same circuit structure multi-functional, the need for additional separate circuits is eliminated, reducing overall complexity while maintaining high gain precision.
Data Source
AI summary
In accordance with an embodiment, a variable gain amplifier includes: a first differential transistor pair coupled to a signal input; a first current source configured to provide a first bias current to the first differential transistor pair; a pair of diodes coupled to an output of the first differential transistor pair; a second differential transistor pair having an input coupled to the pair of diodes; a second current source configured to provide a second bias current to the second differential transistor pair; and a current control circuit coupled to the first current source and the second current source.