Variable Gain Amplifier Current Control for Constant Bandwidth
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Solution Overview
Problem
Conventional variable gain amplifiers, such as those with Cherry-Hooper arrangements, face a challenge in maintaining frequency bandwidth as the gain is adjusted, leading to degradation of the output signal when the input level increases.
Innovation Solution
A variable gain amplifier design that incorporates a current supplier and current extractor, connected in series with a constant current source and load element, allows for adjustment of gain without degrading the frequency bandwidth by maintaining constant current flow through the amplifying transistor and varying current flow through the load element.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the equivalent impedance of the inserted transistor is varied to adjust gain, then the voltage gain of the first stage is varied, but the frequency bandwidth of the amplifier is narrowed
Solution Approach 1:
The patent divides the current path into multiple independent controllable segments: a first current path through the amplifying transistor with constant current, and a second current path through the load element with variable current. This segmentation allows independent control of gain (via load element current) while maintaining bandwidth (via constant amplifying transistor current).
Solution Approach 2:
The patent changes the current parameter independently in different circuit paths. The current through the amplifying transistor is kept constant to maintain bandwidth, while the current through the load element is varied to adjust gain. This parameter separation resolves the contradiction between gain adjustment and bandwidth maintenance.
2Speed
If the feedback amount is increased to widen frequency bandwidth, then the bandwidth of the amplifier is widened, but the degradation of the output signal becomes large as the input level increases
Solution Approach 1:
The patent independently controls the current parameter in the load element to adjust gain without affecting the current through the amplifying transistor. This allows optimization of feedback amount for bandwidth while maintaining stable operating conditions to prevent output signal degradation at high input levels.
3Power
If the current flowing in the series circuit is varied to adjust gain, then the gain of the amplifier is adjusted, but the operating condition of the feedback transistor and load resistor becomes disordered from the designed condition
Solution Approach 1:
The patent segments the current control into two independent paths: one path controls the amplifying transistor current (maintained constant for stable operating conditions), and the other path controls the load element current (varied for gain adjustment). This segmentation prevents disruption of the feedback transistor and load resistor operating conditions.
Solution Approach 2:
The patent introduces a current extractor as an intermediary component that extracts current from the main series circuit and directs it through the load element. This intermediary allows gain adjustment through load element current variation while maintaining the original series circuit current and operating conditions of the feedback transistor and load resistor.
Data Source
AI summary
A variable gain amplifier canceling the dependence of the frequency bandwidth thereof on the gain is disclosed. The variable gain amplifier includes a differential stage with a cascade transistor put between an amplifying transistor and a load resistor. The amplifier further includes a current supplier and a current extractor. The current supplier extracts a current flowing in the second transistor by supplying an additional current to the amplifying transistor. The current extractor adds the additional current flowing in the load resistor which is extracted by the additional current by the current supplier. The bias condition of the load resistor is kept substantially constant independent of the equivalent impedance of the cascade transistor.