Variable Gain Amplifier Feedback for Stable S11 and Gain Steps
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Solution Overview
Problem
Prior art variable gain amplifiers experience increased DC current and power consumption as gain is reduced, leading to degraded input match (S11) due to varying drain voltage and input capacitance, and gain steps deviate from design values due to process, temperature, and supply voltage variations.
Innovation Solution
The proposed variable gain amplifier circuit employs a current diversion transistor to maintain constant drain current and voltage, using cascode and differential pair configurations with feedback loops to control gain stages independently, ensuring consistent S11 and gain settings across different states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If current diversion transistors are used to reduce gain, then gain control is achieved, but drain voltage increases causing drain current to increase and power consumption to increase
Solution Approach 1:
A feedback circuit is employed that senses the drain voltage of the amplifying transistor and adjusts the gate voltage of the current diversion transistor accordingly. This feedback mechanism compensates for the voltage increase caused by current diversion, maintaining constant drain current and power consumption across different gain settings.
Solution Approach 2:
The invention dynamically adjusts multiple parameters including gate voltages of both the amplifying and current diversion transistors based on the desired gain setting. By changing these electrical parameters in a coordinated manner through the feedback circuit, the system achieves gain control while maintaining constant power consumption.
2Adaptability or versatility
If current diversion transistors are used to reduce gain, then gain control is achieved, but input capacitance changes degrading input match (S11)
Solution Approach 1:
The feedback circuit maintains constant drain voltage, which in turn stabilizes the input capacitance of the amplifying transistor. This constant capacitance ensures that the input match (S11) remains consistent across all gain settings, preventing degradation of the input matching.
Solution Approach 2:
Instead of allowing drain voltage to vary with gain control and accepting the resulting input capacitance changes, the invention inverts the approach by using feedback to actively maintain constant drain voltage. This inverted control strategy prioritizes voltage stability over direct current control, thereby preserving input matching characteristics.
3Adaptability or versatility
If current diversion transistors are used for gain control, then gain variation is achieved, but gain steps deviate from design values due to process, temperature, and supply voltage variations
Solution Approach 1:
The feedback circuit continuously monitors the actual gain by sensing drain voltage and adjusts the transistor operating points accordingly. This closed-loop control compensates for variations caused by process tolerances, temperature drift, and supply voltage changes, ensuring that the actual gain steps match the designed values.
Solution Approach 2:
The invention transitions from a static gain control approach to a dynamic one where the feedback circuit continuously adapts the transistor operating conditions. This dynamic adjustment ensures that gain steps remain accurate under varying environmental and manufacturing conditions.
Data Source
AI summary
A variable gain amplifier includes multiple gain stages. Each gain stage includes a gain transistor and a cascode transistor to form a cascode amplifier, and a current diversion transistor to divert current away from a cascode transistor to reduce gain in the stage. A control circuit is included to maintain substantially constant drain-to-source voltage and drain current in the gain transistor.