CMOS Variable Gain Amplifier With Linear-in-dB Control
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Solution Overview
Problem
Designing a linear-in-dB variable gain amplifier using a CMOS process is challenging due to the square function relationship between the current of an MOS device and its bias voltage, making it difficult to achieve a linear gain response.
Innovation Solution
The implementation of an exponential DC converter and a linear voltage multiplier, where the exponential DC converter generates an exponential voltage proportional to the control voltage, coupled with a Gilbert-type or current-steering type voltage multiplier, allows for a gain that is exponential to the control voltage, facilitating the construction of a linear-in-dB variable gain amplifier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a bipolar process is used to design a linear-in-dB VGA, then the gain control linearity is improved, but the manufacturing process complexity increases
Solution Approach 1:
The patent transforms the control voltage through exponential and logarithmic conversion stages, changing the voltage parameters to achieve linear dB gain control. The exponential voltage generator converts the control voltage to an exponential voltage, which is then processed through current mirrors and logarithmic conversion to produce output currents with linear dB characteristics, resolving the contradiction between gain linearity and manufacturing complexity by using CMOS-compatible circuit transformations.
Solution Approach 2:
The patent introduces intermediate conversion stages including an exponential voltage generator and logarithmic conversion circuits as mediators between the control voltage and the final output. These intermediary circuits transform the control signal through multiple stages (voltage to exponential voltage, current mirroring, logarithmic conversion) to achieve the desired linear dB gain characteristic using standard CMOS devices, avoiding the need for specialized bipolar processes.
2Ease of manufacture
If a CMOS process is used to construct the VGA, then the manufacturing ease is improved, but the gain control precision deteriorates
Solution Approach 1:
The patent divides the VGA into multiple functional segments: an exponential voltage generator stage, current mirror stages, and logarithmic conversion stages. Each segment performs a specific transformation function, allowing the overall system to achieve linear dB gain control through the cumulative effect of individual CMOS-compatible circuit blocks, thereby maintaining both manufacturing ease and gain control precision.
Solution Approach 2:
The patent employs parameter transformations through exponential and logarithmic conversion circuits to compensate for the square-law characteristic of MOS devices. By converting the control voltage through these mathematical transformations, the circuit achieves linear dB gain control precision while using standard CMOS transistors, resolving the contradiction between manufacturing ease and gain control precision.
Data Source
AI summary
A variable gain amplifier (VGA) with a gain thereof exponential to a control voltage thereof. The variable gain amplifier (VGA) comprises an exponential DC converter, and a linear voltage multiplier. The exponential DC converter receives the control voltage and generates an exponential voltage which is exponential to the control voltage. The linear voltage multiplier is coupled to the exponential DC converter and has a gain proportional to the exponential voltage of the exponential DC converter.