MOS Transconductance Circuit With Wide gm Control at Low Voltage
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Solution Overview
Problem
Conventional variable transconductance circuits in semiconductor integrated circuits face challenges in achieving wide-range transconductance variation while maintaining low power consumption and compact circuit size, especially under low power supply voltages, which is crucial for optical disk devices that require handling a wide range of signal speeds and amplitudes.
Innovation Solution
A variable transconductance circuit design that utilizes MOS transistors with two control parameters (Ia and Ib) to vary transconductance, incorporating voltage-current conversion circuits, operational amplifiers, and current sources to achieve a wide range of transconductance variation up to 20 times the minimum value with a power supply voltage as low as 3V, and further enhances this range up to 100 times by using parallel-connected MOS transistors and a transconductance control circuit with a current mirror.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If Io is increased by 100 times to allow gm to vary up to 10 times its minimum value, then gm variation range is improved, but power consumption increases and low power supply voltage operation becomes difficult
Solution Approach 1:
The patent changes the control parameter from direct Io variation to Vgs variation through a control voltage Vc. By using the exponential relationship between Vgs and Io in MOS transistors, a small voltage range (0.2V to 2V) can control Io over a wide range, achieving gm variation up to 10 times with much lower current variation requirements.
Solution Approach 2:
The patent introduces a control voltage Vc as an intermediary between the power supply and the transconductance control. This Vc is applied to the gate of the MOS transistor to indirectly control Io, avoiding the need for direct 100-fold current variation and enabling efficient voltage-controlled transconductance adjustment.
2Adaptability or versatility
If a plurality of transconductors are connected in parallel to enable wide-range gm variation, then gm variation range is improved, but on-board circuit area increases
Solution Approach 1:
Instead of using multiple parallel transconductors, the patent uses a single transconductor with voltage-controlled parameters. By varying Vgs through control voltage Vc, the transconductance gm can be adjusted over a wide range, achieving the same effect as multiple parallel circuits but with much smaller area.
3Adaptability or versatility
If Io is increased by 100 times to achieve wide-range gm variation, then gm variation range is improved, but current consumption increases
Solution Approach 1:
The patent changes from current-controlled (Io variation) to voltage-controlled (Vc variation) transconductance adjustment. The exponential I-V characteristic of MOS transistors allows a small voltage change (0.2V to 2V) to produce large current variation, achieving gm control with minimal current consumption.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design allows for a wide range of transconductance variation with reduced current change requirements, achieving high transconductance with low power consumption and compact circuit size, suitable for optical disk devices, enabling efficient signal processing and gain amplification.
Implementation Method 1
a voltage-current conversion circuit for outputting a current signal linear with an input voltage signal (Vi)
Implementation Method 2
first and second MOS transistors (M1, M2) for converting the current signal received to a square-root compressed voltage signal
Implementation Method 3
controls transconductance by varying the current (Ia) from the first and second current sources (1, 2) and the current (Ib) from the third current source (3)
Data Source
AI summary
The variable transconductance circuit includes: a voltage-current conversion circuit for outputting a current signal linear with an input voltage signal; first and second MOS transistors for converting the current signal received to a square-root compressed voltage signal; and third and fourth MOS transistors for converting the square-root compressed voltage signal to a linear current signal. A bias current at the first and second MOS transistors and a bias current at the third and fourth MOS transistors are varied to control transconductance.