Transistor Circuit Impedance Transformation
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Solution Overview
Problem
Existing amplifier devices require very high input impedance, particularly for capacitive sensors, but existing impedance transformation techniques are impractical due to the large silicon area consumption and impractical current requirements for achieving giga-ohm ranges.
Innovation Solution
The use of impedance transformations involving pairs of NMOS and PMOS transistor circuits with different channel width to length ratios and overdrive voltages to achieve high impedance levels, minimizing silicon area usage while maintaining accurate impedance values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a very large resistor is inserted between the input of the amplifier device and a bias point to generate an appropriate operating point, then the input impedance requirement is met, but the silicon area consumption becomes excessive
Solution Approach 1:
The patent changes the operating parameters of transistor circuits, specifically utilizing sub-threshold gate-source voltages to exponentially increase impedance. By operating transistors in the sub-threshold region where gate-source voltage is below the threshold voltage, the circuit achieves giga-ohm range impedance without requiring large physical resistors, thus resolving the contradiction between high impedance and small silicon area.
2Reliability
If transistor circuits with different W/L ratios are used for impedance transformation, then impedance transformation is achieved, but picoamp-level currents are required making the circuit impractical
Solution Approach 1:
The patent modifies the voltage parameter by applying sub-threshold gate-source voltages to the transistor circuits. This parameter change enables the circuits to operate in a regime where impedance is exponentially increased without requiring impractically low picoamp currents, making the impedance transformation practical while maintaining reasonable current consumption levels.
3Reliability
If transistor circuits operate with sub-threshold gate-source voltages, then impedance increases exponentially, but the circuits require precise voltage control
Solution Approach 1:
The patent employs feedback mechanisms through current mirror arrangements where transistor circuits are coupled such that their operation is interdependent. The current mirrors provide automatic regulation and stabilization of the sub-threshold operating points, reducing the need for external precise voltage control while maintaining the exponential impedance increase benefit.
Data Source
AI summary
In one implementation, an apparatus may include a first negative channel metal oxide semiconductor (NMOS) transistor circuit coupled to a first voltage source, a second NMOS transistor circuit coupled to the first voltage source, the second NMOS transistor circuit having a smaller channel width to channel length ratio than the first NMOS transistor circuit, a first positive channel metal oxide semiconductor (PMOS) transistor circuit coupled to a second voltage source and coupled to the second NMOS transistor circuit, and a second PMOS transistor circuit coupled to the second voltage source, the second PMOS transistor circuit having a larger channel width to channel length ratio than the first PMOS transistor circuit.