Variable Substrate Bias Circuit for Low Voltage Linearity
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Solution Overview
Problem
At low power supply voltages (e.g., two volts and below), CMOS driver circuits face difficulty in achieving linear output impedance, leading to increased high-frequency noise and ringing, as existing technologies struggle to maintain impedance linearity.
Innovation Solution
A circuit with a variable threshold voltage (VT) is implemented, utilizing a bias stage with a resistive element and a bias switch to dynamically adjust the substrate bias of the drive transistor in response to input signals, thereby maintaining more linear output impedance and reducing leakage current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If power supply voltage is reduced to two volts and below, then power consumption is reduced, but output impedance linearity deteriorates
Solution Approach 1:
The patent applies dynamics by making the threshold voltage of the drive transistor variable rather than fixed. A bias circuit dynamically adjusts the substrate bias voltage based on the input signal state: applying a first bias voltage when the input is low to reduce threshold voltage and maintain drive strength, and a second bias voltage when the input is high to increase threshold voltage and improve linearity. This dynamic adjustment resolves the contradiction between low power consumption and output impedance linearity.
Solution Approach 2:
The patent changes the parameter of threshold voltage from fixed to variable by modifying the substrate bias voltage. The bias circuit changes the bias voltage parameter in response to input signal transitions, thereby changing the effective threshold voltage of the drive transistor. This parameter change enables the circuit to maintain good output impedance linearity even at reduced power supply voltages of two volts and below.
2Use of energy by moving object
If drive transistor size is reduced to enable lower voltage operation, then power consumption is reduced, but drive current capability deteriorates
Solution Approach 1:
The patent uses dynamic threshold voltage adjustment to compensate for the reduced drive current capability of smaller transistors. By lowering the threshold voltage through substrate biasing during the low input state, smaller transistors can achieve sufficient drive current to charge the output node, enabling lower power consumption without sacrificing drive capability.
Solution Approach 2:
The patent changes the threshold voltage parameter of the drive transistor to optimize its performance for low-voltage operation. By adjusting the substrate bias voltage parameter, the transistor's electrical characteristics are modified to maintain adequate drive current even when the transistor size is reduced for lower power consumption.
Data Source
AI summary
In one form a circuit has a bias stage having an input signal terminal for receiving an input signal. The circuit modifies the input signal with a drive stage to provide an output signal in complement form. A drive transistor in the drive stage of the circuit has a bulk that is connected to a terminal of a load and to a control electrode coupled to the input signal terminal. A bias transistor in the bias stage of the circuit has a bulk that is directly connected to the terminal of the load and to the bulk of the drive transistor. The bias transistor has a control electrode coupled to the input signal terminal. The input signal biases the bulks of the drive transistor and the bias transistor and reduces transistor threshold voltage. Linearity of circuit output impedance is improved and RF interference reduced. Lower voltage operation is also provided.