Temperature-Coefficient Bias Circuit for Stable Amplifier Gain
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Solution Overview
Problem
Electrical amplifiers face challenges in maintaining constant gain over a broad temperature range due to variations in transconductance of field effect transistors, especially when operating in uncontrolled environments or with components that generate heat, making it difficult to achieve stable gain without using 'like-kind' devices which can be inaccurate or inefficient.
Innovation Solution
A controllable temperature coefficient bias circuit that provides independent control over current levels and temperature coefficients, using a combination of current digital-to-analog converters and proportional-to-absolute-temperature circuits to maintain constant gain by adjusting the relative contributions of currents with different temperature coefficients, allowing for precise temperature compensation without relying on 'like-kind' devices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a proportional-to-absolute-temperature circuit is used to maintain constant gain over temperature, then the gain stability is improved, but the device complexity increases due to multiple current mirrors and interface circuitry
Solution Approach 1:
The patent extracts the temperature compensation function from a complex multi-circuit implementation into a single dedicated temperature compensation circuit. This circuit uses a temperature-sensitive element (such as a bandgap reference or PTAT circuit) that directly generates a compensation signal proportional to temperature, which is then added to the bias current. This eliminates the need for multiple current mirrors and interface circuitry while maintaining gain stability over temperature variations.
2Reliability
If like-kind devices are used for temperature compensation, then the transconductance matching is improved, but the manufacturing precision requirements increase
Solution Approach 1:
The patent introduces a temperature compensation circuit as an intermediary between the temperature variations and the amplifier bias. This circuit uses a temperature-sensitive element (such as a bandgap reference voltage or PTAT current source) that senses temperature changes and generates a compensating signal. This intermediary approach allows temperature compensation without requiring direct matching of like-kind devices, thereby reducing manufacturing precision requirements while maintaining transconductance stability.
3Reliability
If the bias current is adjusted to compensate for temperature variations, then the gain constancy is improved, but the settling time increases in rapidly switched applications
Solution Approach 1:
The patent implements preliminary temperature compensation by using a temperature-sensitive element that continuously tracks temperature variations and pre-adjusts the bias current before the amplifier operates. The temperature compensation circuit is designed to have a fast response time, with compensation signals prepared in advance based on predicted temperature trends. This preliminary action allows the amplifier to maintain constant gain without requiring slow adjustments during operation, thereby reducing settling time in rapidly switched applications.
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
Enables the amplifier to maintain constant gain over varying temperatures by adjusting the temperature coefficient of the bias current, ensuring accurate and efficient operation across a broad temperature range without the need for matching 'like-kind' devices, and allows for quick settling of bias values in rapidly switched applications.
Implementation Method 1
The circuit 100 is a sometimes referred to as a proportional-to-absolute-temperature (PTAT) circuit. In the case of the circuit 100, a current I1 flows through the transistor 106 and a current I2 flows through the FET 110. The currents I1 and I2 change over temperature in order to maintain a constant gm for the FET 108.
Data Source
AI summary
A controllable temperature coefficient bias (CTCB) circuit is disclosed. The CTCB circuit can provide a bias to an amplifier. The CTCB circuit includes a variable with temperature (VWT) circuit having a reference circuit and a control circuit. The control circuit has a control output, a first current control element and a second current control element. Each current control element has a “controllable” resistance. One of the two current control elements may have a relatively high temperature coefficient and another a relatively low temperature coefficient. A controllable resistance of one of the current control elements increases when the controllable resistance of the other current control element decreases. However, the “total resistance” of the current control circuit remains constant with a constant temperature. The VWT circuit has an output with a temperature coefficient that is determined by the relative amount of current that flows through each current control element of the control circuit. A Current Digital to Analog Converter (IDAC) scales the output of the VWT and provides the scaled output to an amplifier bias input.