Thermal Compensation of Exponential Bipolar Transistor Pairs
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Solution Overview
Problem
Conventional differential amplifiers using bipolar junction transistors experience variations in bias currents due to temperature changes, which is undesirable in certain applications where constant current is required.
Innovation Solution
A resistor network with resistors having varying temperature coefficients is used to provide a differential voltage to the transistors, making the differential voltage proportional to temperature, thus maintaining a constant ratio of bias currents across temperature variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional differential amplifier uses bipolar junction transistors with a common current source, then the circuit provides differential amplification functionality, but the bias currents drift with temperature variations
Solution Approach 1:
The patent changes the resistance values of the voltage divider resistors as a function of temperature. Specifically, the resistors are selected with temperature coefficients such that their resistance changes compensate for the thermal voltage drift, maintaining a constant differential voltage and thus constant bias currents over temperature variations.
Solution Approach 2:
The voltage divider network is designed to produce a differential voltage that pre-compensates for temperature effects. By selecting resistors with appropriate temperature coefficients, the circuit proactively counteracts the thermal drift before it affects the bias currents, rather than attempting to correct it after the fact.
2Reliability
If the differential voltage is made proportional to temperature to compensate for thermal voltage drift, then the bias current ratio remains constant, but the circuit requires a voltage divider with temperature-dependent resistors
Solution Approach 1:
The voltage divider network serves multiple functions: it provides the necessary differential voltage to the transistor bases and simultaneously performs temperature compensation. By making the differential voltage proportional to temperature through the resistor selection, the same circuit structure achieves both biasing and compensation without requiring separate compensation circuits.
3Temperature
If resistors with varying temperature coefficients are used in the voltage divider, then temperature compensation is achieved, but the manufacturing precision requirements increase
Solution Approach 1:
The patent utilizes resistors with specific temperature coefficients as a design parameter. By selecting resistors whose resistance changes with temperature in a controlled manner, the circuit achieves temperature compensation. The resistor values and temperature coefficients are chosen to match the thermal characteristics of the bipolar transistors, creating a compensated system.
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
The resistor network effectively compensates for temperature changes, ensuring that the ratio of bias currents remains constant, as illustrated by the asymptotic behavior of differential voltage matching thermal voltage over a relevant temperature range.
Implementation Method 1
The voltage divider may include a set of resistors having a resistance that changes with temperature
Implementation Method 2
the thermal voltage, VT, where VT is equal to Boltzmann's constant, k, multiplied by temperature (in degrees Kelvin) divided by the charge of an electron, q
Data Source
AI summary
To compensate for changes in temperature, a pair of bipolar transistors is connected to a voltage divider and receives a differential voltage that varies with temperature. The voltage divider includes a set of resistors placed in parallel. The set of resistors has a resistance that changes with temperature. As the resistance changes with temperature, the differential voltage provided by the voltage divider changes in proportion to a change in thermal voltage.