Constant Current Circuit With Matched Transistors for Low Thermal Drift
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Solution Overview
Problem
Conventional constant current circuits suffer from high drop-out voltage, thermal drift, and poor initial accuracy due to limitations in Zener diodes and transistor junction voltage variations, which are unsuitable for precision applications and increase power consumption.
Innovation Solution
A circuit design using a matched-pair dual transistor configuration with a Schottky diode and a shunt-type voltage reference, along with a grounding bias resistor and optional capacitor, to stabilize the current and reduce noise, achieving precise and stable current output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a Zener diode-based circuit is used for constant current generation, then the circuit can be implemented with conventional components, but the drop-out voltage becomes high and power consumption increases
Solution Approach 1:
The patent changes the voltage reference mechanism from Zener diode breakdown voltage to transistor base-emitter junction voltage, which operates at lower voltage levels. This parameter change reduces the drop-out voltage and consequently lowers power consumption while maintaining constant current functionality
Solution Approach 2:
The patent replaces the Zener diode voltage reference mechanism with a transistor-based voltage reference using base-emitter junctions. This substitution enables operation at lower voltages and reduces the mechanical/stress constraints associated with Zener diode breakdown characteristics
2Use of energy by stationary object
If precision Zener diodes are used to reduce drop-out voltage, then the voltage can be lowered, but the thermal drift increases and initial accuracy deteriorates
Solution Approach 1:
The patent changes the reference voltage mechanism from Zener breakdown voltage to transistor base-emitter voltage, which has superior thermal stability characteristics. This parameter change simultaneously achieves lower drop-out voltage and improved initial accuracy
Solution Approach 2:
The patent uses matched pairs of transistors where one transistor's base-emitter junction serves as a reference for the other. This copying approach with matched devices ensures that thermal drift affects both transistors equally, maintaining accuracy while operating at lower voltages
3Reliability
If the nominal output current is increased to guarantee minimum current over temperature range, then the current stability improves, but the dissipated power increases
Solution Approach 1:
The patent changes the voltage reference mechanism to operate at lower voltages, which reduces the power dissipation for a given current. This enables maintaining reliable current output over temperature ranges without proportionally increasing power consumption
Solution Approach 2:
The patent employs feedback mechanisms through the matched transistor configuration where the reference transistor's base-emitter voltage compensates for thermal drift in the output transistor. This feedback maintains current stability over temperature without requiring excessive current margins
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 design significantly reduces thermal drift and improves accuracy, lowering drop-out voltage and power consumption, ensuring stable current output over a wide temperature range.
Implementation Method 1
the base of the first transistor is biased by the shunt-type voltage reference in series with the base-emitter junction of the second transistor
Implementation Method 2
The drop-out voltage of the traditional circuit is relatively high because of the voltage of the available precision Zener diodes. Lower voltage Zener diodes are available but with high tolerances
Implementation Method 3
the arrangement includes an individual diode which will compensate the drifts to some degree but will have much higher accuracy uncertainties and higher thermal drifts due to the unequal thermal behavior and thermal condition of the diode with respect to the base-emitter junction of the transistor
Data Source
AI summary
An electrical constant current circuit (CCC) in particular for supplying electrical power to a sensor, the circuit (CCC) including a power supply input terminal (INT), a constant current output terminal (OUT), a first transistor (TRF), and a shunt-type voltage reference (RVS), wherein the base of the first transistor (TRF) is biased by the shunt-type voltage reference (SVR) in series with the base-emitter junction of the second transistor (TRS), wherein the emitter of the first transistor (TRF) is connected to the power supply input terminal (INT) via a current set resistor (RST), wherein the collector of the first transistor (TRF) is connected to the output terminal (OUT) through a Schottky diode (SKD). For the improved temperature stability, initial accuracy, drop out voltage and power consumption embodiment provides that the circuit (CCC) includes a second transistor (TRS) in the line between the base of the first transistor (TRF) and the shunt-type voltage reference (SVR), wherein the second transistor's (TRS) base and collector are connected to the first transistor's (TRF) base.