BJT Current Mirror Base Compensation via MOSFET Switches
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Solution Overview
Problem
Current mirror circuits using bipolar junction transistors (BJTs) face challenges in accurately controlling the magnitude of output current due to charge injection errors when multibit digital control signals change and output current legs are deactuated, leading to inaccuracies in current generation.
Innovation Solution
Incorporating metal oxide semiconductor field effect transistors (MOSFETs) adjacent to BJTs to control base current compensation, ensuring equal base voltages for BJTs and minimizing charge errors through capacitors and MOSFET devices, even in the absence of a transistor common base connection line.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If multibit digital control signals are used to deactuate output current legs, then current mirror versatility is improved, but charge injection errors occur leading to reduced measurement precision
Solution Approach 1:
MOSFET devices are introduced as intermediary components between the digital control signals and the BJT base terminals. These MOSFETs act as voltage-controlled switches that compensate for charge injection effects by controlling the base voltage of BJTs, thereby maintaining current precision while enabling versatile digital control of the current mirror circuit.
2Measurement precision
If MOSFETs are added for base current compensation, then output current precision is improved, but device complexity increases
Solution Approach 1:
The MOSFET compensation circuit is merged with the existing current mirror structure by sharing common elements such as the reference voltage supply node and base terminal connections. This integration approach adds the necessary compensation functionality while minimizing the increase in overall circuit complexity through shared components and streamlined architecture.
Data Source
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AI summary
A current mirror circuit includes an input current leg (112) and an output current leg (114). The input current leg includes: a first bipolar junction transistor (BJT) (120) having a collector terminal configured to receive an input current sourced at a current node (104) and a first metal oxide semiconductor field effect transistor (MOSFET) (102) having a gate terminal coupled to the current node and a source terminal coupled to a base terminal of the first BJT (120). The output current leg (114) includes: a second BJT (122) having a collector terminal configured to supply an output current and a second MOSFET (106) having a gate terminal coupled to the current node and a source terminal coupled to a base terminal of the second BJT.