Floating Current Source Using Self-Biased Transistors
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Solution Overview
Problem
Existing floating current sources are complex and costly due to the use of multiple operational amplifiers and transistors, consuming excessive circuit board area and component count, while true floating current sources require a minimum external voltage, deviating from ideal behavior.
Innovation Solution
A simple two-transistor circuit with a single-transistor current sink and source, where the source pin is self-biased to a high impedance DC float voltage, using AC shunts to prevent AC fluctuations, allowing for a high-impedance bias current to a variable resistance load while floating it at a known DC voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple operational amplifiers and transistors are used to create a true floating current source, then high impedance is achieved at both terminals, but device complexity and cost increase significantly
Solution Approach 1:
The circuit is divided into two independent single-transistor current sources, each responsible for one terminal. This segmentation allows each transistor to be optimized independently while maintaining overall high impedance at both terminals, avoiding the need for complex multi-transistor configurations.
Solution Approach 2:
Each transistor automatically biases itself to the correct operating point through its own circuit configuration, eliminating the need for external biasing circuits or multiple operational amplifiers. The transistors self-regulate to maintain high impedance at their respective terminals.
2Device complexity
If a single-transistor current source is used, then device complexity is reduced, but the ability to maintain high impedance at the source terminal deteriorates due to AC fluctuations
Solution Approach 1:
AC coupling capacitors are introduced as intermediary elements between the transistor sources and the external circuit. These capacitors block AC fluctuations from affecting the transistor operating points while allowing DC bias currents to pass, thereby maintaining high impedance at both terminals despite the simplicity of single-transistor configuration.
3Reliability
If AC coupling is implemented to block AC fluctuations, then high impedance is maintained, but DC biasing becomes more complex
Solution Approach 1:
The DC biasing function and AC blocking function are merged into a single integrated circuit configuration. The resistors and capacitors work together in a unified manner to establish both the DC operating point and the AC isolation, eliminating the need for separate biasing circuits and reducing overall complexity.
Data Source
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Figure 3A~3C
AI summary
As taught herein, a floating current source outputs a load biasing current from a source terminal into an external load which may have a variable resistance, and sinks the load biasing current from the load into a sink terminal. Advantageously, the floating current source includes a single-transistor current sink having a bias control that sets the magnitude of the load biasing current desired, and further includes a single-transistor current source that self-biases from the float voltage developed on the external load to an operating point at which the single-transistor current source sources the desired magnitude of load biasing current. One or more AC shunts within the self-biasing network prevent any AC fluctuations present or impressed on the source terminal of the floating current source from changing the operating point of the single-transistor current source, thereby imparting a high effective impedance to the single-transistor current source.