Constant Ratio Current Source Using Linear Negative Resistance
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Solution Overview
Problem
Existing current sources face challenges in maintaining a constant current ratio between two currents, especially when differential voltage varies, often requiring additional headroom or non-linear negative resistance circuits that degrade accuracy.
Innovation Solution
A constant ratio current source is achieved by incorporating a first and second current branch with transistors and resistors, coupled with a linear negative resistance circuit that increases output impedance, maintaining a nearly constant current ratio without additional headroom, using cross-coupled transistors and bias currents to correct for differential voltage variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If degeneration resistors are added to reduce current ratio variation with differential voltage, then the current ratio stability is improved, but the headroom requirement increases
Solution Approach 1:
The patent introduces an intermediate negative resistance element that mediates between the differential voltage variation and the current ratio. This negative resistance counteracts the effect of differential voltage changes on the current ratio without requiring additional headroom, effectively decoupling the stability improvement from the headroom penalty.
Solution Approach 2:
The patent changes the electrical parameter of the circuit by introducing a negative resistance element. This parameter change allows the circuit to compensate for differential voltage variations and maintain stable current ratio without the need for physical degeneration resistors that would consume headroom.
2Stability of the object's composition
If cascode transistors are added to reduce current ratio variation, then the current ratio stability is improved, but the headroom requirement and circuit complexity increase
Solution Approach 1:
The patent uses a negative resistance element as an intermediary to achieve current ratio stability without requiring cascode transistors. This single element provides the necessary compensation while keeping the circuit topology simple and avoiding the complexity of additional transistor stages.
3Device complexity
If a basic current mirror is used, then the circuit simplicity is maintained, but the output impedance is low causing significant current ratio variation with differential voltage
Solution Approach 1:
The patent adds a negative resistance element as an intermediary to the basic current mirror. This element increases the effective output impedance without changing the fundamental current mirror structure, thereby maintaining simplicity while improving current ratio stability.
Solution Approach 2:
The patent changes the effective output impedance parameter of the current mirror by introducing negative resistance. This parameter change allows the basic current mirror structure to maintain its simplicity while achieving the desired current ratio stability.
4Stability of the object's composition
If emitter degeneration resistors are added to the current mirror, then the current ratio stability is improved, but the headroom requirement increases
Solution Approach 1:
The patent introduces a negative resistance element as an intermediary that provides the stability improvement without the physical presence of degeneration resistors. This negative resistance acts virtually to increase output impedance without consuming the headroom that physical resistors would require.
5Stability of the object's composition
If a negative resistance circuit is added to increase output impedance, then the current ratio stability is improved, but non-linearities are introduced degrading accuracy
Solution Approach 1:
The patent uses a carefully designed negative resistance element that acts as an intermediary to increase output impedance while maintaining linearity. The element is configured to provide the necessary compensation for differential voltage variations without introducing significant non-linearities, thus preserving current ratio accuracy.
Data Source
AI summary
A constant ratio current source comprises a first current branch which includes a first transistor that conducts a current I1 from a first current input to a first node and a resistor R1 connected between the first node and a circuit common point, and a second current branch which includes a second transistor that conducts a current I2 from a second current input to a second node and a resistor R2 connected between the second node and the circuit common point. The current branches are arranged such that I2 varies with I1. A linear negative resistance circuit connected between the first and second nodes provides an apparent negative resistance that increases the differential output impedance at the first and second current inputs, such that the ratio of I2:I1 is maintained approximately constant for a varying differential voltage applied across the first and second current inputs.