Current Source Circuit Stabilizing Output Against Process and Temperature Variations
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Solution Overview
Problem
In semiconductor integration circuits, achieving stable current against process variations, power supply voltage variations, and temperature variations in a simple circuit configuration is challenging due to the instability of resistance values and transistor operation points.
Innovation Solution
A current source circuit is designed with a reference current source, a reference voltage source, and transistors of specific conductive types to generate currents proportional to thermal voltage, canceling out variations by employing a circuit configuration that changes the basic current determination from resistance to transistor-based, thereby stabilizing the output current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a resistance R1 is used to generate reference current, then the circuit configuration is simple, but the current stability deteriorates due to process variation and temperature variation
Solution Approach 1:
The patent changes the fundamental parameter used for current generation from resistance value to transistor threshold voltage and thermal voltage. By using the relationship I ∝ (kT/q) * (W/L) * (1/Vth^2), the circuit achieves current stability that is independent of resistance variations. The current is now determined by transistor parameters (W/L ratio, threshold voltage) and thermal voltage, which have better process consistency and temperature compensation characteristics.
Solution Approach 2:
The patent employs multiple transistors with identical or proportional W/L ratios to create current mirrors. By copying the gate voltage and current characteristics across matched transistors, the circuit achieves stable current generation without requiring precise resistance matching. The current mirror technique allows the reference current to be replicated accurately across different parts of the circuit.
2Reliability
If the resistance R1 value is varied to compensate for temperature, then the temperature dependency of current is reduced, but the semiconductor manufacturing process is restricted
Solution Approach 1:
Instead of varying resistance values to compensate for temperature, the patent changes the temperature compensation mechanism by using the inherent thermal voltage relationship (Vth = kT/q). The circuit naturally compensates for temperature variations through the proportional relationship between thermal voltage and absolute temperature, eliminating the need for temperature-dependent resistance selection and simplifying the manufacturing process.
Solution Approach 2:
The circuit achieves automatic temperature compensation through the intrinsic relationship between thermal voltage and temperature. As temperature changes, the thermal voltage (kT/q) changes proportionally, and the transistor parameters adjust accordingly to maintain stable current. This self-compensating mechanism eliminates the need for external temperature control or specialized manufacturing processes.
3Reliability
If an inverting circuit and variable resistance are used for feedback, then the output current variation is suppressed, but the operation points become difficult to set and unstable
Solution Approach 1:
The patent implements a feedback mechanism where the gate voltage of the transistor is controlled by the drain current through a feedback capacitor. This creates a stable operating point where the transistor automatically adjusts its gate-source voltage to maintain the desired current, eliminating the need for complex external feedback circuits with variable resistances. The feedback is inherent in the transistor's self-biasing characteristics.
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 solution provides a stable current that is not influenced by process variations, power supply voltage variations, or temperature variations, with reduced temperature dependency and easy circuit constant setting, eliminating the need for feedback mechanisms.
Implementation Method 1
a reference voltage source circuit configured to generate a voltage proportional to a thermal voltage based on the reference current
Data Source
AI summary
A current source circuit includes a reference current source circuit; a reference voltage source circuit generating a voltage proportional to a thermal voltage based on the reference current; a first transistor connected between the reference voltage source circuit and the second power supply voltage and through which a first current flows; a second transistor which has a gate applied with a voltage as a result of addition of the voltage generated by the reference voltage source circuit and a voltage between a source and a drain of the first transistor and through which a second current flows; a current source supplying a third current of a current value proportional to that of the first current; and a third transistor through which a difference current between the second current and the third current flows. An output current is supplied based on the difference current.