Circuit Function
A current source is a circuit that delivers a steady and constant current to a load, regardless of the load's resistance. This feature makes it ideal for applications where accurate measurement or control is required, such as in sensor circuits. By applying a known constant current, the resistance or voltage of the load can be easily determined. Current sources are also widely used in driving semiconductor sensors due to their stability and precision. In this specific design, the reference voltage is connected directly to the power supply, making it difficult to adjust externally. Therefore, the output current is controlled using a variable resistor, allowing for fine-tuning of the current level.
Circuit Working Principle
To better understand how the circuit operates, let’s take +VCC as the reference voltage. The input voltage to the operational amplifier A1 should not be too close to +VCC, so it is reduced by approximately 5 volts, resulting in a non-inverting input voltage of around 10 volts. For proper operation, the inverting input of A1 must also be maintained at about 10 volts. The voltage across resistor R4 is regulated to 4.7 volts, which results in a constant output current of IO = 4.7 V / 470 Ω = 10 mA. The variable resistor VR1 allows for precise adjustment of this current, ensuring it remains exactly at 10 mA.
The P1 FET is used in place of TR1 to eliminate any base current error in TR2. This is because the gate current of a FET is effectively zero, making it an ideal choice for high-impedance and low-error applications. This configuration improves the overall accuracy and stability of the current source.
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