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2023/07/03

Analysis and Application of Four Common Constant Current Source Circuits

The basic constant current source circuit is mainly composed of an input stage and an output stage, where the input stage provides a reference current and the output stage outputs the required constant current. The constant current source circuit is the foundation for providing a stable current to ensure the stable operation of other circuits. The constant current source circuit is required to output a constant current, so the device used as the output stage should have a saturated output current volt ampere characteristic.

This can be achieved by using bipolar junction transistors or MOSFETs operating in a saturated state of output current. In order to ensure the current stability of the output transistor, two conditions must be met:

The input voltage should be stable - the input stage needs to be a constant voltage source;

The output resistance of the output transistor should be as high as possible - the output stage needs to be a constant current source.

Analysis of four types of constant current source circuits:

In the improved differential amplifier, the emitter resistance RE is replaced by a constant current source, which not only sets a suitable static working current for the differential amplifier circuit, but also greatly enhances the common mode negative feedback effect, making the circuit have a stronger ability to suppress common mode signals, and does not need a very high source voltage. Therefore, the constant current source and the differential amplifier circuit are a perfect match!

The constant current source can not only provide a suitable static current for the amplification circuit, but also act as a Active load to replace the resistance with high resistance value, thus increasing the voltage amplification factor of the amplification circuit. This usage is widely used in integrated operational amplifier circuits. This section will introduce common constant current source circuits and their applications as Active load.

Mirror constant current source circuit

The mirror constant current source circuit is simple and widely used. However, when the power supply voltage is constant, if IC1 is required to be large, IR will inevitably increase, and the power consumption of resistor R will increase, which should be avoided in integrated circuits; If IC1 is required to be small, then IR will inevitably be small, and the value of resistance R will be large, which is difficult to achieve in integrated circuits. Therefore, people have thought of using other methods to solve this problem, which can lead to other current source circuits.

Proportional constant current source circuit

It consists of two tubes VT0 and VT1 with identical characteristics, with their emitters connected in series with resistors Re0 and Re1, respectively. The proportional constant current circuit source changes the relationship between IC1 ≈ IR, resulting in a proportional relationship between IC1 and IR, thus overcoming the shortcomings of the mirror constant current source circuit. Like typical static operating point stable circuits, Re0 and Re1 are current negative feedback resistors, so compared to mirror constant current source circuits, the output current IC1 of proportional constant current sources has higher stability.

Micro variable constant current source circuit

If Re0 is very small or even zero, then Re1 can obtain a smaller output current by only using a smaller resistor, and this circuit is called a micro variable constant current source. The static current of the input stage of the integrated operational amplifier is very small, often only tens of microamps, or even smaller. Therefore, the micro variable current source is mainly applied to the Active load of the input stage of the integrated operational amplifier.

Multi-channel constant current source circuit

The integrated operational amplifier is a multi-stage amplification circuit, which requires multiple constant current source circuits to provide appropriate static current for each level. Multiple different output currents can be obtained using a single reference current to adapt to