Constant current source is a widely used component in the circuit. Here I sort out the structure and characteristics of the more common constant current source.
The constant current source is divided into two forms: CurrentSource and CurrentSink.
The simplest constant current source is a constant current diode. In fact, the application of constant current diodes is relatively small, except that the constant current characteristics of the constant current diodes are not very good, the current specifications are relatively small, and the price is relatively expensive.
The most commonly used simple constant current source is shown in Figure (1). Two identical transistors are used, and the relatively stable be voltage of the triode is used as the reference. The current value is: I=Vbe/R1.
The advantages of this constant current source are simple and easy, and the value of the current can be controlled freely without using special components, which helps to reduce the cost of the product. The disadvantage is that different types of tubes, the be voltage is not a fixed value, even if the same model, there are certain individual differences. At the same time, under different working currents, this voltage will also have some fluctuations. Therefore, it is not suitable for precise constant current requirements.
In order to accurately output the current, an op amp is usually used as feedback, and the FET is used to avoid the error caused by the be current of the triode. A typical op amp constant current source is shown in Figure (2). If the current does not need to be particularly accurate, the FET can be replaced by a triode.
The current calculation formula is:
I=Vin/R1
This circuit can be considered as a standard circuit of constant current source. In addition to sufficient accuracy and adjustability, the components used are also very common, easy to set up and debug. However, Vin also requires additional users.
As can be seen from the above two circuits, the constant current source has a fixed form, which uses a voltage reference to form a fixed current on the resistor. With this formula, the construction of the constant current source can be extended to all devices that can provide this "voltage reference".
The simplest voltage reference is the Zener diode, which uses a Zener diode and a triode to create a simpler constant current source. As shown in Figure 3:
The current calculation formula is: I=(Vd-Vbe)/R1
The TL431 is another commonly used voltage reference. The constant current source built by the TL431 is shown in Figure (4). The replacement of the transistor with the FET can achieve better accuracy.
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