Comparative circuit diagram of various VI conversion and constant current sources

As shown in the figure, several circuits can obtain a constant current output on the load resistor RL. The first type is rarely used because the RL is floating. The second type of RL is virtual land, which is not often used. In the third type, although RL is floating, one end of RL is connected to the positive power supply terminal, which is more commonly used. The fourth type is the positive feedback balanced type, which is popular because the load RL is grounded. The fifth principle is the same as the fourth one, except that the current output capability is expanded. In use, people often make the resistor R2 much larger than the load RL, and omit the follower op amp. The fifth type is the circuit I want, which is also a ground load, and the latter two are constant current source circuits.

Comparing several V/I circuits, any unidirectional device without a triode can achieve AC constant current. After adding a triode, it can only achieve unidirectional DC constant current. The fourth and fifth are based on the balance of positive and negative feedback. If the balance is lost due to resistance errors, it will affect the constant current output characteristics, that is, the output current will change with the load. The errors of other resistors will only affect the value of the output current, but not the output characteristics. If the output current is large, or if the collector current and emitter current of the triode are not equal, the triode can be replaced with a MOSFET.

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