Constant current source circuit diagram of operational amplifier and Darlington transistor

The output current of the circuit is larger due to the use of Darlington transistor BSY86. The maximum value of the output current is limited by the resistor R=150 ohms. The size of the output current is adjusted by the potentiometer RP1 and remains constant regardless of the load resistance Rl. The potentiometer RP1 in the picture uses 10k ohms, and the current can be adjusted in the range of 5uA ~ 40mA.

The part surrounded by the input terminals A, B, and C of the operational amplifier can be replaced by the potentiometer RP1. At this time, the emitter potential of the BSY86 can be adjusted to -0.6V relative to ground through RP2, and the sliding contact of the potentiometer is relative to the potential of the positive end of the voltage regulator tube. is -12V, so the resistor R and potentiometer RP1 can be cancelled.