This is a circuit of precision current source. This circuit uses two kinds of transistor, the 2N2219 bipolar and the 2N3069 JFET. Both of them are used as voltage devices between the current sensing resistor and the output. As the current sensing resistor this circuit uses R1. Here is the circuit :
This circuit is called current source because this circuit use LM101 that provides a large amount of loop gain. The 10K resistor and 2N2219 can be removed, if we use this circuit for a small current values. The output will appear at 2N3069′s source. The current output is determined by following equation :
Io= Vin/RI with Vin <= 0V
[Source: National Semiconductor Application Note]
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