The 2N5457 JFET and PN2222
bipolar have inherently high output impedance. Using R1 as a current sensing resistor to
provide feedback to the LM101 op amp provides a large amount of loop gain for negative
feedback to enhance the true current sink nature of this circuit. For small current
values, the 10 K resistor and PN2222 may be eliminated if the source of the JFET is
connected to R1. |