Op-Amp - PNP-NPN BJT Feedback Pair Current Source
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Op-Amp - Compound PNP-NPN BJT Pair Current Source.
Rrefhlim (reference high limit) forbids very low reference current.
Rrefllim (reference low limit) prevents NPN's Vce from going lower than about 1.5 V.
Compared with just a PNP at the output, the advantage of using the compound pair is that the output current is much closer to the reference current. The drawback is the higher minimum voltage needed across the output transistor to maintain proper operation. This limits the range of operation of the circuit.
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