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Voltage controlled current source
This circuit uses an LM3900 "Norton" op amp to form a voltage-controlled current source. This circuit is also called a transconductance amplifier. The transconductance is -(1/R2) as the voltage gain from the input terminal to the emitter of Q1 is -1. For a Vin=0 vdc the output current is essentially zero mA DC. The resistors R1 and R6 guarantee that the amplifier can turn off transistor Q1. The output current is given as: Io=Vin*(1ma/volt).