Op-Amp - Current Mirror Voltage To Current Converter

This circuit uses an operational amplifier and a current mirror to implement a voltage to current converter (voltage-controlled current source). The diode-connected transistor Q1 is in the feedback loop of the operational amplifier. The input voltage appears at the junction of the reference transistor's collector and the current setting resistor Rset. The current (Iset) through Rset is equal to Vin / Rset. Ideally the collector current of Q1 should be equal to Iset but because the base currents of the transistors flow to Rset, the collector current of Q1 is slightly less than Iset. The collector current of Q1 is mirrored by Q2 and this mirrored current is taken as the output quantity. An LED serves as a load for the output current. The operational amplifier drives the emitters of the current mirror, it should be capable of supplying the required current. The operational amplifier used here has an output short circuit current (Isc) of 25 mA, for proper operation this rating should not be exceeded by the sum of Iset and the output/LED current.