TransconductanceAmplifier

IOut = Current Through Load VOut = [VIn (R1+R2)] / R2 IOut = VOut / (R1+R2) IOut = VIn / R2

This circuit is a voltage-to-current converter. Here's how it permits an input voltage to control the brightness of an LED:

R3 controls VIn. Vary R3 to alter IOut, hence the brightness of the LED.