6.1 Voltage-Controlled Current Source

A voltage-controlled current source is a type of circuit where the current passing through a load is set by an input voltage. This type of circuit is especially useful for non-linear loads, such as LEDs (which have an exponential current-voltage relationship). An example circuit is shown below:

In this circuit, the LED from the output to the inverting input acts as the constant-current load. The resistor from the inverting input to ground sets the current response of the circuit, according to this formula.

I_{L O A D} = \frac{V_{I N +}}{R}

The ideal output response comes with a few caveats. This assumes the voltage at the inverting input is equal to the voltage at the non-inverting input, V_IN+, which is only true if the maximum output of the operational amplifier minus the voltage drop across the load is equal to V_IN+. If this condition is not met, the output will be current limited to:

\frac{V_{O H} - V_{L O A D}}{R} = I_{M A X}

Where V_OH is the max output voltage of the operational amplifier and V_LOAD is the drop across the load at I_MAX.

This example is shown in Microchip Application Note AN3632, "Constant-Current Driver Using the Analog Signal Conditioning (OPAMP) Peripheral" (DS00003632A).