Draw the schematic of the current source design you plan to use. For this example, the current source design is based on an op amp in a inverting amplifier configuration (negative feedback). The op amp uses the feedback resistor as the load resistor (connected between the output of the op amp and the negative amplifier signal input) and a current source setting resistor connected between the negative amplifier signal input and ground. The positive terminal of the voltage supply, used to control the amount of current that the current source supplies to the load resistor, is connected directly to the positive amplifier input of the a feedback resistor.
Define the maximum current level that the current source will supply. Use a value that is lower than the maximum output current specification listed on the op amp data sheet. For this example, use a value of 5 milliamperes (mA).
Set a value for the the maximum value of the voltage that will be applied to the positive input of the op amp. For this example, use a maximum voltage of 5 volts.
Divide the maximum voltage source value by the maximum current source value to obtain the value for the current setting resistor. For a maximum voltage of 5 volts and a maximum current of 5 mA, the current setting resistor would have a value of 1000 ohms, since 5 volts divided by 5 mA (0.005) is 1000.
Calculate the current-to-voltage gain of the voltage-to-current op amp design, also known as a operational transconductance amplifier (OTA). Divide the maximum voltage by the maximum current to obtain 1 mA per volt. Record that the transconductance (the voltage-to-current gain) of the op amp is 1 mA per volt. Use this number to calculate the specific value of current that the op amp will supply for a given input voltage. For example, if the input voltage is 3 volts, the OTA will supply 3 mA, since 3 volts multiplied by 1 mA per volt is 3 mA.
Determine the upper limit for the value of the load resistor that can be used with the current source. Divide the supply voltage that is used to power the op amp by the maximum current of the current source. For an op amp that uses a 10 volt supply with a maximum current source value of 5 mA, the maximum value of the load resistor would be 2000 ohms, since 10 volts divided by 5 mA (0.005) is 2000 ohms.