Voltage to Current

The VFC (voltage-to-frequency-converter) circuit in Figure 1 achieves a wider dynamic range and a higher full-scale output frequency—100 MHz with 10% overrange to 110 MHz—by a factor of 10 over any commercially available converter. The circuit's 160-dB dynamic range spans eight decades for a 0 to 5V input range and allows continuous operation down to 1 Hz.

Figure 1 shows a classic voltage-to-current (V/I) converter. You can select the resistor values such that the output current in the load, RL, varies only with the input voltage, VIN, and is independent of RL. The circuit is widely used in industrial instruments for supplying a 4- to 20-mA signal. The circuit has its limitations, however, because the resistor values must be quite accurate to obtain a true current source.

The voltage-to-current (V/I) converter in Figure 1 uses three common op amps, two medium-power transistors, and only a few passive components. The first op amp (IC1) inverts the sum of voltages VIN and VOUT to V1=-(VIN+VOUT). The second op amp (IC2) and transistors Q1 and Q2 invert this voltage to produce VIN+VOUT.

The voltage-to-current converter in Figure 1 can both source and sink current. The circuit is more flexible than some traditional current references that require different topologies for current sourcing and sinking. Also, you can easily adjust the value of the current reference by simply adjusting the circuit's input voltage.

You sometimes need to drive a white LED from one 1.5V battery. Unfortunately, the forward voltage of a white LED is 3 to 4V. So, you would need a dc/dc converter to drive the LED from one battery. Using the simple circuit in Figure 1, you can drive one white LED or two series-connected green LEDs, using only a few components. The circuit is a voltage-to-current converter, which converts the battery voltage to a current that passes through the LED.

The circuit in Figure 1 performs active voltage-to-current conversion or acts as a variable-gain current mirror with high precision and bandwidth. A typical application is testing high-speed ICs or other devices that have inputs designed to be driven from current-steering DACs to enable a modulated voltage source to control the devices. The circuit thus simplifies the testing of such devices in isolation, because modulated voltage sources are readily available, but modulated current sources generally are not.