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
V2 = VIN + VOUT.
The formula for calculating the output current is thus:
|Figure 1.||A versatile voltage-to-current converter provides a handy current source in many analog applications.|
The formula shows that the value of IOUT depends only on VIN and R6. Voltage follower IC3 reduces to a negligible level the current from the circuit output to IC1. The advantages of the circuit are:
- Load-grounding possibility;
- Simple control of IOUT/VIN ratio;
- High precision, linearity, stability, and bandwidth;
- Wide IOUT range, approximately 1 µA to IC(max) of Q1 and Q2; and
- High output resistance of approximately 50 MΩ.