V/I converter accommodates grounded load

Analog Devices OP77

Michele Frantisek


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:

A versatile voltage-to-current converter provides a handy current source in many analog applications.
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.

Materials on the topic

  1. Datasheet Analog Devices OP77
  2. Datasheet ON Semiconductor 2N3019
  3. Datasheet STMicroelectronics 2N4033


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