V/I converter accommodates grounded load

Analog Devices OP77

Michele Frantisek

EDN

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

EDN

You may have to register before you can post comments and get full access to forum.
User Name