V/I converter accommodates grounded load
Michele Frantisek, Brno, Czech Republic -- EDN, 6/10/1999
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 formula for calculating the output current is thus:
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 Mµ. (DI #2365).
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