V/I converter accommodates grounded load

Michele Frantisek, Brno, Czech Republic -- EDN, 1999-06-10 00:00:00

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 V_IN and V_OUT to V₁=-(V_IN+V_OUT). The second op amp (IC₂) and transistors Q₁ and Q₂ invert this voltage to produce V_IN+V_OUT. The formula for calculating the output current is thus:

The formula shows that the value of I_OUT depends only on V_IN 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 I_OUT/V_IN ratio;

• high precision, linearity, stability, and bandwidth;

• wide I_OUT range, approximately 1 µA to I_C(max) of Q₁ and Q₂; and

• high output resistance of approximately 50 Mµ. (DI #2365).