*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 (IC_{1}) inverts the sum of voltages V_{IN} and V_{OUT} to

V_{1} = –(V_{IN}+V_{OUT}).

The second op amp (IC_{2}) and transistors Q_{1} and Q_{2} invert this voltage to produce

V_{2} = V_{IN} + V_{OUT}.

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 I_{OUT} depends only on V_{IN} and R_{6}. Voltage follower IC_{3} reduces to a negligible level the current from the circuit output to IC_{1}. 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 IC(max) of Q_{1}and Q_{2}; and - High output resistance of approximately 50 MΩ.