V/I converter accommodates grounded load

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 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

V₂ = 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 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 IC(max) of Q₁ and Q₂; and
• High output resistance of approximately 50 MΩ.

Materials on the topic