One challenge in circuit design is building a good current source, especially when the load is variable or the current must be controlled with a voltage source. The figure shows a simple, low-cost voltage-controlled current source using two op amps that provides a good range of current and maximum load.

The idea is to apply a voltage on a reference resistor (or resistors) having a low thermal coefficient. The current passing through this resistor will be the output current. U1 adds the input voltage with V2, and U2 buffers the load voltage, so we have:

V1 = V_IN + V2

Obviously, U1's output current is amplified by Q1.

Applied voltage on the resistor network R is (V1 - V2), which will be equal to:

V_R = V1 - V2
= (V_IN + V2) - V2
= V_IN

So the output current will be:

I_OUT = V_R/R = V_IN/R

If R is a constant value (low thermal coefficient), the output current will be a linear function of the input voltage. Four resistors in series reduce the effect of thermal dependency.

We also have to consider some other constraints. Limited supply voltages cause a limited maximum output current. And if the load is large, transistor Q1 must be able to handle the maximum current.

If U2 is a rail-to-rail amplifier, then the zener diode isn't required. Otherwise, it's needed to prohibit the current source from malfunctioning with low current outputs. (When V2 is very close to -V_CC, the buffer's output may not exactly follow input-voltage changes.)

Here is the calculation: Suppose we want to calculate the maximum output current with ±V_CC, and R as the resistor value, when the input voltage has a maximum value of V_IN(MAX). Then:

I_MAX = [2 x V_CC - (2 x V_z) - V_BE - V_IN(MAX)]/R

For the circuit shown, a maximum current of 20 mA is feasible for a maximum load of 1100 Ω. Using higher-voltage op amps and larger power transistors can increase these values if cost isn't a concern.

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