Current source has high compliance

Frank Vitaljic, Bellingham, WA -- EDN, 9/1/2000

Current sources are useful in many areas of electronics, such as voltage and current division, current transmitters, excitation of thermistors, RTDs, bridges, potentiometers, and circuit biasing. Current sources are either fixed- or adjustable-current types. For the dual-op-amp current source in Figure 1, the load current, I_L, is adjustable by varying V_IN (0 to ±1.2V), according to the following equation:

 

 

 

The maximum variation in load current is ±6 mA for a 200W current-sensing resistor, R_SENSE. This variation yields a maximum load voltage of ±12V (3V less than the supply voltages). The maximum current-offset error is 0.01 mA for V_OS1 and V_OS2 of 1 mV. This amount of offset is normal for lower grade op amps. Note that you connect one end of the load to virtual ground (VG). Amplifier IC₁ acts as a voltage-error driver, and the transconductance amplifier, IC₂, senses the load current and converts it to V_A. The control equations are:

 (1)

 (2)

where G equals open-loop gain; and

 (3)

where G₀=dc gain, W=unity-gain bandwidth, and s=j complex frequency.

Solving equations 1 and 2 for I_L and substituting G in Equation 3 for G_AG_B, the load current becomes:

 

 

 

which demonstrates the load current's insensitivity to the load, Z_L. (DI #2579)