Answer 3—The original Howland circuit requires exact matching of the two pairs of resistors in order to achieve the high output impedance of a true current source. However, the resistor values (especially R3) need to be fairly low in order to get practical output current values. These two requirements are difficult to meet simultaneously.

The modified Howland source decouples these issues by adding the fifth resistor. Typically, R1–R4 are in the 10- to 100-kW range. This keeps the control currents manageable, and R5 is a few tens or hundreds of ohms (validating the assumption that nearly all of the load current flows through it).

Also, in the modified circuit, it is a simple matter to put a small capacitor across R4 to provide a dominant low-pass pole for stability. Similarly, a small capacitor across R2 makes sure that the positive feedback is less than the negative feedback during startup, at which time the load current is zero. This is particularly important for nonlinear loads.

Finally, the common-mode swing of the op-amp inputs is equal to the load voltage in the original circuit. This makes the performance of the circuit depend directly on the CMRR of the op-amp. In the modified circuit, this common-mode swing can be reduced, typically to half or less of the load voltage, easing the op-amp’s equirements.

Contributor: David Tweed