Answer 1—This circuit is known as a “Howland current source.” The load current is derived as follows. First, write down the equations for the voltages at the inputs of the op-amp, using superposition to account for the various voltage sources and the load current in the circuit:

 Set them equal to each other and isolate the term containing I_LOAD:

 Note that if R3/R2 = R4/R1, the following expressions are also equal:

 This makes it easy to solve for I_LOAD:

In other words, the circuit is a voltage-controlled current source. The load current is essentially independent of load voltage.

Contributor: David Tweed