An instrument for measuring the characteristics of a circuit element has a plurality of SMUs for applying test signals to the terminals of the circuit element and measuring the resulting output signals. A control circuit compares the output signal value to a reference value while varying the test signals. The test signal is varied by an integrator which operates in two modes, a search mode with no feedback to quickly bring the output signal to the approximate value of the reference, and a feedback mode to hold the output signal at the reference value while a measurement is made. The parameters for setting stable operating conditions for the feedback loop with the circuit element being tested are stored in a memory of the control circuit.