The basics of testing op amps, part 4:Testing op amps requires stable test loops

In previous installments, we covered some of the basic tests and identified the sources of errors that occur when designing and testing operational amplifiers (op amps). We advise that you read these articles first before making assumptions based on the knowledge and use of the test circuits that we will present in this final installment.

In this article we cover compensation issues when using the suggested test circuits. If the loop in a test circuit isn't stable, then it's not useful. Always monitor the output of the DUT test loop during test development. If the loop is oscillating and you don't know it, you may be reporting bad results. Worse yet, you may not know it until much later when correcting the problem becomes more difficult.

Self-test compensation
When distilled to its simplest form, the self-test circuit in Figure 1 is essentially a closed loop system with a gain of 1201. If R1 is reduced to 5 kW, then the closed-loop gain is 301. Thus, it's inherently stable, even with decompensated op amps that aren't unity-gain stable. When we modify the loop for IB testing, however, the circjuits can become unstable. Exercise care when configuring the DUT for IB testing. You can achieve stability by adding a compensation capacitor (CCOMP) around resistor RF in Figure 1.


Figure 1. A self-test loop circuit tests for the DUT amplifier's gain over frequency.

To jump directly to the rest of this article and find links to parts 1-3 and a spreadsheet for calculating C_COMP, head to Test&Measurement World.