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 CCOMP, head to Test&Measurement World.
A common convention in Europe is to always use range multipliers, including instead of decimal points, which can easily be lost especially for photocopied documents. So, 50 (ohms) becomes 50R or 50R0, and 21.6k becomes 21k6. Unambiguous.
I suspect that somewhere in the editing and layout somebody assumed that the "K" had been forgotten on R2. I had a similar problem with drafting detailers at one job, they assumed that the "dumb engineer" had forgotten to add the "K" notation on many of the resistors. The result was that when the technician built up the pre-production units they just did not work right. Correcting the values, the drawings, and the detailers fixed the problem.