D. T. Nguyen, T. Dudar, Texas Intruments, Inc., Dallas, TX

Summary: A typical failure of a differential amplifier can be measured by Output Offset (OO) voltage. However, when the offset voltage is well within spec, the failure can be difficult to detect. Moreover, in a case where emitters are connecting in parallel to make up a large BJT for a differential amplifier, a leakage in one of the hundred of parallel emitters would degrade the performance of the whole differential pairs. In one case, single emitter failure caused OO, but in another, the only way to detect the failure was to monitor the final stage of the output. The paper will outline the test techniques to detect the differential OpAmp failure, circuit analysis (design and simulation), fault isolation, and root cause analysis with data from the fabrication to support plasma charging on emitter-base junction. This differential OpAmp is a typical one with feed back loop, current/voltage bias for the differential pairs, and outputs to the second stage, etc. In a case of a large BJT, which was laid out by the used of 240 emitters connecting in parallel. When one of the 240 parallel emitters failed, it degraded the performance of the differential OpAmp.