Out-of-spec problem with a long tail

摘要

In the mid-1960s, early in my career as an engineer, I found myself facing a problem that at the time appeared impossible to solve. My employer had been awarded a contract to design, develop, and produce specialized surveillance equipment related to the war effort in Vietnam. Field testing on the initial units had gone well, and the company was in a full production swing when an unusual problem appeared. The returned modules contained a specially designed video amplifier showing inadequate gain. The only clue as to what might be wrong was a slightly out-of-spec dc bias level at the joined emitters of a long-tail pair of differential amplifiers housed in the amplifier module.