Small-Signal Distortion in Feedback Ampliers for Audio1

We examine how intermodulation distortion of small two-tone signals is aected by adding degenerative feedback to three types of elementary am-pli er circuits (single-ended, push-pull pair, and dierential pair), each im-plemented with three types of active device (FET, BJT and vacuum triode). Although high precision numerical methods are employed in our analysis, the active devices are modeled with rather simple models. We have not investigated the consequences of more elaborate models. Though negative feedback usually improves the distortion characteristics of an amplier, we nd that in some cases it makes the distortion \messier." For instance, a common-source FET amplier without feedback has a distor-tion spectrum displaying exactly four spurious spectral lines; adding feedback introduces tier upon tier of high-order intermodulation products spanning the full bandwidth of the amplier (as suggested by Crowhurst in 1957). In a class-B complementary-pair FET amplier, feedback mysteriously boost