Transient intermodulation distortion--Part I: Hard nonlinearity

Otala´s suggested rules for eliminating transient intermodulation distortion (TIMD) in a low-pass feedback amplifier have been widely and wrongly interpreted as being necessary. In fact, the necessary and sufficient condition, for avoiding gross TIMD with a broad-band input signal, is that the forward-path stages before the dominant pole should not clip on a signal input which is twice the amplitude of full rated input to the complete amplifier including feedback. When the input signal is band limited, this dipping criterion can be relaxed by a factor approximately equal to the ratio of the (signal 3 dB bandwidth) to the (amplifier closed-loop 3 dB bandwidth). In both cases the condition is essentially independent of the low-frequency loop gain. There is no TIMD penalty whatsoever in using a large amount of feedback, provided the forward-path gain is concentrated in stages that do not precede the dominant pole. All the usual benefits of feedback accrue with increasing loop gain.