Negative Feedback
Douglas SelfIt is diffi cult to convince people that this frequency is of no relevance whatsoever to the
speed of amplifi ers and that it does not affect the slew rate. Nonetheless, it is so, and
any fi rst-year electronics textbook will confi rm this. High-gain op-amps with sub-1-Hz
bandwidths and blindingly fast slewing are as common as the grass (if somewhat less
expensive) and if that does not demonstrate the point beyond doubt then I really do not
know what will.
Limited open-loop bandwidth prevents the feedback signal from immediately following
the system input, so the utility of this delayed feedback is limited. No linear circuit can
introduce a pure time delay; the output must begin to respond at once, even if it takes a
long time to complete its response. In the typical amplifi er the dominant-pole capacitor
introduces a 90° phase shift between input pair and output at all but the lowest audio
frequencies, but this is not a true time delay. The phrase delayed feedback is often used
to describe this situation, and it is a wretchedly inaccurate term; if you really delay the
feedback to a power amplifi er (which can only be done by adding a time constant to the
feedback network rather than the forward path) it will quickly turn into the proverbial
power oscillator as sure as night follows day.
12.1 Amplifi er Stability and Negative Feedback ..........................................................
In controlling amplifi er distortion, there are two main weapons. The fi rst is to make the
linearity of the circuitry as good as possible before closing the feedback loop. This is
unquestionably important, but it could be argued that it can only be taken so far before the
complexity of the various amplifi er stages involved becomes awkward. The second is to
apply as much negative feedback (NFB) as possible while maintaining amplifi er stability.
It is well known that an amplifi er with a single time constant is always stable, no matter
CHAPTER 12