Audio Engineering

334 Chapter 10

open-loop bandwidth and slew rate have nothing to do with each other and may be
altered independently. Open-loop bandwidth is determined by compensation Cdom , VAS ,
and resistance at the VAS collector, whereas slew rate is set by the input stage standing
current andCdom • Cdom affects both, but all the other parameters are independent.

In an amplifi er, there is a maximum amount of NFB you can safely apply at 20 kHz;
this does not mean that you are restricted to applying the same amount at 1 kHz, or
indeed 10 Hz.The obvious thing to do is to allow the NFB to continue increasing at
6 dB/octave—or faster if possible—as frequency falls so that the amount of NFB applied
doubles with each octave as we move down in frequency, and we derive as much benefi t
as we can. This obviously cannot continue indefi nitely, for eventually open-loop gain runs
out, being limited by transistor beta and other factors. Hence the NFB factor levels out at
a relatively low and ill-defi ned frequency; this frequency is the open-loop bandwidth and,
for an amplifi er that can never be used open loop, has very little importance.

References .............................................................................................................

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