
Active signal processing in the time domain 175
must act or the higher the low-frequency limit of
the audio band must be to enable the loop gain to
be rolled off. In principle, loop stability can be
predicted analytically as in the case of an NFB loop
round any other amplifier, but often in the case of
an AGC loop the law of gain reduction versus
control voltage is unknown or incompletely speci-
fied.
Another point to note in connection with an
AGC circuit such as Figure 7.15b is that the AGC
attack time - the rate at which the gain is turned
down following a sudden rise in input level - is
determined by the capacitor C and the charging
circui ...