Analog Electronics

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 ...