
Audio-frequency signals and reproduction 95
without any need for
V i
at all! This corresponds to
an infinite closed loop peak in Figure 4.6b and to
closed loop poles actually on the
±] ω
axis in Figure
4.6c. The result is a self-sustaining oscillation at
ωρ ;
the amplifier is unstable. If the phase shift exceeds
180°,
so that the locus in Figure 4.6a passes to the
north of the point -1.0, the amplitude will rapidly
increase until limiting occurs, resulting in a non-
sinusoidal oscillation. Since at each peak, positive
or negative, the amplifier has to recover from
overload, the frequency of the non-sinusoidal
oscillation is lower than
ωρ ;
thi ...