Environmental Engineering Reference
In-Depth Information
The separation factor is strictly related to a parameter of the loop gain
commonly
used
to
measure
the
degree
of stability
of a
feedback
system
margin
2
,
Indeed, the phase margin is defined as 180°
plus the phase of the return ratio evaluated at the transition frequency,
Figure 4.8 illustrates how the phase margin is determined on the Bode plots
3
of a second-order transfer function.
namely,
the
phase
For a second-order system with negative poles we have
In a well-designed amplifier
is larger than unity and the condition
must
also
hold.
Thus,
is
about
equal to
the
transition
frequency,
and
Then (4.14) is reduced to
2
Another parameter, less frequently utilised by electronic designers, is the
gain
margin,
defined as the difference between the gain 20 log
and 0 dB,
where is the frequency at which the phase equals -180°.
3
We assume that the reader is familiar with the Bode plot technique. For a review of
this method see for instance [SS91], [G85].
