Environmental Engineering Reference
In-Depth Information
feedback network on the basic amplifier, and the determination of
A
and
f
is
usually a non-trivial task (especially because
A
is rarely independent of
f
in
practical electronics)
Nevertheless, this scheme is suitable for a simple
focus on the main feedback properties.
.
Assuming blocks
A
and
f
are linear and unidirectional, the transfer
function of the feedback amplifier is given by
where is the closed-loop gain and quantity
fA
is called the
loop gain.
Note that for negative feedback, the loop gain must be positive. Therefore,
the closed-loop gain is smaller than the open-loop gain (by a factor 1+
fA
). If
fA
< 0 the feedback is positive. If
fA
is much greater than 1, the closed-loop
transfer function falls to 1/
f
, which is independent of the open-loop gain. To
better represent this concept we can rewrite (3.
1
a) as
If the feedback network contains circuit elements that are not susceptible
to uncertainties we can achieve a closed-loop gain which is desensitised to
parameter variations of the open-loop function. This property can be
formally expressed by evaluating the sensitivity of the closed-loop gain
versus the open-loop one
