Environmental Engineering Reference
In-Depth Information
loop gain setting the output variable to zero. This means that the null return
ratio is the loop-gain after short-circuiting the port considered.
From the above discussion, and representing the loop gain as an explicit
function of and we can specify (3.22) to represent the closed-loop
input and output resistances,
and
as follows
where and are the corresponding driving point input and output
resistances with the critical parameter
P
equal to zero, and
and are the return ratios under the specific conditions for
the source resistance, and load resistance,
We have already discussed the ability of feedback to provide gain
desensitisation. However, such desensitisation of the input and output
resistances cannot be achieved. In fact, it can be shown through relations
(3.23) and (3.24) that I/O resistances depend on open loop parameters.
The application of Blackman theorem to the common Z amplifier in Fig.
3.3 is now described. Assume transconductance
as the control parameter
P.
The input and output
4
resistances with
are
Both
and
are equal to zero, whereas
and
result
Hence, input and output resistances are given by
4
As usual, the output resistance is calculated excluding
resistor
