Page 119 - Op Amps Design, Applications, and Troubleshooting
P. 119
102 AMPLIFIERS
A v (overall) = A Vl x A V2 x Ay 3
= 0.922x0.857x10
= 7.9
As with the AC-coupled inverting amplifier, we have chosen to ignore the
frequency-dependent effects of open-loop op amp gain. This is generally a reason-
able approach because our calculations are performed at the lowest input frequen-
cies, where the open-loop gain is the closest to its ideal value.
Input Impedance. The input impedance for the AC-coupled inverting ampli-
fier circuit shown in Figure 2.27(a) is equal to the net impedance of R/ and Q.
Recall that the (-) input of the op amp is a virtual ground point. The source, there-
fore, sees the input impedance offered by C/ and R/. Because this is a frequency-
dependent value, we must discuss input impedance at a particular frequency of
interest. For purposes of our present discussion, let us compute the highest and
lowest values for input impedance if the input frequency range is 800 hertz to
3 kilohertz. The following input impedance at 800 hertz is computed. First we
find the reactance of C/ at 800 hertz.
Now we can compute the impedance of R/ and C/.
The minimum value for input impedance occurs at the highest input fre-
quency. In most cases, the input impedance approaches the value of R f ; however,
the computations are similar to those as shown:
