Page 120 - Op Amps Design, Applications, and Troubleshooting
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AC-Coupled Amplifier 103
Now we can compute the impedance of R { and C/.
The input impedance for the AC-coupled noninverting amplifier circuit
shown in Figure 2.27(b) is essentially equal to the impedance offered by R l and Q.
Technically, the input impedance of the (+) input terminal appears in parallel with
RI. We can generally ignore this impedance, however, since it is usually an
extremely high value. For our present example, we begin by finding the reactance
of Q at the lowest input frequency.
Next we find the net impedance of RI and C/.
The minimum value for input impedance occurs at the highest input fre-
quency, as it did with the inverting circuit. In most cases, the input impedance
approaches the value of R^ The following computations, however, are shown:
Now we can compute the impedance of R t and Q:
