Page 126 - Op Amps Design, Applications, and Troubleshooting

AC-Coupled Amplifier 109

We will use a standard value of 6.2 kilohms.

Compute the Value of C/. To simplify subsequent calculations, we will choose
a value for Q that produces a reactance that is much less than the resistance Rj. We
will design for a reactance of one-tenth RI, or 620 ohms in this case, at the lower cut-
off frequency. The value for C/ is computed from the capacitive reactance equation.

We will select the next higher standard value of 0.56 microfarad.

Select the Value of R t. In many cases, R L is the input resistance of a subse-
quent stage. In these cases, R L is not selected; it is already defined by the nature of
the problem. For our example design, we will choose K L as 27 kilohms. This is suf-
ficiently large to eliminate any concerns of output loading on the op amp, but it is
low enough to facilitate coupling to a subsequent op amp circuit if needed,

Compute the Value of Co* By following the guidelines given for the selection
of the RjQ combination, we have assured ourselves that the lower cutoff frequency
will be primarily determined by the Rf,C 0 network. We use the fundamental for-
mula for capacitive reactance to compute a value for CQ that produces a reactance
equal to the value of R L at the lower cutoff frequency (500 Hz).

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