Page 250 - Op Amps Design, Applications, and Troubleshooting

232 ACTIVE FILTERS

value, we will have to select a different value for Q and C 2 and recompute the
resistance values. Because of the wide range of resistance values typically required
in a given design, it is not uncommon to have resistance values ranging from 1000
ohms or less to well into the megohm ranges. Nevertheless, it should remain a
goal to keep the resistance values above 1 kilohm and below 1 megohm if practi-
cal. The lower limit is established by the output drive of the op amp and the effects
on input impedance. The upper limit is established by the op amp bias currents
and circuit sensitivity. That is, if the resistance values are very large, then the volt-
age drops due to op amp bias currents become more significant. Additionally, if
the resistances in the circuit are excessively high, then the circuit is far more prone
to interference from outside noise, nearby circuit noise, or even unwanted cou-
pling from one part of the filter to another. For our initial selection, let us choose to
use 0.001-microfarad capacitors for Q and C 2.

Compute the Value of Jt|. The value of resistor ^ is computed with Equa-
tion (5.18).

In the case of the present design, we compute the value for RI as

We will plan to use a standard value of 200 kilohms. It should be noted, however,
that the component values in an active filter are generally more critical than in
many other types of circuits, so if close adherence to the original design goals is
required, use either variable resistors for trimming or fixed resistors in a series
and/or parallel combination, or use precision resistors.

Compute the Value for R 2. Resistor K 2 is calculated with Equation (5.19).

a
For our present design, we compute R 2 § follows:

Compute the Value for & 3. Resistor R 3 is computed by simply doubling the
value of RI. That is,