Page 259 - Op Amps Design, Applications, and Troubleshooting

Band Reject Filter 241

For our purposes, we compute C 2 as

Capacitor C 3 is twice the size of Q, or simply

In our particular case

The nearest standard value is 1500 picofarads, which is close enough for many
applications, but would undermine the performance of our filter. The simplest
way to obtain more precise values in this case is to parallel two 680-picofarad
capacitors. This will give us the value we need.
are
Compute RI and R^. Resistors RI and R 2 equal in size and are twice the
value of R 3. We can express this as an equation.

In the present design case, we compute these resistors as

The nearest standard value is 43 kiiohms. We might be able to get by with this, but
in general we must try to obtain the exact values required. To this end, let us
choose either a combination of fixed resistors (e.g., 39 kQ in series with a 3.6 kO) or
a fixed resistor and a variable resistor in combination (e.g., a 39-kQ fixed resistor
in series with a 5-kO variable resistor).

Compute RQ. Resistor R4 helps to compensate for the voltage drop across Rj
and R 2 that is produced by the op amp bias currents. To minimize the effects of the
bias currents, we set R 4 equal to the series combination of R 5 and R 2. That is,