Page 251 - Op Amps Design, Applications, and Troubleshooting

Bandpass Filter 233

For our design, we compute R 3 as

The nearest standard value is 390 kilohms. As previously stated, if the application
requires greater compliance with the original design goals, use either a variable
resistor or a combination of fixed resistors to achieve the exact value required. In
our case, we will use two 200-kilohm resistors in series for R 3.

Determine the Value for R 4. Resistor R 4 has no direct effect on the fre-
quency response of the filter circuit. Rather, it is included to help compensate for
the effects of the op amp bias current that flows through R 3. You will recall that
we try to keep equal the resistances between ground and the (+) and (-) input
pins of the op amp. Therefore, we will set R 4 equal to R 3. In equation form, we
have

In this case, it is probably not necessary to use a variable resistor or fixed resistor
combination to obtain an exact resistance. We will simply use the nearest standard
value of 390 kilohms for R 4.

Select the Op Amp. We will pay particular attention to the following op amp
parameters when selecting an op amp for our active filter:

1. Bandwidth
2. Slew rate

If the resistance values turn out to be quite high, then an op amp with particu-
larly low bias current will be important. If the capacitance values must be below
about 270 picofarads, then select an op amp with minimum internal capaci-
tances.

Bandwidth. The required bandwidth of our op amp is determined by the
highest frequency that must pass the circuit. This is, of course, the upper cutoff
frequency (f H) and can be approximated (for our purposes) with Equation (5.22).