Page 248 - Op Amps Design, Applications, and Troubleshooting
P. 248
230 ACTIVE FILTERS
analytical method assumes that the filter was designed according to standard prac-
tices. The checks below will provide you with a reasonable degree of assurance that
the filter design is compatible with the analytical procedure to be described:
1. Is K 3 approximately twice the size of RI?
2. Are Q and C 2 equal in value?
3. Is RI at least 10 times the size of R 2?
If the answer is yes to all of these questions (which is the typical case), then the filter
can be analyzed as described below. We will compute the following characteristics:
1. Resonant frequency
2. Q
3. Bandwidth
4. Voltage gain
Filter Q. The Q of the filter shown in Figure 5.12 can be computed with the fol-
lowing equation:
Substituting values gives us
Since the Q is less than 10, we will classify this circuit as a wide-band filter.
Resonant Frequency. The resonant frequency of the filter shown in Figure
5.12 can be computed with the following equation:
where C is the value of either Q or C 2. Calculations for the present circuit are
It is this frequency that should receive the most amplification (or least attenuation)
from the filter circuit.