Page 260 - Op Amps Design, Applications, and Troubleshooting

242 ACTIVE FILTERS

For purposes of our present design, R 4 is computed as

For many applications, this is not a critical value. In our case, let us use the nearest
standard value of 82 kilohms.

Select the Op Amp. As with previous filter designs, there are two op amp
parameters that we want to focus on in order to select an appropriate op amp.

1. Bandwidth
2. Slew rate

As mentioned before, if the resistance values turn out to be quite high, then an op
amp with particularly low bias current will be important. And, if the capacitance
values must be below about 270 picofarads, we select an op amp with low internal
capacitances.

Bandwidth. The required bandwidth of our op amp is determined by the high-
est frequency that must pass the circuit. The design specifications for our present
case specify the highest input frequency as 18 kilohertz. The required bandwidth
for the op amp is computed in the same manner as described in Chapter 2:

This is well within the capabilities of the standard 741 op amp.

Slew Rote. The minimum slew rate for the op amp can be computed with Equa-
tion (5.7).

This exceeds the capability of the standard 741, which has a 0.5-volts-per-
microsecond slew rate. We will use an MC1741SC for our design, as it satisfies the
design's bandwidth and slew rate requirements.
This completes the design of our 5500-hertz band-reject filter, whose
schematic is shown in Figure 5.18. The oscilloscope displays in Figure 5.19 indi-
cate the performance of the circuit at resonance, at the two cutoff frequencies, and
at two far removed frequencies in the passband of the filter. Table 5.4 contrasts the
design goals for the filter with the actual measured performance of the final
design. It should be noted that 5-percent tolerance components were used to con-