Page 326 - Op Amps Design, Applications, and Troubleshooting

304 SIGNAL PROCESSING CIRCUITS

Applying these equations to our present design gives us the following results:

Select Jtf. The minimum value for RI is determined by the minimum required
input impedance (8 kft in the present case). The upper limit depends on the op
amp selected, desired output impedance, and required immunity to nonideal
characteristics. It would be unusual to choose RI as anything greater man a few
tens of kilohms, since the resulting high-output impedance of the circuit would
make it difficult to interface. Let us select RI as a 9.1-kilohm standard value.

Select the Op Amp. The op amp can be selected by assuming that the ampli-
fier is passing strictly sinusoidal waveforms. Chapter 2 discussed this in more
detail, but the primary consideration is op amp slew rate. The required slew rate
can be estimated by applying Equation (7.5):

This, of course, exceeds the 0.5-volts-per-microsecond rating of the standard 741, but
it does fell within the 10-volts-per-microsecond slew rate limit of the MC1741SC. Let
us design around this device.

Select PI* Diode Di must have a reverse breakdown voltage that is twice the
value of the supply voltage. In the present case, it must withstand 30 volts. The
current rating for DI is more difficult to determine, since it is partially determined
by the input impedance of the circuit being driven by the clipper. As we are deal-
ing with low currents, and as all of the diode current must be supplied by the op
amp, it is reasonable to select a diode with a current rating that is greater than the
short-circuit op amp current. The manufacturer's data sheet lists the maximum
short-circuit current for the MC1741SC as ±35 milliamps.
Many diodes will perform well in this application. Let us select the com-
mon 1N914A diode, which should be adequate under any probable circuit con-
ditions.

Compute Ci. Capacitor Q helps to ensure that the reference voltage on the (+)
input of the op amp remains constant. Its value is not critical and may even be
omitted in many applications. A reasonable value can be computed by ensuring
that the reactance of Q is less than 10 percent of the resistance of the smaller of R 2
or R 3 at the lowest input frequency. In our present case, these resistors are both 20
kilohms, so we compute the value of Q by applying the basic equation for capaci-
tive reactance.