Page 340 - Op Amps Design, Applications, and Troubleshooting

318 SIGNAL PROCESSING CIRCUITS

the DC voltage. Having made this definition, we can apply a simplified equation
to determine the lower frequency limit:

In the case of the circuit in Figure 7.18, we estimate the lower frequency limit as

Response Time. Response time describes how quickly Q can respond to
decreases in the amplitude of the input signal. Here again, this can be computed
from the basic discharge equation. However, if we assume that the capacitor is
charged to peak and discharges toward an eventual value of 0, then we can use the
simplified form, Equation (7.14).

where v PK[old) is the peak input signal amplitude before the decrease, and v PK(new)
is the peak input signal amplitude after the decrease. For example, let us deter-
mine how quickly the circuit shown in Figure 7.18 can respond if the input signal
drops from 2.5 to 1.2 volts peak. We apply Equation (7.14) as follows:

7.5.3 Practical Design Techniques

Let us now design an ideal peak detector circuit similar to the one shown in Figure
7.18. It should satisfy the following design goals:

1. Input frequency range 300 to 3000 hertz
2. Peak input voltage 1 to 5 volts
3. Response time <200 milliseconds
4. Ripple voltage <3 percent

Select the Clipper Op Amp. The minimum unity gain bandwidth is the same
as the upper input frequency, since A x is essentially operated at a closed-loop gain
of 1 (when the rectifier conducts).
The minimum slew rate for the op amp is computed by applying Equation
(2.11). On the negative alternation of the input cycle, the output of A } will go to