Page 225 - Op Amps Design, Applications, and Troubleshooting

Triangle-Wove Oscillator 207

We now select either R x or Q and compute the other. In either case, we want
RI to be in the range of 1.0 to 470 kilohms, if practical. Similarly, Q should be
greater than 470 picofarads and nonpolarized. For our present example, let us
select Ci as 0.0047 microfarad. We now compute R^ by dividing Q into the RjQ
time constant.

Let us use a standard value of 150 kilohms for Rj,

Select the Op Amps. Other than obvious things like supply voltage ratings,
the most critical op amp parameter is slew rate. In order for our calculations
regarding frequency of operation to be valid, the rise and fall time in the output of
A 2 must be a small part of the time for either alternation. The greater the switching
times, the greater the error in calculations. If we accept rise and fall times of 10
percent of one alternation of the triangle wave, we can apply Equation (4,34),

This exceeds the 0.5-volts-per-microsecond rating of the standard 741, but falls well
within the capability of the MC1741SC We will select mis device for our op amps.
Figure 4.12 shows the schematic diagram of the completed design. The oscil-
loscope plots in Figure 4.13 indicate the actual performance of the circuit Finally,
Table 4.3 contrasts the original design goals with the measured performance of the