Page 205 - Op Amps Design, Applications, and Troubleshooting

188 OSCILLATORS

amperes for the 1N5226 diodes. The design value of the zener current is then com-
puted, as in Equation (4.17):

The value of R 2 can now be computed, as in Equation (4.18):

We will select a standard value of 3.6 kilohms for R 2. However, because this
design method inherently uses a zener current that is less than the test current
(I ZT), we can expect the regulated voltage to be less than the stated value.
Appendix 5 includes a graph that allows us to estimate the error. In the present
case, the zeners will have about 2.65 volts instead of the rated 3.3 volts, which
will in turn cause the ramp output to have an amplitude of 6.5 volts instead of
the design goal of 8. If this is an important circuit parameter for a given applica-
tion, we should select a zener with a higher voltage rating but continue to oper-
ate it below its rated current. We continue with our present selection, since the
reduced voltage is still within the tolerance stated as part of the original design
goals.

Compute RI and Cj. Once the zeners have been selected, the values of R^ and
Q determine the frequency for a given voltage. The required RiQ product can be
found with Equation (4.19):

where V MMp is the amplitude of the ramp output voltage, y w(max) is the highest
input voltage, and/ HJ is the highest frequency of oscillation. Calculations for our
present design example are

At this point, we can either select Q and calculate Rj or vice versa. 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 pF and nonpolarized. Because it is essential that R 4 be
exactly one-half the value of R\ f and because there are a limited number of resis-
tor pairs that have exactly a 2:1 ratio, it is generally easier to select R l and com-
pute Q.
For purposes of this design, let us select RI as 2 kilohms. We can then com-
pute Q by transposing the results of Equation (4.19):