Page 214 - Op Amps Design, Applications, and Troubleshooting

196 OSCILLATORS

Note that the actual measured zener voltages will vary somewhat from their nom-
inal value. Nevertheless, we will use our computed values for the remainder of
the analysis.
We are now in a position to compute the operating frequency, duty cycle,
pulse width, and so on. Equation 4.27 is used to determine the duration of the pos-
itive output alternation.

Similarly, Equation (4.28) is used to determine the time for the negative output
alternation.

For illustrative purposes, let us compute the minimum and maximum time
for both alternations by repeatedly applying Equations (4.27) and (4.28).

We can already see that the circuit gives us a 101:1 range of control on the positive
alternation. A similar calculation for the negative alternation gives us

Since Equations (4.27) and (4.28) ignore the forward voltage drops of D lf D 2f and
+
the effective resistance of the zener regulator circuit, the actual times for t and t~
will be somewhat longer than our calculations predict.
Because the components are matched, the results are the same for each alter-
nation. In practice, however, the two alternations do not have to have equal ranges.
Now let us extend our analysis to determine the minimum and maximum fre-
quency of operation. These two extremes are given by Equations (4.29) and (4.30).