Page 213 - Op Amps Design, Applications, and Troubleshooting

Variable-Duty Cycle 195

approximate reference voltage can be found by applying Equations (4,22) and
(4.23) using the rated voltages for the zeners. In our case, we have

+V REF = 3.3 V + 0.6 V = 3.9 V, and

-VREF = -3.3 V - 0.6 V = -3.9 V

If we compute the actual zener current, we can make a closer approximation.
Since the circuit is utilizing similar zeners, we can calculate either one. Suppose
we work out the current during the positive output alternation:

Substituting values gives us

Of course, even this is not an exact value because we know that the 3.3-zener drop
is actually less, but our overall result will be very close to the actual value. If we
now refer to the manufacturer's data sheet in Appendix 5, we can estimate the
actual zener voltage for a 1N5226 with about 2 milliamperes of current. The graph
in Figure 2 of Appendix 5 indicates that our zener will have a voltage of about 2.6
volts. If we use this value and recompute the reference voltages with Equations
(4.22) and (4.23), we will get more accurate values.

Let us now perform a similar calculation for the D 5 and D 6. We will use the
nominal values for the zener voltages and apply Equations (4.25) and (4.26).

Substituting values gives us the following estimates: