Page 201 - Op Amps Design, Applications, and Troubleshooting

184 OSCILLATORS

Now let us consider the combined effects of the two inputs to A 3. Recall that
the combined effect is found by adding the outputs caused by the two individual
inputs. During times when the output of A 2 is positive, it has no effect on the out-
put of A 3 (D 3 is reverse-biased), and the output of A 3 is solely determined by the
+V IN signal, as computed previously. During the times when A 2 is at its -V SAT
level, the output of A$ clearly will be driven to its +V SAT level Even in the best
case, when +V m is at its most positive (+5-volt) level, the output of A 3 will be

V 0 = V 0l + V 02
= +25.4 V + (-5.5 V)

= +19.8 V
an< are me
where V 0l 3 ^o 2 effective output voltages produced by A 2 and +V !N,
respectively. As you can readily see, this combined value still exceeds the +V SAT
level of A 3, so we will expect the output of A 3 to remain at +13 volts anytime the
output of A 2 is at the -V SAT level. On the other hand, when the output of A 2 is at the
+VSAT l eve l the output of A 3 will be between -1.6 and -5.6 depending on the value
of input voltage (+V IN).
Finally, let us examine the operation of A! more closely. During times that the
output of A 3 is at the + V SAT level, diode D 4 will be reverse-biased and will isolate
or remove that input path for A^. During these times, AI is controlled by the effects
of +V/ N only. Let us examine the charging rate of Q at the two extremes of +V !N.
If + VJN is at its lower limit (+1 volt), then the current through RI is computed as

Similarly, the maximum input current is computed as

Since D 4 is effectively open (i.e., reverse-biased), and since no significant current
can flow into or out of the (-) input terminal of the op amp, we can infer that all of
the input current goes to charge Q. More specifically, electrons flow from the out-
put of A l through Q (i.e., charging Q), and through RI to +F IN. Further, because
this current is constant (unless +V IN changes), capacitor Q will charge linearly
according to the following expression: