Page 204 - Op Amps Design, Applications, and Troubleshooting

Voltage-Controlled Oscillator 187

of ^ 3 is -1 and -2 for the +V m and A 2 signals, respectively. The diodes may be sub-
stituted with any general-purpose diode, but they should all be of the same type
in order to have similar voltage drops.

Select the Zener Diodes. The voltage rating of D 2 plus the forward voltage
drop of DI determines the upper limit of the ramp output voltage. Similarly, DI
plus the forward voltage drop of D 2 determines the lower limit of the ramp output
voltage. We can express this as an equation for selecting the voltage ratings of D }
and D 2:

For our present design, the required zener ratings are computed as shown:

V Di = -4 V + 0.6 V = -33 V, and
V D2 = +4 V - 0.6 V = +3.3 V

Appendices 5 and 8 provide a manufacturer's listing of several zener diodes.
Either 1N5226 or 1N4728 zeners should work for our application. Let us select the
1N5226 device for this example.

Compute #2" Resistor R 2 is a current limiting resistor that keeps the current
through DI and D 2 within safe limits. Although the circuit will work well with a
wide range of values for R 2, a good choice is to design for a current through the
zener diodes given by the following expression:

Resistor JR 2 can then be determined from the following equation:

where V SAT is the highest expected saturation voltage for A 2f V z is the lower of the
two zener voltages (if not equal), and I z is the zener current calculated with Equa-
tion (4.17). The data sheet in Appendix 5 lists a knee current (I ZK) of 0.25 milli-