Page 183 - Op Amps Design, Applications, and Troubleshooting
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166 VOLTAGE COMPARATORS
We will now select jR/ and compute R F. We will choose 10 kilohms for resistor R,.
The feedback resistor R F can now be computed, from Equation (3.15), as shown:
The factor 7.5 in the above equation is simply the R F/Rj ratio previously computed,
Select the Output Zener Diodes. The voltage rating of the two zener diodes
is determined by the stated output voltage swing. That is,
Substituting values eives us
Similarly, the voltage rating for D 3 is computed as
Values for the present circuit are
The power ratings for the zeners are not critical, but must be noted for subsequent
calculations. By referring to Appendix 5, we can select a 1N5229 and a 1N5226 for
diodes D 2 and D 3/ respectively. We also observe that both of these diodes are rated
at 500 milliwatts.
Compute Jtj. Resistor RI is a current limiting resistor for the zener diodes. First,
we will compute the maximum allowable currents through each of the zeners,
using Equation (3.19).
