Page 174 - Op Amps Design, Applications, and Troubleshooting

Window Voltage Comparator 157

+Vjtfp> The value of +VREF is established by the 1N5230 zener diode. Appendix 5
indicates that this is a 4.7-volt zener, so +V REF is approximately +4.7 volts. If desired,
we can determine the amount of zener current with Equation (3.20) (transposed):

r +V CC - V Z

-YMF. The 1N5225 zener diode is used to establish the -Vg£f source. Appendix 5
lists the 1N5225 as a 3.0-volt zener. Its current can be calculated with Equation (3.20)
(transposed) as

Output Voltage (VQ). The upper limit of v 0 occurs when both of the isolation
diodes are reverse-biased or effectively open. This means that the pull-up resistor
(JRp) has essentially no current flow and therefore no voltage drop across it. Since
RP drops no voltage under these conditions, the output will be at a +15-volt level.
As mentioned previously, if +V SAT is sufficiently low, the isolation diode will not
be reverse-biased and the output voltage (v 0) will be less than +15 volts (+V SAT +
V F, where V F is the forward voltage drop of the diode).
If either of the op amp outputs is forced to its -V SAT level, then the associated
isolation diode will be forward-biased. Appendix 1 indicates that the -V SAT will be
about -11 volts. The current through R P can now be estimated as follows:

where V F is the forward voltage drop of the isolation diode (typically 0.7 volts). In
our present case, we can compute I Rp as

The actual output voltage (v o) under these conditions is determined as follows: