Page 167 - Op Amps Design, Applications, and Troubleshooting

150 VOLTAGE COMPARATORS

FIGURE 3.11 A voltage comparator
with hysteresis.

Upper Threshold Voltage. The upper threshold voltage is the value of the
voltage that appears across RI when the output is at its maximum positive level,
plus the value of the reference voltage. Appendix 1 lists a minimum value of 10
volts for +Vs AT. The value of the threshold voltage is computed by applying the
basic voltage divider formula to R F and R l and then adding the result to the refer-
ence voltage (V REF).

For this particular circuit, we have

As with the zero-crossing detector previously discussed, the threshold voltages
vary with V SAT. If this variation is objectionable, then the output can be regulated
by zeners, as discussed in Section 3.6.

Lower Threshold Voltage. The lower threshold voltage for the circuit shown
in Figure 3.11 is computed using the same method discussed for the upper thresh-
old:

or, more specifically,

The lower threshold suffers from the same variations described for the upper
threshold. Also, notice that in this particular circuit, the upper and lower thresh-