the output of a 741 operational amplier that controls a TTL input. The output of the opera-
tional amplier swings between +15 V and −15 V. The positive swing closes the transistor
switch, producing a TTL input of approximately 0 V. The negative swing drives the tran-
sistor into cut-off, producing a TTL input of +5 V. Diode in the base circuit protects the
base against excessive reverse voltage. The diode clamps the base voltage at approximately
−0.7V on the negative swing.
14.8.5 | Comparator Drive
Figure 14.26 shows the connection of a comparator LM339 to a TTL load. Because of the
open-collector output, the ...
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