Digital Electronics: Principles, Devices and Applications

Flip-Flops and Related Devices 359VccR c1CR c2R2VoVinQ1Q2ReR1Figure 10.2Schmitt trigger circuit.with the Schmitt trigger circuit of Fig. 10.2, we find that coupling from Q 2 collector to Q 1 base in thecase of a bistable circuit is absent in the case of a Schmitt trigger circuit. Instead, the resistance R eprovides the coupling. The circuit functions as follows.When V in is zero, transistor Q 1 is in cut-off. Coupling from Q 1 collector to Q 2 base drives transistorQ 2 to saturation, with the result that V o is LOW. If we assume that V CE2 (sat.) is zero, then the voltageacross R e is given by the equationVoltage across R e = V CC R e /R e + R c2 (10.1)This is also the emitter voltage of transistor Q 1 . In order to make transistor Q 1 conduct, V in must beat least 0.7 V more than the voltage across R e . That is,V in min = V CC R e /R e + R c2 + 07 (10.2)When V in exceeds this voltage, Q 1 starts conducting. The regenerative action again drives Q 2 to cut-off.The output goes to the HIGH state. Voltage across R e changes to a new value given by the equationVoltage across R e = V CC R e /R e + R c1 (10.3)V in = V CC R e /R e + R c1 + 07 (10.4)Transistor Q 1 will continue to conduct as long as V in is equal to or greater than the value given byEquation (10.4). If V in falls below this value, Q 1 tends to come out of saturation and conduct lessheavily. The regenerative action does the rest, with the process culminating in Q 1 going to cut-offand Q 2 to saturation. Thus, the state of output (HIGH or LOW) depends upon the input voltage level.