schmitt trigger question

Thread Starter

Ron Potashnik

Joined Dec 27, 2015
16
i have the next circuit
1.png

what will happen if i replace all the 4093nand component with similar component but have no histeresis? what is the need of histeresis component here?
 

WBahn

Joined Mar 31, 2012
25,062
The resistor and capacitor result in a relatively slowly changing waveform. As a result, the circuit does not pass through the transition region (which is rather poorly defined to begin with) quickly. The noise in the signal can result in the output of the logic gate flailing (switching back and forth many times which it should only change once).
 

Thread Starter

Ron Potashnik

Joined Dec 27, 2015
16
The resistor and capacitor result in a relatively slowly changing waveform. As a result, the circuit does not pass through the transition region (which is rather poorly defined to begin with) quickly. The noise in the signal can result in the output of the logic gate flailing (switching back and forth many times which it should only change once).
thank you, great explain
 

RBR1317

Joined Nov 13, 2010
512
It appears that Circ1 and Circ2 are configured as 4093 oscillators. As such, the input voltage is meant to hover between the switching threshold points. The time it takes for the capacitor voltage to slowly traverse the middle region is what determines the oscillator frequency. A regular input gate does not have this wide region between the switching points (since there is generally only a single switching threshold) so any oscillations will be of much higher frequency and totally unpredictable.

Figure is taken from Fairchild Semiconductor Application Note 140, June 1975.

4093-oscillator.png
 
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