Schmitt Inverter

Posted on Feb 4, 2014

The Schmitt trigger is a special circuit which acts like a switch that changes state at two different thresholds. These are called the upper and lower threshold or the positive and negative going threshold. The difference in these two threshold levels is called the hysteresis voltage. The Schmitt trigger does not react to any input voltage leve

Schmitt Inverter
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l in the range between the two thresholds, which for a 74C/HC14 corresponds precisely to the " forbidden zone ". A Schmitt trigger can also be likened to two comparators controlling an RS flip-flop at the output. In fact, the schematic of the 74C14 shows it to be designed that way. The upper threshold comparator sets the output latch and the lower threshold comparator resets the output latch. Each of the six inverters in a 74C14 Schmitt trigger uses 12 MOSFETs, so by comparison the discrete version of the Schmitt trigger using 3 transistors and 11 other components is about as complex. Normally the 74HC14 threshold parameters are a fixed ratio of Vcc. This keeps the device functionally simple and as a result the 74HC14 Schmitt trigger is one of the most popular devices for interfacing real world signals to digital electronics. It just doesn`t get any simpler compared to the other versions we will discuss. However it is a little known fact that it is possible to alter the thresholds of a 74HC14 by using negative feedback from output to input. For example, by adding a 5. 1M resistor from output to input and a 1M input resistor this will provide about 15% negative feedback. That is subtracted from the internal 30% positive feedback and with Vcc =5V, it effectively changes the thresholds at the input of the 1M resistor to approximately 2. 1V and 2. 9V. Very useful if the signal of interest has smaller transistions than the...

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