MICROWIND Tutorial Part 6 : Design and VLSI layout of Schmitt Trigger circuit.

Schmitt Trigger. (Never omit the "c" and "t" from the name of this circuit).
Yes, very soon we are back with a hot tutorial on Schmitt trigger. As usual, we will be providing the layout and simulation waveforms. Some of them are really sleeping off by the time we are done with the explanation, so we are cutting short the explanation and just giving pictorial hints so that you can draw on your own. Reffer back to the previous tutorial incase you need some basic guidance.

In electronics, a Schmitt trigger is a comparator circuit that incorporates positive feedback.

When the input is higher than a certain chosen threshold, the output is high; when the input is below another (lower) chosen threshold, the output is low; when the input is between the two, the output retains its value. The trigger is so named because the output retains its value until the input changes sufficiently to trigger a change. This dual threshold action is called hysteresis, and implies that the Schmitt trigger has some memory. In fact, the Schmitt trigger is a bistable multivibrator.

Schmitt trigger devices are typically used in open loop configurations for noise immunity and closed loop positive feedback configurations to implement multivibrators.(Courtesy: Wikipedia)

You can read the entire article over here.

Here is the schematic. Please go through the schematic properly. The naming conventions used in the schematic(for naming the MOS devices) are retained in the layout for readability.

The desing is very simple. Using the MOS generator place few n and p device and provide proper routing.

Here is the explainatory layout of Schmitt Trigger Circuit:

The output waveforms (voltage vs time) can be observed as follows:

But we are less interested in this. We want to see the hysteresis curve. To viewe the hysteresis curve follow the instructions:

1. Click on Voltage vs Voltage at the bottom of the simulation screen. (Marked with red circle in the next figure)

2. Tick the Hysteresis right at the right corner of the screen. (Again it is highlighted in the next figure)

3. Click on Reset followed by More.

You are done with Schmitt Trigger and you should be able to see the hysteresis curve as follows.

Final Schmitt Trigger Layout:

Have a great time