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[SOLVED] Schmitt Trigger transistor sizing

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polygon pusher

Newbie level 5
Sep 15, 2010
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I am designing a Schmitt trigger in Jazz 0.18 cmos. I am having a problem sizing the transistors. My approach was to centre the trip point of the schmitt without the feedback devices and then add in the feedback devices to try and give +/- 100mV Hysteresis. I can get the lower trip point where I want it but cannot seem to increase the upper trip point away from the original centre point no matter what I do to the feedback transistor size.

Any help much appreciated

Do you mean transistor size? Or power? Or gain?

100 mV hysteresis is a small amount. It will take many adjustments to achieve.

Adjusting hysteresis is tricky even with an op amp which is voltage operated.

Using transistors (current operated) may make it harder.

Are you able to achieve 1V hysteresis?

Can you shift your desired voltage range up or down? You may find an operating point where transistors will permit small hysteresis.


I am talking in relation to transistor sizing. I would expect the 2 nmos and 2 pmos in the input stage to have roughly a 4:1 sizing ratio but in this process it appears to be more than 6:1 (without the feedback transistors connected) and sizing for a centred trip point at 0.9V with a 1.8V supply. Actually you are right about the hysteresis it needs to be roughly +/- 200mv around the centre point.


Polygon pusher

The switching points of a simple Schmitt trigger depend on supply, Vth and transistor ratios: what topology are you using? what are the available Vth's for PMOS and NMOS?

Do you have feedback on both pull up and pull down paths ? If you only have the feedback device on the pull down path then it will only affect low trip point no matter what you do, you should be having feedback paths on both the paths.

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