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Design >> Analog Design >> How does this act as a Schmitt Trigger
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Message started by cktdesigner on Dec 6^th, 2011, 4:03am

Title: How does this act as a Schmitt Trigger
Post by cktdesigner on Dec 6^th, 2011, 4:03am

Hi,

Could somebody please explain how the circuit in the attachment behaves like a Schmitt trigger?

Thanks.

Title: Re: How does this act as a Schmitt Trigger
Post by loose-electron on Dec 6^th, 2011, 10:16am

the two invertrers are not matched in size, and the feedback one is weaker.

Two inverter set up like that function as a latching structure, the flipped state, then requires a bigger signal to return to the prior state after flipping.

Therefore you get hysterisis. = schmitt trigger

Title: Re: How does this act as a Schmitt Trigger
Post by cktdesigner on Dec 6^th, 2011, 8:12pm

Thank you.

could you please tell which inverter's aspect ratio will need to be adjusted to modify upper and lower swtiching points?

INVBUF or INVFB?

Title: Re: How does this act as a Schmitt Trigger
Post by Vladislav D on Dec 7^th, 2011, 9:31am

btw, what is kind of PMOS in the circut? Depletion?

Title: Re: How does this act as a Schmitt Trigger
Post by loose-electron on Dec 7^th, 2011, 9:34am

cktdesigner wrote on Dec 6^th, 2011, 8:12pm:

Thank you.

could you please tell which inverter's aspect ratio will need to be adjusted to modify upper and lower swtiching points?

INVBUF or INVFB?

You have 3 inverters in a "pulling" process here, the input device (drawn at the transistor level) and the other 2.

Your hysterisis is set by the relative geometry sizes of the input device and the feedback inverter.

Its voltage division with resistors and switches, draw each of the inverters as 2 switches and 2 resistors, and think of it that way. You will start to understand whats going on better at that point.

Title: Re: How does this act as a Schmitt Trigger
Post by cktdesigner on Dec 15^th, 2011, 1:08am

loose-electron wrote on Dec 7^th, 2011, 9:34am:

cktdesigner wrote on Dec 6^th, 2011, 8:12pm:

Thank you.

could you please tell which inverter's aspect ratio will need to be adjusted to modify upper and lower swtiching points?

INVBUF or INVFB?

I am still unable to understand how hysterisis is present.

I do not understand how the higher switching point (VIH) occurs at a high voltage value and also in the return path why does the circuit switch at a lower level voltage.

Could you please point me to a source where operation of this circuit is explained in detail?

Thanks.

Title: Re: How does this act as a Schmitt Trigger
Post by loose-electron on Dec 18^th, 2011, 2:22pm

draw out the circuit as:

Ideal switches for all NMOS & PMOS elelements.

Resistors in series with those switches which model the
"on resistance" of the CMOS switch.

Look at the voltages and resistances of the system like that.