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comparator with hysteresis

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sarabjeet

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I want to know how to go about designing comparator with hysteresis (ie what steps to follow given a spec).
Considering this to be a comparator using diff amp with pfets as diode load and with them two pfets giving positive feedback. We see hysteresis when W/L of pfets providing positive feedback is greater than W/L of pfets as diode load.

What steps to follow from a spec? How to select bias I, input device W/L, load W/L, etc.

Given hysteresis window size (eg 100 mV), how to select ration of W/L of +ve feedback pfets to W/L of diode load pfets.

Thanks
 

u can find what u want to know in Allen's book
 

Thanks windmillkity.

But what I'm looking for is:

Given that I need hysteresis window of lets say 100mV (ie V_TRIPp - V_TRIPm).
then how to select how much positive feedback is needed (for diff amp with internal feedback with cross coupled pfets).

This is not discussed in Allen's Book. Book just says that this circuit has hysteresis and that why hysteresis is needed.

In other words for this circuit what is dependency of Ibias, gm (load diode), gm (feedback pfets), etc on TRIP points?

Sarab
 

I think that Jacob. Baker's book is very good:

CMOS: Circuit Design, Layout, and Simulation, 1st Edition
 

Hi Sarab,

i am sure you have a through understanding of the circuit.
I would not be giving straight answer to you, but would just point to the direction.
Try analysing the circuit as a schmitt triger (with no opamp).
The MOS ratio would decide the trip point.
Hope it helps
 

u can do the some trial analysis is that increase the postive feedback width and see at which w u r getting ur hysterisys..
first u put all transistors sizes same
 

Hi, sarabjeet. I've simulated and study the structure in P.Allen's book. As for your question. the width of the hysteresis window is determined by mainly 3 factors. (1) the W/L of the input transistor pairs (2) the value of the tail current (3)the positive feedback factor, in Allen's book it is represents as (W/L)6/(W/L)3,
You can go on to deduce the equations given in the book to get what you want.
Best wishes!!! :D
 

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