how the positive feedback in the hys comparator occor

[manissri]

i am attaching the comparator schematic.
i wud like to know how the positive feedback occour in this comparator.

 

[doho]

Consider the case when the differential input voltage to the comparator is 0V. Then half the tail current flows in either transisto, as in the picture above. In this state, the circuit actually has three solutions.

Solution 1: half the tail current flows in either current mirror
Solution 2: all tail current flows through one current mirror
Solution 3: all tail current flows through the other current mirror

It turns out that Solution 2 is metastable so that it in practice will not exist.

Now consider what happens when differential input moves slowly from the condition Vid=0V. As soon as it moves a little, the positive feedback in the loop will force all current to flow through one current mirror so that the circuit ends up in Solution 1 or Solution 3. Hence, does the comparator essentially have infinite DC gain.