Bandgap Feedback Polarity

[twonsr]

feedback polarity

Hi all :

I have a problem when I read razavi's analog circuit book P.389.

It said "to ensure an overall negative feedback, Bp must be less than Bn, preferably by roughly a factor of two......"

What is "a factor of two" ?
Does it mean Bn/Bp must be larger than 2 ?

TKS

 

[happy2005]

that's right. But the important thing is Bp must be less than Bn. The user usually selects the ratio Bn/Bp aproximately 2.

 

[twonsr]

But why is it "2" , why not 3 or 4 , etc ?

Any theory for this ?

 

[happy2005]

i don't have razavi's analog circuit book. if you scan it or give info about pg. 389 , may be i can say something more. And, tell me the topic of the chapter, oscillators or stability? give me more info pls.

 

[nile_king]

Hi

when u focus on the ct u will find that there is 2 loops one for +ve FB and the other for -ve, So as there is +ve FB we have 2 make sure that this +ve FB will not make the ct oscillate so we need the -ve FB 2 more more that +ve FB by any factor
and i think that Razavi say by factor 2 for safety