hi,
  i had a basic question on the following circuit.
in fig 1 -ve feedback loop gain -gm1*R and overall loop gain is  gm1*gm3/(1+gm1*r)*gm2*gm4.So how to find +ve loop gain here? i know that if -ve feedback loop gain is grater than +ve feedback loop gain circuit is stable or +ve feedback loop gain is less than 1.So in this circuit i am getting overall L.G is lees than 1,does it mean overall feedback is +ve but LG is less than 1 and -ve feedback loop gain is less than +ve feedback?  

 Same thing i did for 2nd circuit and here loop gain is >1,so it wont be stable at required bias,it will be reach either of the rail? can any body explain intuitively why 2nd wont work?
i hope this question will clarify lot of doubts in feedback basics for me ,please let me know any thing wrong in the above explanation?

Thanks,
Rajasekhar.

hi buddypoor,
   thanks for your question..i am aware of the things what you have told,ok any how i will re frame my question?
   In fig 1 i am getting loop gain +ve but L.G less than 1,so it works.Where as  fig 2 L.G +ve ,but grater than 1. so it will have multiple operating points and it may reach rail.please tell if any thing wrong in the above.
  So can any body please tell me intuitively why fig 2 wont work? i think now question is  clear.

thanks,
Rajasekhar.

Hi Rajasekhar,

I agree with subgold. I guess you are thinking that in figure two, at low frequencies you get gm2*gm4*(1+gm1R)/(gm3*gm1). If you use the proper sizing for each transistor then you can get a +ve loop gain < 1 at low frequencies.

Intitively you can think that in figure 2 M1 gate voltage variation due to a delta i generated at M4 will be larger (due to R at M1 source)  than M2 gate voltage variation due to M3 in figure 1. Since the other gains involved in the loop are the same then the loop gain is larger for figure 2 assuming same sizing as in figure 1

Tosei

hi nobody,
               its degenerated mosfet only,why you got that doubt.And booth loop gains are different and thats why my opinion is 2nd wont work
hi Tosei,
            thanks for your reply,certainly it will if you resize..but whole point in doing in this specific values  is to get gm=1/R, so i  should n't.Finally i have one question..what is the +ve feedback loop gain.I got overall loop gain and -ve feedback loop gain.So i think if i subtract it may come.So please clarify my doubt.

Thanks,
rajasekhar.

Hi Rajasekhar,

The overall loop gain is +ve. The -ve gain is just a local feedback on the degen stage. So when you consider the overall gain gm2*gm4*(1+gmR)/(gm3*gm1) you are already considering the -ve local feedback loop. The net result is the +ve feedback.

Tosei

Hi Rajasekhar,

Look at it in this way:
The two circuits are +ve feedback one. By using -ve feedback in the circuit 1 you ensure the +ve feedback is < 1 and therefore stable.
For circuit 2, if you keep the local negative feedback then you need resizing.
But bottom line is there is only one main (+ve) loop with an inner -ve feedback loop that is useful for adjusting the loop gain

Hope this helps
Tosei