Hi Mayank,

Generally speaking, the +ve feedback loop has to have less gain than the -ve one. In that way a stable operating point can be achieved for the ckt.
Both loops share a common branch: the opamp´s output. so you can "break" the loop right there and analyze it as if it was a single loop (this is one of the scenarios that a multiloop ckt can be analyzed as a single loop one).

Hope this helps
Tosei

Hello all,
             I am simulating the above attached ckt....But something unexpected is happening....
  Opamp Open Loop response is stable with a PM of around 60 deg.  I am breaking the loop at the common point.
  Accroding to my calculations, -ve FB Loop should be stronger than +ve FB Loop...I am confused, esp by looking at the phase response...Is this way of measurement correct ??

Can't attach two files in one post :--- 1st image --> opamp Open Loop Response

Better use stability ("stb") analysis of spectre. Insert a vdc = 0 volt at the branch connecting OPMAP output to the PMOS gate. (bcz this is the branch which is common to both the loops).

[quote author=Mayank_Singh link=1258556820/0#2 date=1258605890]Thanx Tosei,
           
          The input at Vin will change the current in 1st branch, changing the voltage at Vx...Opamp will try to settle Vy to the same voltage, & current in second branch will be decided by Vy/Rload  & NOT  VIN.....This action doesnt reflect a current source, instead a voltage source
      Am i mistaken somewhere ?
[quote]

Hi Mayank,

In my opinion you do have a voltage controlled current source, since Vx is controlled by Vin, and Iout is a function of Vy=Vx.
The observation is that the output current is not a linear function of the input voltage, but still it is controlled by it.
What it looks to me is that if you want a more linear control on the current you then need to size the controlled NMOS such that it operates in triode region....


Best Tosei

Thanx Tosei,
                 Found the problem --> The ckt works fine if i replace the i/p Mos + src-Degeneration res with one single resistance whose value is in coordination with my math. High ro is causing a problem i suppose....
                 But if i try to increase the BW of the opamp, -->
1.   Error Amplifier is supposed to be of as low a BW as possible....Here, i am using the opamp as an error amplifier b/w vx & vy nodes....Increased BW would allow more noise to be pumped in...
2.   Increased Power Consumption...

Using input MOS in triode region is a nice suggestion, would try that for sure....But i still didnt understand how increasing opamp UGB would improve PM ?

--mayank

Hello Tosei,
                 Thanx for your answer....but i thought we need a narrow band  opamp for some other reasons which i stated in my post above....
 
   Could you pls emphasize more on what you meant by     Quote:


I didnt understand this statement quite well....

thanx,
mayank