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Design >> Analog Design >> replacing the ideal tail current in a differential pair with active transistors
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Message started by salma shabayek on Jun 12^th, 2009, 1:27am

Title: replacing the ideal tail current in a differential pair with active transistors
Post by salma shabayek on Jun 12^th, 2009, 1:27am

Dear all,
I'm having a trouble in replacing the ideal tail current in the differential pair with an nmos current sourc transistors mirrored to an nmos diode connected transistor.
I did so by placing the nmos transistor with double the size of the nmos input transistor but I dont get amplification.
before placing the tail current I had large gain but now I dont..
so please if anyone knows what's the problem please tell me.

I also have another question ; since I'm working with 1uA current per branch and from using the current density charts and gm/id curves in Spectre I always get (W/L) ratio< 1

Is that acceptable???
thank you alooooooottttttttttttt

Title: Re: replacing the ideal tail current in a differential pair with active transistors
Post by subgold on Jun 12^th, 2009, 2:47am

it is because your dc biasing point of the input transistors are too low.

Title: Re: replacing the ideal tail current in a differential pair with active transistors
Post by raja.cedt on Jun 12^th, 2009, 3:05am

hi,
did you probe the tail transistor drain?i think it is at very low hence tail transistor is in triode so degeneration. I am surprising how cum you are working with 1u, what you will get 1u?

Thanks,
rajasekhar.

Title: Re: replacing the ideal tail current in a differential pair with active transistors
Post by oermens on Jun 12^th, 2009, 4:21am

In order to achieve Ids_NM1=Ids_NM4 you need

(W/L)_NM1=(W/L)_NM4
Vgs_NM1=Vgs_NM4
Vds_NM1=Vds_NM4

you are probably not meeting the last criteria. if you can spare the headroom, use cascode or wilson current mirror. refer to razavis analog book.

also I think it is difficult to achieve 1uA from tail transistor in saturation. you might have to consider weak inversion current mirror. but this suffers from poor matching... see this paper: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=4263360

Title: Re: replacing the ideal tail current in a differential pair with active transistors
Post by ic_engr on Jun 12^th, 2009, 8:36am

Salma,

What is your supply voltage. What is the VT of the NM0 & NM8.?
You definitely need to raise the DC levels on the diff pair (NM0 & NM8). Remember, the lower the DC on the diff pair, the lower VDS will be on the current source NM4 in question.

More importantly, looking at the model names (n_12_hs130e), it seems you are using high speed devices, which will definitely suffer with poor output impedance thus excerbating your problem.

Good luck.

Title: Re: replacing the ideal tail current in a differential pair with active transistors
Post by salma shabayek on Jun 13^th, 2009, 3:33am

ok ...thank you alll soooo muchhh