Designer’s Guide Community

New Member

Offline

Posts: 6

replacing the ideal tail current in a differential pair with active transistors
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

Back to top

scematic2.png

IP Logged

subgold Community Member Offline Posts: 97	Re: replacing the ideal tail current in a differential pair with active transistors Reply #1 - Jun 12^th, 2009, 2:47am it is because your dc biasing point of the input transistors are too low.
Back to top	IP Logged

raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: replacing the ideal tail current in a differential pair with active transistors Reply #2 - 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.
Back to top	IP Logged

oermens

Community Member

Offline

Posts: 86

Re: replacing the ideal tail current in a differential pair with active transistors
Reply #3 - 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

Back to top

IP Logged

ic_engr

Senior Member

Offline

Posts: 135

Re: replacing the ideal tail current in a differential pair with active transistors
Reply #4 - 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.

Back to top

IP Logged

salma shabayek New Member Offline Posts: 6	Re: replacing the ideal tail current in a differential pair with active transistors Reply #5 - Jun 13^th, 2009, 3:33am ok ...thank you alll soooo muchhh
Back to top	IP Logged