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Design >> RF Design >> gain and linearity
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Message started by sivacha on Nov 15^th, 2006, 11:10pm

Title: gain and linearity
Post by sivacha on Nov 15^th, 2006, 11:10pm

i am facing a big trouble with gain and p1dB of my active mixer.
I need a gain of 20dB and a p1dB of -12dBm for a low voltage application.
I am not sure whether it is possible or not.

Is there any technique to improve linearity without losing the gain??

Thanks..

Title: Re: gain and linearity
Post by ACWWong on Nov 16^th, 2006, 2:18am

There some design techniques out there which can improve the gain.linearity product, but best way to improve linearity with out losing gain is to increase power consumption (and area).
Given I don't know the freq/topology of your proposed mixer its difficult to comment further, but your specs don't seem unachieveable given a realistic power budget.

Title: Re: gain and linearity
Post by sivacha on Nov 16^th, 2006, 2:27am

thanks to ACWWong.

but there is a serious constraint on current consumption also. iam already using the peak current.
mixer topology is folded mixer with pmos gm stage.
tech is tsmc13rf and it is for 2.4GHz range.

iam unaware of any of the other techniques except increasing the power, which is not favorable in my case.
hope i have provided enough info.

Title: Re: gain and linearity
Post by ACWWong on Nov 16^th, 2006, 2:51am

You should really be using NMOS gm stage, to get best gain to power consumption ratio. There should be no reason that you should use PMOS over NMOS. Assuming you are doing down conversion in a receiver, the input should be ac coupled from the previous stage, so DC bias level should be setup within the mixer itself.
Using NMOS will definately help you get more gain.linearity product for the same power.
If you have headroom issues (i.e < 1V) then you can have a NMOS input gm stage with current mirrored (with ratio) into an NMOS LO switching stage.

Title: Re: gain and linearity
Post by sivacha on Nov 16^th, 2006, 6:52am

thanks to ACWWong.

i was using NMOS gm stage before. but the LNA output is very weak. so i was suggested to use a PMOS gm stage to avoid ac coupling .
i am facing the same problem with the nmos gm stage also. gain is high, good noise figure but i am losing the linearity. i dont find any difference whether i use pmos gm stage or nmos gm stage.

Title: Re: gain and linearity
Post by ACWWong on Nov 16^th, 2006, 8:30am

sivacha wrote on Nov 16^th, 2006, 6:52am:

This does not make sense. here are a few point which might help you.
1) The LNA output signals are always small, ac coupling at 2.4GHz with MIM capacitors should introduce little loss as you couple with a high pass filter (with cut off less than 2.4GHz). I have done this on many occasions.
2) nmos mobility is normally about 3 times greater than in pmos, as a consequence you get much more gm for the same current and device geqometry. this is fact.
3) The linearity of the gm stage if it is just a differential pair is dependant on the overdrive (vgs-vt) you are operating the device at. If you have excess gain in your nmos implemenation, you can decrease gain and increase linearity by reducing W/L to increase overdrive given the same bias current. Other standard linearing techniques like degeneration or offset coupled pairs (like bjt multitanh) exist.
4) You should confirm that your linearity is truly limited by your input stage (by simulating it standalone) and not due to output headroom or LO switching issues.

cheers
aw

Title: Re: gain and linearity
Post by loose-electron on Nov 18^th, 2006, 11:44am

AW:

I have seen multi-tan structiures used in the differential pairs of Gilbert cell mixers to improve linearity a bit.

Care to comment on that? (Like I said, seen them, but not designed one using that architecture)

Jerry

PS - in total agreement on NMOS gain structure, and AC coupling into the mixer. Don't deal with DC coupling and offset control unless you have to.

Title: Re: gain and linearity
Post by ACWWong on Nov 18^th, 2006, 3:41pm

loose-electron wrote on Nov 18^th, 2006, 11:44am:

Yes, the classic Gilbert paper "the Multi-tanh principle" explains it all very well...
Anyway, i have used them in MOS and bjt forms in gilbert cell style cells for modulator and mixers... because of the increased input capacitance & fast gm roll off outside of the flat (linearised) gm section, perhaps not the best method for linearising high frequency first downconversion mixer in a wireless receiver....
More suited for well defined signal excursions in a modulator of a wireless transmitter... extremely flat gm characteristic gives for very low distortion products ... pretty robust to corners, mismatch etc.
Used them extensively in gmC filters (triplet forms) for receiver channel filtering.
All in all I like the technique...

Used them extensively in gmC continuous-time filters (in triplet form) for receiver channel filtering.

Title: Re: gain and linearity
Post by loose-electron on Nov 20^th, 2006, 9:48pm

Hm... My last gmC filter was for a UWB filterdesign and I did the "throw more current at it" approach to make linearity. I will have to keep it in mind the next time I have to go there.

UWB is "plug into the wall" due to to the 250MHz OFDM channel. The ADC to process the OFDM sucks a lot of juice.