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Design >> RF Design >> CMOS Gilbert cell with current bleeding : IIP3
https://designers-guide.org/forum/YaBB.pl?num=1160251975

Message started by Prabhu on Oct 7^th, 2006, 1:12pm

Title: CMOS Gilbert cell with current bleeding : IIP3
Post by Prabhu on Oct 7^th, 2006, 1:12pm

I am designing a double balanced Gilbert cell with current bleeding to increase the conversion gain and to reduce flicker noise, to use smaller switches .

My problem is this:

Every one says that linearity of a Gilbert cell is primarily determined by the linearity of the transconductor.
In my design, the transconductor alone has IIP3 = -10 dBm.
Overdrive of the switches = 50 mV, LO drive = 0.2 V peak single ended.
I_bleeding = 0.5 * I_transconductor.
But my mixer IIP3 = -18 dBm.
If the switches switch properly, they should not affect the linearity.
What could be the reason for this IIP3 degradation?

Title: Re: CMOS Gilbert cell with current bleeding : IIP3
Post by ACWWong on Oct 9^th, 2006, 4:28am

I would probably look at whats going on at the transconductor drains, to enusre you aren't going into triode. Easiest thing to do is to increase Vbias to the LO quad, and increase vdd in your simulation to see if there is an improvement.

Also for sanity you should conifrm the small signal gain is as expected (~gm*Zload*(2/pi)) and that the compression of the transconductor is similar to that of the full mixer. Doing this could save you time in debugging your problem before having to waste time consuming IIP3 simulations...