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Design >> RF Design >> Terrible bondwire effect. How could I fix it?
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Message started by wccheng on Feb 27^th, 2007, 12:17am

Title: Terrible bondwire effect. How could I fix it?
Post by wccheng on Feb 27^th, 2007, 12:17am

Dear all,

I design a LNA. Firstly, I don't added any bondwire effect (1nH in series with 1ohm to pretend a bondwire) on each pad for the simulation. Afterwards, I added the bondwire model at each pad and do the simulation again. I found the NF is 3dB higher compared to the LNA without bondwire model. It is terrible. I have searched each bondwire model. I found the major contribution of the NF is the bondwire connect between bottom current source (core differential LNA) of the transistor's source and ground. Actually, how could I fix this problem? I have tried to use two pads to decrease the bondwire's inductance value. However, it is fail.

Another interesting thing is that the LNA is same in ADS and Cadence. I have added all the bondwire effect in both platform. However, the Cadence will give wrose result.

Thanks

wccheng

Title: Re: Terrible bondwire effect. How could I fix it?
Post by Visjnoe on Feb 27^th, 2007, 1:29am

Dear wccheng,

before we continue, I would advise you to simulate the LNA with a 1nH bondwire and 0.1-0.2 Ohm series resistance. The rule of thumb for bondwires is 0.1-0.2 Ohm/nH bondwire.

Please report on the resulting NF you see then and let's take it from there.

Kind Regards

Peter

Title: Re: Terrible bondwire effect. How could I fix it?
Post by wccheng on Feb 27^th, 2007, 2:05am

Dear Sir,

I have tried to use 1nH in series with 0.1ohm to pretend a bondwire effect. The result is same. If I removed this bondwire effect and directly connect to gnd, the performance is good. :'(

Thanks
wccheng

Title: Re: Terrible bondwire effect. How could I fix it?
Post by Visjnoe on Feb 27^th, 2007, 3:15am

Dear wccheng,

OK, so if I understand this correctly, even with 1nH/0.1 Ohm the NF is increased by 3dB?

Could you post your LNA topology?
As far as your testbench is concerned: do you use a port @LNA input (impedance = 50 Ohm?) ?

Since you are pretty sure what's the main contributor, remove all other bondwires from your simulation, just include the current source's transistor bondwire and re-simulate (if necessary): please report the NF.

Regards

Peter

Title: Re: Terrible bondwire effect. How could I fix it?
Post by ACWWong on Feb 27^th, 2007, 3:20am

I guess the moral of the story is that when designing external world interfaced circuits, especially at high frequency (like LNA or PA), one must include all parasitic effects (pads, ESD, Layout, bondwire, packaging even pcb effects/components), so not to get a nasty shock when the chip comes back.
Anyway, the bondwire inductor is acting as degeneration in your circuit, so it reduces gain and so increases noise, but makes it more linear/higher P1dB, higher IIP3). Knowing that the bondwire will be present should now allow you go round the design loop again to re optimise the gain/noise and linearity trade-off to meet your specification targets.
cheers
aw

Title: Re: Terrible bondwire effect. How could I fix it?
Post by aaron_do on Feb 27^th, 2007, 4:37am

Hi wccheng,

you said you are using a differential structure right? In that case theoretically the bondwire should have little if any effect on the LNA. This is because it is contributing common-mode noise, and is connected to AC gruond. You should ensure the that the source terminals of the plus and minus branches are connected together before being connected to ground. Also, when you chose the output nodes for the noise measurement are you selecting the difference of the two output nodes or one output node and ground? You should chose the difference of the two nodes to eliminate the common-mode noise. Alternatively you could add a balun at the output.

If you are using a single ended structure then i suggest you allocate more than one pad for ground so that the bondwires appear in parallel. Anyway I suggest you post your schematic if you want a good answer.

Aaron

Title: Re: Terrible bondwire effect. How could I fix it?
Post by mg777 on Feb 27^th, 2007, 9:45am

A source bondwire inductance would Widlar the current mirror.

Anyways, the 3 dB number is kinda funny, esp if it doesn't move with the number of bond wires. Definitely sounds like some half section gain definition business.

M.G.Rajan
www.eecalc.com