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Design >> RF Design >> How to choose capacitance value?
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Message started by saam on Nov 14^th, 2010, 1:24am

Title: How to choose capacitance value?
Post by saam on Nov 14^th, 2010, 1:24am

How to select the values of the dc-bloking, coupling and bypass capacitors at RF frequencies? which parameters we need to consider?

ThankU
saam

Title: Re: How to choose capacitance value?
Post by saam on Nov 14^th, 2010, 1:37am

Hello pancho
Can you please elaborate....

ThankU
saam

Title: Re: How to choose capacitance value?
Post by rfmagic on Nov 15^th, 2010, 1:28am

please post the answers to all the forum members... thanks

to your question - it depends.

you can use the decoupling capacitor to be a part of a matching network between 2 block and in that case the capacitance value will be detaemined by the matching

if you want the decoupling to be transparent to the input block thenyou have to choose the capacitance to be big enough not to present substential impedance at the frequency that you are working at

Title: Re: How to choose capacitance value?
Post by saam on Nov 15^th, 2010, 10:00pm

Hello,
ThankU but my coupling capacitor is not in input network
I have used coupling capacitor to couple active inductor as a load but here due to parallel resonance (i think in wire which is connecting coupling capacitor to the input stage at output node) in S21 plot i am getting peak at lower frequencies (in MHz) also but actually i have to 2 get peak only @4GHz for 2-6GHz wide band (-dB bandwidth).
If i use coupling capacitor=1nF there is no peak
if i use coupling capacitor=3pF (in pico range) i am getting peak at lower frequency (in MHz) also ....
but 1nF is very high value,,while doing layout that capacitor occupying more area than passive inductor.

Can anyone suggest me how to choose coupling capacitor value

thank U
saam

Title: Re: How to choose capacitance value?
Post by rfmagic on Nov 15^th, 2010, 11:36pm

try simulating the peak frequency versus capacitor value - it is interesting to see what minimum value of decoupling capacitor that dont effect the peak frequency

Title: Re: How to choose capacitance value?
Post by saam on Nov 16^th, 2010, 12:07am

Ya i have simulated for different values of coupling capacitor from 1pF to 1nF, at high value of capacitance i.e nearly 300-500pF..i am not getting peak...that is the problem i posted,,because that high cap occupying more area than passive inductor. :-[

Title: Re: How to choose capacitance value?
Post by rfmagic on Nov 17^th, 2010, 12:23am

So I assume that you are using this decoupling cap at the output of an amplifier. in that case I would use the active inductor to resonate this capacitance along with the rest of the capacitance that you have on this node

Title: Re: How to choose capacitance value?
Post by saam on Nov 17^th, 2010, 8:11am

Sorry i didnt get u..can u please elaborate it
for more clarification i am attaching the circuit which i have used ( fig.2), please note that in that circuit i havn't used that series resistance Rc and directly coupled to the output node.

ThankU
saam

Title: Re: How to choose capacitance value?
Post by rfmagic on Nov 22^nd, 2010, 3:43am

I have simulated the impedance loading the first stage of the LNA - take a look at the attached schematic. C4 represents Cgs3 in the paper you attached, SRC2 and R4 represent the current source IL and the rest of the components are simplified model of the active inductor and the series decoupling components. I have taken aproximate values just to feel the circuit and it seems that you can get a peak in your band and control it very easily with the active inductor inductance and the series Cc and Rc.

Title: Re: How to choose capacitance value?
Post by saam on Nov 23^rd, 2010, 4:37am

Hello
Thank You so much for analysis given, and in these days there is some network problem and away from internet, so unable to give reply.. sorry for that..
I analyzed with the circuit which you have given, in that high frequency peak can be controlled using only active inductor inductance because i don't have Rc in my circuit.
but here my inductor is giving high value of inductance ie 9n-18nH in 2-6GHz frequency range and i have used decoupling capacitor as 1nF at both input and output. Is this gives any effect??

Title: Re: How to choose capacitance value?
Post by rfmagic on Nov 24^th, 2010, 1:37am

I am not sure I understand your question but I will try to answer anyways.
You can simply raplace the actice inductor in your circuit with a simplified model and then you can sweep the inductance of the active inductor and see the effect.
if you want to get a good feeling about the active inductor model you may reffer to
"CMOS active inductors and transformers principle implementation and applications" by Fei Yuan

Title: Re: How to choose capacitance value?
Post by saam on Nov 27^th, 2010, 7:24pm

Ya i already hv dat book.
And i got solution for my question,,that lower frequency undesired band, it depends upon the frequency of blocking capacitance at the input and coupling capacitance. And if u select proper values of that capacitance it was suppressing the peak. Now my circuit is working properly.
and thank U so much for your valuable analysis given, it helps me to think in that way.

ThankU