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21st August 2005, 03:24 #1
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dc blocking capacitor
Hi all,
I want to design a RF AMP.(freq.1900MHz),and how can i define the capacitor value for DC block and inductor value for DC feed? What is the rule to select this type component for DC?
Thanks
John

21st August 2005, 03:24

21st August 2005, 06:54 #2
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dc block capacitor
In general DC blocking capacitor shall behave like a short at working frequency
Calculate the reactance in ohms of the DC blocking capacitor for a minimum value at your working frequency:
Usually Xc(ohms) = 1/(2*3.14*f*C) shall be less than 2 ohms at working frequency.
where
f = frequency in Hz
C = Capacitance in Farad
For example 100pF at 900MHz has Xc = 1.8ohms
Another option for DC blocking cap is to choose a capacitor with SRF at the working frequency.
The RF choke is a circuit element designed to present high impedance to RF energy while offering minimal resistance to direct current.
Usually the choke reactance XL(ohms) = 2*3.14*f*L shall be greater than 500ohms at working frequency.
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21st August 2005, 16:28 #3
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dc blocking capacitor value
You also need to look at self resonance frequency of the capacitor also. The operating frequency must be less than self resonance frequency otherwise the capacitor will act as an inductor.
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21st August 2005, 16:28

23rd August 2005, 09:43 #4
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blocking capacitor calculation
The free software Agilent AppCad performs that calculation in a exaustive and simply to understand manner.
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23rd August 2005, 09:43

28th August 2005, 09:04 #5
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dc blocking cap
there r two opinions about SRF dc block capacitor?
which one is the better , SRF at operating frequency or SRF higher than operating freq?

14th March 2011, 02:49 #6
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Re: dc blocking cap
SRF higher would be better as all caps have tolerances. It will ensure that the capacitor has capacitive effects rather then inductive at operating frequency.
 Post added at 02:49  Previous post was at 02:44 
Vfone could you please provide references, I want to quote them in one of my paper. Thanks

14th March 2011, 02:49

14th March 2011, 04:24 #7
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Re: dc blocking cap
You might be able to find language like that in some amplifier cookbook's... but in general, those concepts are something taught in school, and are known to those "skilled in the art" of RF design. They are generally learned from senior designers as a ruleofthumb by most RF people that I know.

14th March 2011, 06:02 #8
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Re: How to define the Capacitor value for DC block?
There is always theoretical or practical evidence behind a statement.

15th March 2011, 03:15 #9
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Re: How to define the Capacitor value for DC block?

15th March 2011, 04:14 #10
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Re: How to define the Capacitor value for DC block?
I agree, thanks for helping.
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