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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, 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 rule-of-thumb 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, 03:15

- 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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