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# How to define the Capacitor value for DC block?

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#### John_li

##### Full Member level 5
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

#### vfone

##### Advanced Member level 6
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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### ant_ebg

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#### boy

##### Advanced Member level 1
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.

ant_ebg

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### ant_ebg

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#### sergio mariotti

##### Advanced Member level 1
blocking capacitor calculation

The free software Agilent AppCad performs that calculation in a exaustive and simply to understand manner.

### John_li

Points: 2

##### Member level 4
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?

#### MobiNaz

##### Full Member level 3
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

#### enjunear

##### Advanced Member level 3
Re: dc blocking cap

Vfone could you please provide references, I want to quote them in one of my paper. Thanks

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.

#### MobiNaz

##### Full Member level 3
There is always theoretical or practical evidence behind a statement.

#### enjunear

##### Advanced Member level 3
There is always theoretical or practical evidence behind a statement.

Of course you can determine the circuit values from the fundamental equations, but the "why" is often learned from practice/experience (and it's usually obvious that you don't want to put large, unwanted parasitics into your nice, ideal design).

#### MobiNaz

##### Full Member level 3
I agree, thanks for helping.

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