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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

    •   Alt21st August 2005, 03:24

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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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    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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    •   Alt21st August 2005, 16:28

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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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    •   Alt23rd August 2005, 09:43

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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?



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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



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    Re: dc blocking cap

    Quote Originally Posted by MobiNaz View Post
    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.



    •   Alt14th March 2011, 04:24

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    Re: How to define the Capacitor value for DC block?

    There is always theoretical or practical evidence behind a statement.



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    Re: How to define the Capacitor value for DC block?

    Quote Originally Posted by MobiNaz View Post
    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).



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    Re: How to define the Capacitor value for DC block?

    I agree, thanks for helping.



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