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    RF choke, why not turned-off MOS?

    Why we don't use a turned-off mosfet to be the rf choke, it provides
    very large resistance and small parasitics for rf application. But we rarely see
    this, why?

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    Re: RF choke, why not turned-off MOS?

    Gee, that is a great idea. Next time you need an RF choke that has extremely high dc resistance, feel free to use it!



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    Re: RF choke, why not turned-off MOS?

    But how about the settling time constant, while you power down or power up
    your circuit.



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    Re: RF choke, why not turned-off MOS?

    RF Chokes are meant to have Very High RF impedance and
    no DC impedance. They also need to be able to handle any
    where from 1mA to 10 Amps of current.
    An Inductor it the perfect candidate.



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    Re: RF choke, why not turned-off MOS?

    If you connect a rf choke to a gate of mos, it needn't to handle the dc current.



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    Re: RF choke, why not turned-off MOS?

    If there is no inversion layer in the channel, the DC bias of the substrate is equal to GND, so it is impossible to conduct the VB to RF_IN.



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    Re: RF choke, why not turned-off MOS?

    The Vb still can conduc to RF_in because of the leakage current. It's not really infinite
    ohms when a transistor turned of. It may be around Gohms.



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    Re: RF choke, why not turned-off MOS?

    Quote Originally Posted by hebu
    The Vb still can conduc to RF_in because of the leakage current. It's not really infinite
    ohms when a transistor turned of. It may be around Gohms.

    The voltage conducted to RF_in is divided by two huge resistors, one is the
    turned- off channel, and the other one is reversed p-n juction. So, we need
    to make sure the resistance of channel is smaller enough than junction
    resistance.



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    Re: RF choke, why not turned-off MOS?

    What is the advantage of an turned off MOS over a RF Choke? Can you get stand alone MOS devices to put in a power supply line? How much does it cost? How about static zaps?

    There are hundreds of ways to do an arbitrary design task. If you are proposing a new way to do something, there should be advantages that you can enumerate. Just because you can do something doesn't mean that you should.

    Dave



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