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small pF capacitors - their uses?

Discussion in 'Electronic Basics' started by andrew_h, Feb 12, 2006.

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  1. andrew_h

    andrew_h Guest

    Sorry for the very basic question - I am currently learning

    What is the purpose of these tiny, pF capacitors? like 47 pF? What are
    they commonly used for?
  2. Ken

    Ken Guest

    High frequencies.
  3. andrew_h

    andrew_h Guest

    As a high-pass filter sort of thing, i.e. to eliminate them? ('absorb'
  4. Pooh Bear

    Pooh Bear Guest

    Stability compensation in op-amp circuits quite often.

  5. PeteS

    PeteS Guest

    They are used anywhere we need a small capacitance (obviously) even at
    low frequencies.

    Two that come to mind at relatively low frequencies are in the
    compensation loop of a switchmode power supply where the dominant pole
    is set by (primarily) the output of a current source, which means it
    has a very high output resistance - the supply is switching at about
    300kHz and the compensation cap is 33pF. (That's not the only
    compensation component there).

    Another is the loading caps on crystals (as part of a crystal
    oscillator) as low as 1MHz (perhaps lower) - typical values for this
    are in the range of 10pF to 33pF.

    There are a lot of reasons for using small caps - as always, it's
    application dependent.


  6. andrew_h

    andrew_h Guest

    Actually, I was looking at a 27 Mhz RF transmitter my father had made
    in the late 70's (discussed in another post).

    That had alot of small caps thinking about it - very often connected to
    a transistor.

    Would they have been loading caps for the 27.445mhz crystal?
  7. PeteS

    PeteS Guest

    Possibly, but more likely they were part of the RF system. Opportunity
    abounds for a 47pF cap in a 27MHz transmitter


  8. Guest

    Normaly this type of cap like disc shape, i use this disc cap ( because
    it is cheaper and save my money) to make my 6W FL. power by dry far no problem. But they ( the commercial ) use polyester film
    cap in this circuit, reason not know
  9. They are used in circuits where currents are very small, and/or where
    things happen very fast.

    An example of a circuit with very small currents might be an
    integrator for the photo current from a diode with the image of a star
    focused on it. An example of a circuit where things happen fast might
    be filter tuned to 100 MHz or a logic delay timer with a delay of a
    few nanoseconds.
  10. Tim Williams

    Tim Williams Guest

    I used a small, well somewhat larger but still small (220pF) capacitor, with
    a pair of 22k resistors, to add a small phase shift before a phase detector.
    This fixed the problem of having the two input signals very close in phase
    at high frequencies, since a capacitor shifts phase as frequency goes up.

  11. andrew_h wrote...
    The most recent small capacitor I have designed was a precision
    stable 0.1pF unit serving as the gain-setting feedback element in
    a capacitance bridge. It consists of a small hole in a shield.
  12. Tim Williams

    Tim Williams Guest

    I can do you one better. My 6AU6's are rated at 0.005pF or so, grid to
    plate. If you shield the pins at the socket.


  13. Rich Grise

    Rich Grise Guest

    Typically at higher frequencies, yes, but they're commonly used for
    thousands of things, where you only need a little capacitance. One notable
    usage is to provide the proper loading for the crystal in a crystal
    oscillator. Another is RF bypass in RF amplifiers - that's basically
    a power supply filter. Another is in tuned circuits, another is filters,
    another is coupling, and so on.

    Keep reading, though! The question shows that you're thinking, and that's
    a _GOOD_ thing! :)

  14. Rich Grise

    Rich Grise Guest

    Uh, isn't that what the "screen grid" is for? ;-)

  15. Tim Williams

    Tim Williams Guest

    Suppressor, too. Have to get the wires out of the envelope though :)

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