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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
    electronics!

    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'
    them??)
     
  4. Pooh Bear

    Pooh Bear Guest

    Stability compensation in op-amp circuits quite often.

    Graham
     
  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.

    Cheers

    PeteS
     
  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

    Cheers

    PeteS
     
  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
    cells.so 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.

    Tim
     
  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.

    :eek:)

    Tim
     
  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! :)

    Cheers!
    Rich
     
  14. Rich Grise

    Rich Grise Guest

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

    Cheers!
    Rich
     
  15. Tim Williams

    Tim Williams Guest

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

    Tim
     
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