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small pF capacitors - their uses?

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andrew_h

Jan 1, 1970
0
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?
 
K

Ken

Jan 1, 1970
0
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?

High frequencies.
 
A

andrew_h

Jan 1, 1970
0
As a high-pass filter sort of thing, i.e. to eliminate them? ('absorb'
them??)
 
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Pooh Bear

Jan 1, 1970
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andrew_h said:
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?

Stability compensation in op-amp circuits quite often.

Graham
 
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PeteS

Jan 1, 1970
0
Ken said:
High frequencies.

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
 
A

andrew_h

Jan 1, 1970
0
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?
 
P

PeteS

Jan 1, 1970
0
andrew_h said:
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?

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

Cheers

PeteS
 
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
 
J

John Popelish

Jan 1, 1970
0
andrew_h said:
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?

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.
 
T

Tim Williams

Jan 1, 1970
0
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
 
W

Winfield Hill

Jan 1, 1970
0
andrew_h wrote...
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?

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.
 
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Tim Williams

Jan 1, 1970
0
Winfield Hill said:
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.

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
 
R

Rich Grise

Jan 1, 1970
0
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?

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
 
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Rich Grise

Jan 1, 1970
0
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.

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

Cheers!
Rich
 
T

Tim Williams

Jan 1, 1970
0
Rich Grise said:
Uh, isn't that what the "screen grid" is for? ;-)

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

Tim
 
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