small pF capacitors - their uses?

High frequencies.

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

Stability compensation in op-amp circuits quite often.

Graham

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

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

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.

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

-- Deep Fryer: a very philosophical monk. Website:

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.

--
 Thanks,
    - Win

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.

:o)

Tim

-- Deep Fryer: a very philosophical monk. Website:

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

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

Cheers! Rich

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

Tim

-- Deep Fryer: a very philosophical monk. Website: