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Old 31st March 2009, 11:19 AM   #1
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Default Are there some problems if i decide to use two 100pf capacitors to obtain 50pf


I'm needing to make some experiences into my standard amplifier, some tweaks, and i have not small values of capacitors, i have only ceramics...and...for this purpose ceramics are not a very good idea.... in special as Miller, or compensation capacitor into the Voltage Amplifier Stage.... so...there are many others i can use made with other dieletric materials to test..but not with the small values i need...from 5 pf to 27pf.

Are there problems to do this?... say, to use them in series to reduce value?... some consequence?... some strange side effect?

Say... some strange inductive, resistive or capacitive effect because size or something?... are there Radio Frequency problems related that stuff.... i remember that series condensers were not used in Radio Frequency transmitters, we use to substitute them for the good value... i can remember that i have used, many times, this series capacitor mid point as an impedance divider....... and the antenna, the aerial, for transmitting pursposes, were soldered into this point.

I am producing question because i really do not know if there are any kind of problems with this.

The question is more directed to scientist alike Roender (Mihai)and others, but forum folks may know something i do not know.

My worries are related to physics...the science that studies the physics of solids.

regards,

Carlos
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Old 31st March 2009, 11:46 AM   #2
roender is offline roender  Romania
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Hi Carlos,

Where do you wanna use this series capacitors and why NPO ceramics are not good (those have a black dot on the capsule)?

Regards,
Mihai
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Old 31st March 2009, 11:49 AM   #3
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Default Some folks have said the ceramic capacitor react when facing variable levels


...that they are very good into oscilators where you have fixed frequency and fixed level into the carrier....but they change value in accordance to the potencial into the dieletric.

Well.... i have just accepted those informations from forum skilled folks.. but i have never watched that effect behavior..so.. i cannot guarantee nothing.... i am just repeating the same way parrot does.

I have those black dots and red dots too.....i have thousands of nice ceramics units.... and all them tested and guarantee..were used into the past...came from junk...the guaranteed skilled components..were used and passed all tests during operation.

ahahahahahh!

regards,

Carlos

.................................................. ............

But dear Mihai...please....answer big fat Charlie question in the place to make another.
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Old 31st March 2009, 12:00 PM   #4
roender is offline roender  Romania
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Dear Carlos,

The NPO ceramic is made from a very stable dielectric material, almost at the same quality level as our beloved silvered mica capacitors.
Now, if you are forced to use a series combination of identically small values capacitors, the only precaution you must take is to use the same voltage tag for both if you use them in VAS as Miller compensation. When you'll get the right value, replace the combined capacitor.
IMHO, the RF effect is to small to mater ...

Cheers,
Mihai
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Old 31st March 2009, 12:10 PM   #5
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i have a circuit i use with my dummy loads to get a 10:1 reduction in voltage going into an o-scope input. i didn't have variable trimmer caps available so i just "kluged" a few 5pf caps together in series and parallel combinations until i got the right compensation for the scope input. works just fine but takes up a bit more space than a trimmer cap.
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Old 31st March 2009, 12:39 PM   #6
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Air is the best dielectric. Why not use a tiny trimmer?

Or, for a hard-core approach:
gimmick
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Old 31st March 2009, 01:01 PM   #7
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Default Thank you Roender...so... i gonna use my ceramics ones


Unclejed and traderbahn

I think this trimmer option is nice too.

thank you folks.

regards,

Carlos

.................................................. ........

I have used those twisted wires for low capacitance..they are fine..but you have to fix using something...because if it bent or vibrates or turn near the ground the value will change.

Also air core capacitores have problems with humidity too.
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Old 31st March 2009, 01:25 PM   #8
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I assume you already know about the different dielectric effects on linearity. The better dielectrics come in larger values and sizes. I would say the primary consideration when putting caps in series is the doubled inductance. This may or may not matter in you particular circuit; I'm sure you have thought of this.
BTW, it is possible, if you are cunning as a fox, to combined two caps of different dielectric materials in order to achieve a more linear combination.
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Old 31st March 2009, 02:30 PM   #9
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Default Oh!... double inductance...this is interesting


so i gonna have twice the inductance....are there more effects?

I do not know everything.... just something.... it has always something i have never realised.

thanks,

Carlos
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Old 31st March 2009, 03:21 PM   #10
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Any real capacitor has parasitic inductance and resistance. In simplest form, these appear in series with the cap. So every cap is really a C R L series circuit. If you Google for "capacitor parasitics" you'll find more detailed info.

When you put two identical caps in series you halve the C and double both L and R. When you put two identical caps in parallel, you double the C and halve the L and R.

The R will usually be negligibly small. The L will depend upon the physical size and shape of the cap. There are other parasitics that may be important, especially leakage R in electrolytic caps, but you don't need to worry about this for such small caps. I suppose humidity will introduce a parasitic R in a trimmer in Brazil

A straight bit of thin wire is an inductor and has, very roughly, 5nH/cm. You can estimate the cap inductance from the average length of the signal path through the two caps. Note: inductance is increased much more if the signal path forms a loop: the smaller the loop area, the less L - so how you physically arrange the two caps matters.

Does this make sense?

I am curious as to why you are interested in using such low capacitance values. Transistor junction capacitance is in the region you are interested in and it is extremely non-linear. Valves are a different matter.

Brian
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