B C pF Cap

[Phend]

Hello,
I see on a few circuits, a small cap in the pF range, connected across a transistors B C leads.
Is this for noise reduction ? What do diffent values do ie 47 vs 330 pF ?
Thanks for any discussion.

[mozz]

Tames the high end a bit. Old germanium had 50pf or more and usually silicon is very low 1pf-10pf which allows for higher frequency usage. Some germanium was limited to 500kHz do to the capacitance where silicon can do 100's of Mhz. I can measure it on my LCR, some Japanese germanium are very consistent while US/Euro can be all over the place.

Some datasheets or the better manuals will list the capacitance spec.

[antonis]

Search for " Miller capacitor compensation".. 

[PRR]

Warning: this topic has not been posted in for at least 120 days.
Unless you're sure you want to reply, please consider starting a new topic.

TESTING replying to a very old stale thread. September 30, 2023 to November 1, 2024.

...seems to work OK?

[GibsonGM]

Visible in this part of the state, Paul...

[fryingpan]

Since this thread is just some sort of experiment.




I don't think you can get any more cyberpunk than that.

(I never know how to properly support veroboard, so I just came up with this).

[amptramp]

You don't need much in the way of B to C capacitance because the capacitance appears to be increased by the gain of the transistor stage.  Suppose the stage has a gain of 5 and you have a 10 pF capacitor.  As the input increases by one volt, the output decreases by 5 volts and the effect seen at the input is that you have six times the capacitance that the capacitor is set for.

You can use a variable gain stage to make a capacitor appear to be larger or smaller in a circuit:



This is an old circuit called the Magic monitor by RCA who installed it in some top-of-the-line radio - record players in the 1940's and 1950's.  The idea here is that the variable gain tube, the 6BA6, has a 27 pF capacitor from the plate to the output and there is another 1000 pF capacitor feeding the audio input to its grid.  When the tube gain is high, the capacitance appears to be high and this circuit filters out the highs to achieve lower noise.  It is fed by an amplifier - detector stage that senses the level of the audio and produces a negative control voltage from it.  When the signal level is high, the negative voltage reduces the 6BA6 gain and the capacitance is small.  When the signal drops, the 6BA6 stays at full gain and the effective capacitance is large, reducing the bandwidth of the signal and limiting noise to the lower bandwidth.

As Solomon once said, there is nothing new under the sun.

[antonis]

the capacitance appears to be increased by the gain of the transistor stage.

Plus unity..
(many people tend to forget/ingore the way Miller effect comes up - no big deal for high gain stages but, as in your example, 6 is 20% bigger than 5..)

It may look negligible but not in case of that capacitance has to be taken into account for LPF calculation and alike..

[GibsonGM]

Since this thread is just some sort of experiment.




I don't think you can get any more cyberpunk than that.

(I never know how to properly support veroboard, so I just came up with this).

I see nothing at all wrong with that. Then, I see a lot of people fill up rotted rocker panels with spray foam.       Hey, if it does the job, and it's not some fire hazard (at 9V, no), have at it!