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Old 10th January 2012, 02:42 AM   #1
Bigun is offline Bigun  Canada
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Default measuring Cob ?

I have some NPN BJTs from ebay, it's not easy to be sure they are genuine. They are known for their low Cob. Question is, can Cob be measured with a simple capacitance meter ?
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Old 10th January 2012, 07:49 AM   #2
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by Bigun View Post
Question is, can Cob be measured with a simple capacitance meter ?
It can, but the value won't be very accurate: ordinary capacitance meters apply an average measurement voltage of 2~3V, and the Cob is specified at a larger voltage, generally 10V.
This means the capacitance measured this way will be somewhat over-evaluated, but it will be in the right range though.

PS:
It is important to respect the polarity, otherwise the reading will be meaningless.
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Old 10th January 2012, 11:42 AM   #3
Bigun is offline Bigun  Canada
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I could bias the device with a higher dc voltage, then measure Cob with a passive capacitor in series wit the device under test. I would measure the passive device separately and back it out of the measured data. What do you think ?
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Old 10th January 2012, 11:51 AM   #4
AndrewT is offline AndrewT  Scotland
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How do you allow for the capacitance of the bias source?
It is effectively in parallel with the DUT
That pair are in series with the DC blocker.
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Old 10th January 2012, 12:03 PM   #5
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by Bigun View Post
I could bias the device with a higher dc voltage, then measure Cob with a passive capacitor in series wit the device under test. I would measure the passive device separately and back it out of the measured data. What do you think ?
Maybe, but in my experience, rudimentary capacitance meters hate the slightest parallel parasitic resistance, and even a bias tee with R= some megohm is sufficient to completely disrupt the measurement.

You should first try your bias arrangement on a known, fixed capacitor of 22pF for example. If it is coherent, you may go on with your real measurement.
Otherwise, use a tantalum capacitor precharged at 10V in series, or even a 9V battery (be careful not to mess up the polarity, otherwise you will fry your meter)
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