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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 ?
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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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 ?
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.
It is effectively in parallel with the DUT
That pair are in series with the DC blocker.
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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