measuring Cob ?

[Bigun]

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 ?

[Elvee]

Originally Posted by Bigun

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.

[Bigun]

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 ?

[AndrewT]

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.

[Elvee]

Originally Posted by Bigun

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)