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In a datasheet like this one, how are \$C_{OBO}\$ and \$C_{IBO}\$ defined?

enter image description here

What I know:

  • C for capacity
  • O for output, respectively I for input
  • B for base
  • O for open, not connected

But with regard to what circuit are in- and output defined? In other words, where can this capacitor be visualized and what is the rationale behind it?

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3 Answers 3

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\$C_{ibo}\$ and \$C_{obo}\$ are composed of parasitic capacitances. A parasitic capacitance is an unavoidable property of all transistors because their terminals are made of conducting materials in very close proximity to each other. Parasitic capacitances start to affect performance when the transistor is operated at high frequencies, which is why they are minimized as much as possible.

A BJT has several parasitic capacitances between all three of its terminals.

Parasitic capacitances of BJT

The input and output capacitance are defined as below according to this application note from Infineon:

cibo cobo

As for the terminology, base means that the capacitance is measured with respect to a common-base configuration, where the input is at the base of the transistor and the output is at the collector.

common_base

Input/Output denotes which terminal the capacitance is being measured from. Open/Short denotes whether or not the output - the terminal opposite to that of the terminal where the capacitance is measured from - is an open circuit or a short.

You can read the application note for more capacitance definitions.

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  • \$\begingroup\$ I guess "base means that the capacitance is measured with respect to a common-base configuration" is the key sentence here. \$\endgroup\$
    – jippie
    Commented Feb 1, 2015 at 19:07
  • \$\begingroup\$ Is there a typo in the equation for (C_obo)? Why is (C_cb) never used? \$\endgroup\$ Commented Feb 17, 2021 at 9:30
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    \$\begingroup\$ See David 's answer below. \$\endgroup\$ Commented Feb 17, 2021 at 18:17
  • \$\begingroup\$ Thanks! I can't believe that such an obvious error would stay published in an app note like that for so long. \$\endgroup\$ Commented Feb 18, 2021 at 2:42
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Comments: Infineon's equation for Cobo is obviously incorrect and should be \$C_{obo} = C_{cb} + \frac{C_{ce} \cdot C_{eb}}{C_{ce} + C_{eb}}\$. And I suppose a big reason for using grounded base config for these parameters is that the equations become easier (grounded emitter would cause miller effect to come into play), not because people so often actually use grounded base setups. I'm thinking that in an ordinary grounded emitter setup, where one rather wants to know cbe and cbc, one could approximate it to \$C_{be} = C_{ibo}\$ and \$C_{bc} = C_{obo}\$. At least that's my reasoning for it to make any sense.

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The above answers are incorrect for use BJT hybrid-pi model in the engineering.

you could find the detailed answer in the following referece:

Book Name: Intuitive Analog Circuit Design Page 83-93 all BJT parameters you needed which could be obtained from the datasheet

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    \$\begingroup\$ This answer would be better, if you could elaborate a little on what that book tells us. As it is standing, the answer is not offering a lot of information. \$\endgroup\$
    – jusaca
    Commented Mar 5, 2021 at 7:15

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