Electronic – How are C(OBO) and C(IBO) defined

datasheettransistors

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?

Best Answer

\$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.