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BJT base-emitter and base-collector junction capacitance
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bonzer:
Hello everyone. Please help me to understand how to find the base-collector junction capacitance in my case. I'm working on an exercise that has an amplifier circuit with two BJTs, as you said previously here in another thread it's an A class and what I'm trying to understand is what would be in this case of a 2N2222 base-emitter and base-collector junction capacitance. I have the values of these zero bias capacitances and the formulas (I attached here two slides).  I use LTSspice for the simulation.
What I want to know is how to find their values in the case of this circuit at like 400kHz? (I need them to apply a method for the superior cutoff frequency estimation)
Allright - CJE is usally equal to 2*CJE0 so there's no problem with it (as shown in the slide).

---MODIFIED-------
And how about Cu? (base collector junction capacitance). I need to measure VCB. In small signal mode I have a control on frequency but it's all amplified so there're huge voltages as it's the differential mode component that we see. I measured it in transient time mode in LTSpice. I see at the second transistor (at right) 7.2V. But then we have negative denominator in the formula and it gives problems because there's a power (m) and even if ti wasn't we can't have a negative capacitance so please help me to understand what's the right way? What would you do?
bonzer:
It looks like these formulas up are wrong, for example for the base collector junction it should be like here in the attachment with a sum at the denominator and not substraction. Then it makes more sense.
In fact my project works well when using the values calculated with this formula attached.

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