BJT dynamics

Audioguru again

Joined Oct 21, 2019
5,427
The simulation shows that "'blocking" immediately does not happen maybe because the reverse emitter-base voltage of the second transistor causes it to conduct with avalanche breakdown and discharge the capacitor for each half-cycle.
I did not know that the Sim software knows about the reverse emitter-base breakdown.
 

Ian0

Joined Aug 7, 2020
6,727
I remember a debate with a colleague as to whether emitter-base avalanche was damaging to the transistor or not. We never did find out for certain, but decided to add another diode to stop it happening.
 

Audioguru again

Joined Oct 21, 2019
5,427
The datasheet for every transistor says, "Absolute Maximum Voltage" which means to NEVER exceed it.
I learned that causing an emitter-baser to have avalanche breakdown causes it to lose hFE.
 

Ian0

Joined Aug 7, 2020
6,727
The datasheet for every transistor says, "Absolute Maximum Voltage" which means to NEVER exceed it.
I learned that causing an emitter-baser to have avalanche breakdown causes it to lose hFE.
Learned from experience? Or learned from an explanation of the theory?
I seem to remember that we were trying to explain some odd partial failures when we discovered a BC212 was undergoing emitter-base avalanche breakdown (albeit current-limted), so that would fit your explanation of loss of hFE. We never did get to the bottom of it, as we added a reverse diode and the problem went away.
 

Wolframore

Joined Jan 21, 2019
2,579
This is why I asked you to try the pull up. Yes the cap can't discharge, but the way I look at is like this: Cap blocks DC so your AC no longer has any reference, you need a reference of it's indeterminate... as it fills the cap and can no longer pass the signal... it's saying the same thing but different perspective. I had this issue when I was trying to make an AM modulator last spring. Much stronger pulse width with a pull up.

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Audioguru again

Joined Oct 21, 2019
5,427
Digressing here, but isn't the avalanching of a transistor emitter-base junction reckoned to be an excellent noise source?
Yes, the same as an avalanching zener diode.

I think the circuit in post #27 destroys the transistor by its signal generator that has no output resistance. Then the base current pulse levels are unlimited.

The floating base shows what I said about the collector signal level slowly getting "blocked" as the capacitor charges.
 

Thread Starter

roxi60

Joined Sep 2, 2018
73
Thank you very much to all your contributions.
I simulated the functionality of the B_E junction with a diode clamper.
I used a square wave to see better what Audioguru again said about the discharging of the capacitor through the resistance, that make possible, during the rising of Vi, a start from higher voltage and a higher positive output voltage too at the end of the rising, that make possible the conduction of the diode.
I hope I understood well the principle behind.
Thank you.
Regards.

With resistor.JPGWithout resistor.JPG
 

Audioguru again

Joined Oct 21, 2019
5,427
Your input signal frequency is too low at 5Hz and its level is too low at 0.5V peak to show the capacitor causing peak detection on the positive swing, and is not showing the charged capacitor to cause the output to go fairly far negative on the negative swing, eventually not giving an output signal anymore when the capacitor becomes fully charged (the "blocking that was mentioned".
 

Thread Starter

roxi60

Joined Sep 2, 2018
73
Thank you.
I simulated with higher frequency and amplitude, but substantially the same behaviour.
Which are values that highlights the "blocking" you said, and where to look on the signal?
Thank you very much.
 

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