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Help requested to identify common emitter, collector base bjt circuits

iouae

May 14, 2023
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May 14, 2023
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I skimmed through the thread and did not see this: Back to the original post, #5 is incorrect. #5 is common collector, just like #2.

There are pedantic, semantic, and qualifying issues with each statement, but I see the kind of truth table you are trying to construct. All of the subtleties of real circuit design notwithstanding, the table is ok for guidance.

Something you might add are the (low / medium / high) input and output impedances of each of the three circuit classes. Very handy in figuring out someone else's design decisions.

ak
Thanks for the correction AnalogKid. Yes #5 is wrong. My new general rule which replaces all the ones in the original post is the pin of the transistor with the least changing voltage is the common pin. If it is an out pin it would be changing.
 

iouae

May 14, 2023
24
Joined
May 14, 2023
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24
My understanding, and I may need correcting here, BUT....as your diagram shows the load below the transistor emitter, as the transistor starts to turn on, it's emitter voltage would start to rise, turning the transistor off again.

These days, it's simpler, easier, quicker, just to use a mosfet.
No real calculations for base current, wattage dissipation etc. etc. as long as there is sufficient current handling capacity.
If using with uC use logic level.
I have hundreds of bjt's and only a handfull of mosfets, because they are more expensive. But I want to live in a mosfet world and plan to use a lot more mosfets in future.

And I do believe that using a load like a motor which keeps changing, if this is attached to an emitter, would keep changing the emitter voltage and in so doing turning the transistor on and off, because it is the voltage of the base RELATIVE to the emitter which turns a transistor on. The voltage of the base relative to the collector does not matter, therefore the motor is attached to the collector. I think that's so.

I wonder if Spice can accurately simulate a motor's changing load, and whether it would accurately simulate the differences between common e and common c when connected to a motor?
 
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