the so called "ac" in bjt's?

[questron]

when bjt biasing, linear active region, the different bjt models, and what not about bjt's are talked about, the term "ac" is always used, such as "ac" signals, and "ac analysis". but, when i look close, there is nowhere in a bjt i could find "ac" currents, currents that actually reverse directions that is, thus a question: are bjt's a kind of devices that are only capable of "dc", and therefore not responsible for real ac currents in output signals at all?

the terms "ac" and "dc" become fuzzy here. i like the terms evb149 used in his posts better, "varying unipolar dc current" and "bipolar ac current", (re https://www.eevblog.com/forum/beginners/video-by-request-bjt-amplifier-topologies/).

evb149:
thank you for the confirmation! that's what i need to be sure about it. it is also most interesting when you mentioned "... and you capacitively couple that voltage then you can end up with bipolar AC since the capacitor will not pass DC, only the AC component." now i realize that it is the capacitor that switches a varying unipolar dc current in a bjt into a bipolar ac current for the next stage, since if current does not reverse directions, voltage across the capacitor cannot change. it is the capacitor that gives rise to ac currents, not the bjt. this is great, thanks a lot for the help!

T3sl4co1l:
"The absolute, instantaneous current can be reverse, but this would be under somewhat unusual circumstances, involving fast signals and junction capacitances, or voltages very different from those indicated." thank you T3sl4co1l, this is most interesting as well! i can sort of relate to it when fast signals and junction capacitance are involved, and would like to find out more about it. is there a research topic as such? i searched for "bjt ac current", and only found "bjt ac ananlysis" which does not involve bipolar ac currents in bjt's at all. would this - "instantaneous current can be reverse" - phenomenon be an annoyance to circuit design, or does it actually have some useful applications?

[Ratch]

when bjt biasing, linear active region, the different bjt models, and what not about bjt's are talked about, the term "ac" is always used, such as "ac" signals, and "ac analysis". but, when i look close, there is nowhere in a bjt i could find "ac" currents, currents that actually reverse directions that is, thus a question: are bjt's a kind of devices that are only capable of "dc", and therefore not responsible for real ac currents in output signals at all?

the terms "ac" and "dc" become fuzzy here. i like the terms evb149 used in his posts better, "varying unipolar dc current" and "bipolar ac current", (re https://www.eevblog.com/forum/beginners/video-by-request-bjt-amplifier-topologies/).

evb149:
thank you for the confirmation! that's what i need to be sure about it. it is also most interesting when you mentioned "... and you capacitively couple that voltage then you can end up with bipolar AC since the capacitor will not pass DC, only the AC component." now i realize that it is the capacitor that switches a varying unipolar dc current in a bjt into a bipolar ac current for the next stage, since if current does not reverse directions, voltage across the capacitor cannot change. it is the capacitor that gives rise to ac currents, not the bjt. this is great, thanks a lot for the help!

T3sl4co1l:
"The absolute, instantaneous current can be reverse, but this would be under somewhat unusual circumstances, involving fast signals and junction capacitances, or voltages very different from those indicated." thank you T3sl4co1l, this is most interesting as well! i can sort of relate to it when fast signals and junction capacitance are involved, and would like to find out more about it. is there a research topic as such? i searched for "bjt ac current", and only found "bjt ac ananlysis" which does not involve bipolar ac currents in bjt's at all. would this - "instantaneous current can be reverse" - phenomenon be an annoyance to circuit design, or does it actually have some useful applications?

AC simply means "alternating cycle" and DC means "defined constant".  Currents in a BJT don't reverse, but they can vary between cutoff and saturation, and can mimic any waveform within the frequency range of the BJT.

Ratch