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Hello, I am not very professional in electronics, jus i am always wondering the flow of AC signal in NPN transistor, i tried to figure out but i confused to know how the transistor react while reversing electron flow through the emitter to base, and the same time what happens emittor to collector side.

I am considering Electron flow, not in conventional flow.

Thank you

In ClassA amplifier, there is a DC offset to which the signal is superimposed. Like a small wave on a lake, the amplitude of the signal to be amplified is smaller than the depth of the lake, or the DC offset, so that the TOTAL NET current is always in one way, the one established by the DC offset.

In ClassB amplifier, two transistors are used, one for each possible direction of the flow.

In general, if the net current can be in either direction (because there is no DC offset, or not large enough to be always in the same direction), once the Vbe or Vbc (Voltage from base to emitter, or base to collector) is high enough so that the diode conduct, (Vbe_on, Vbc_on, in some datasheet), then the transistor can be in saturation mode or in active mode. Generally, we use the FORWARD ACTIVE mode (Vbe > Vbe_on, and Vce > Vbe), with the "beta" value, but the REVERSE ACTIVE mode is possible too ( Vbc>Vbc_on and Veb >Vec), but the "beta" to be used is much much smaller and not tested, generally, by the manufacturer. For a P2N2222 as example, for the forward active mode, beta can be of 200, but only of 5 for the reverse active mode. That make the transistor quite innapropriate as symetry of the amplification is concerned and thus, that configuration is not very often seen for amplification.

Thank you Very much for your clarification, further in Class A, if out put flow is one direction means how the alternate current will be generated? if flow is DC then not possible to create EM Radiation.

i Hope there might be a way to split the DC output into amplitude strong AC signal. Could you please explain to me with any simple Circuit in order to understand.

Thank you

If am not wrong, as you said DC offset means DC audio information or Signal waves ?

if above question is right, I am wondering how this out put will be merged with carrier wave

Please explain me through some circuit diagram

If you put a DC source in series with an AC source, we get an AC signal on top of a DC signal (simply add them: Vdc + Vac * sin( omega * t ) = A0 + A1 sin(w t) ).

The trick would be to have Vdc > Vac, so that the sum is always positive, since then, the flow is always in the same direction. It just get slower (when the sin() is negative), then faster (when the sin() is positive), then slower, ... etc.

That is NOT practical, though, since all the DC signal would have to pass through the AC source which is often a sensor or an antenna, or some gadget which won't support such a voltage, or current intensity, and also, furthermore, your amplification would amplify every thing, including the DC component, not just the AC component.

That is why we often use a coupling capacitor: we put the AC signal behind it (put the AC source between the mass and the capacitor), so that the AC signal could pass through the cap, but the DC current, trying to reach the ground, will be blocked in that portion of the circuit (on its way toward the ground, if we consider the classical current that is), but the other side of that cap, we will get indeed, the sum of the DC and the AC signal.

There is surely such a circuit around, I'll take a look and give the reference... or will draw a rudimentary one if I don't find it.

By DC I mean a constant voltage source, like a 12V battery, or something similar. It is historically called a Direct Current source, while in fact, it is the voltage which is constant, not the current. Don't ask me why it got that name, I simply don't know.

Try: Audio-Amplifier. It is easy to spot the signal and the cap which forbid the passage for the DC toward the AC source. The tension divider in the middle is used to bring the DC offset. It is required (or another arrangement) when we use a SINGLE DC source and for the DC offset and for the AMPLIFICATION functionality (done here by the transistor)..

Thank you very much for your prompt answer. great!