Common Emitter Amplifier - Electron Flow and AC function with transistor

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

by Pandian
January 24, 2021

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.

by vanderghast
January 25, 2021

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

by Pandian
January 25, 2021

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

by Pandian
January 25, 2021

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 vanderghast
January 26, 2021

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.

by vanderghast
January 26, 2021

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)..

by vanderghast
January 26, 2021

Thank you very much for your prompt answer. great!

by Pandian
January 28, 2021

No Answers

No answers yet. Contribute your answer below!

Your Answer

You must log in or create an account (free!) to answer a question.

Log in Create an account

Go Ad-Free. Activate your CircuitLab membership. No more ads. Save unlimited circuits. Run unlimited simulations.

Search Questions & Answers

Ask a Question

Anyone can ask a question.

Did you already search (see above) to see if a similar question has already been answered? If you can't find the answer, you may ask a question.

About This Site

CircuitLab's Q&A site is a FREE questions and answers forum for electronics and electrical engineering students, hobbyists, and professionals.

We encourage you to use our built-in schematic & simulation software to add more detail to your questions and answers.

Acceptable Questions:

  • Concept or theory questions
  • Practical engineering questions
  • “Homework” questions
  • Software/hardware intersection
  • Best practices
  • Design choices & component selection
  • Troubleshooting

Unacceptable Questions:

  • Non-English language content
  • Non-question discussion
  • Non-electronics questions
  • Vendor-specific topics
  • Pure software questions
  • CircuitLab software support

Please respect that there are both seasoned experts and total newbies here: please be nice, be constructive, and be specific!

About CircuitLab

CircuitLab is an in-browser schematic capture and circuit simulation software tool to help you rapidly design and analyze analog and digital electronics systems.