Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

In transistor as an amplifier, we have NPN transistor (base in the middle and emitter and collector at the sides). The collector has electrons in majority because it is N-type. similarly, holes are in majority in base, but as it is lightly doped, it has very little amount of hole. Now, when a small signal of alternating current is supplied to the base, the base current changes and this change is called input current. most of this input current moves into the collector (base has less holes so most of the current moves on when it fills the holes). Now, the collector has its own majority charge carriers (electrons) and some more electrons are added to it by the input signal. So when collector gives current to the load, it will be the combination of input signal and the already present electrons. Hence, small signal is amplified.

Am I right or wrong?

share|cite|improve this question
Shouldn't this be in ? – Garan Jan 23 '13 at 8:46
@Garan this question is about the physics of an electronic device and is OK here. – Larry Harson Jan 25 '13 at 17:25
Cross-posted from after 4 minutes. – Qmechanic Mar 8 '13 at 0:22
Dear Muhammad Rafique. In general, it is frown upon to cross-post simultaneously, because it may waste potential answerer's time. As a minimum OP should mention the cross-post (on both sites!). The preferable procedure is not to crosspost, and if the post hasn't received an acceptable answer after, say, a couple of days, then OP could flag for migration. – Qmechanic Mar 8 '13 at 0:37

You have got the 'amplification' idea wrong.You may add thousands and thousands of electrons to the input signal. But if 'CHANGES' are of same value as input signal , you do not have amplification. You have just changed the voltage level.

The question is not very clear. For example it is not clear what configuration you refer to. I assume you have the common emitter configuration of npn transistor in mind.

The transistor amplifies the 'CHANGES' in signal. Think of the BASE as a sort of 'CONTROL'. For npn transistor, the base signal changes number of holes available. You can imagine that the base that controls the resistance between emitter and collector.


A base signal changes the current flowing through emitter collector terminals. By controlling the amount of current/electrons that flows between emitter and collector in a manner proportional to the input current you get amplification.

share|cite|improve this answer
so, you mean that the input signal reduces the barrier between base and emitter which result in the amplification of current? – Rafique Jan 23 '13 at 14:18

Yes, since more electrons are flowing, there is a greater flow rate of charge; aka, current; and so the signal is amplified. The only thing is, the main voltage with the collector does not add to the small current that passes through. Rather, the voltage is there to keep the electrons moving after they go through the depletion zone of the transistor.

share|cite|improve this answer
This just barely answers the question. You should add more deatil about how the input voltage is used in the amplification process. – Brandon Enright Feb 22 '15 at 18:37

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.