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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?

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    $\begingroup$ Shouldn't this be in electronics.stackexchange.com ? $\endgroup$
    – PixelArtDragon
    Commented Jan 23, 2013 at 8:46
  • $\begingroup$ @Garan this question is about the physics of an electronic device and is OK here. $\endgroup$
    – Larry Harson
    Commented Jan 25, 2013 at 17:25
  • $\begingroup$ Cross-posted from electronics.stackexchange.com/q/55791 after 4 minutes. $\endgroup$
    – Qmechanic ♦
    Commented Mar 8, 2013 at 0:22
  • $\begingroup$ 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. $\endgroup$
    – Qmechanic ♦
    Commented Mar 8, 2013 at 0:37

2 Answers 2

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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.

(Hint: TRANSISTOR = TRANSFER + RESISTANCE)

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.

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  • $\begingroup$ so, you mean that the input signal reduces the barrier between base and emitter which result in the amplification of current? $\endgroup$
    – Rafique
    Commented Jan 23, 2013 at 14:18
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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.

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  • $\begingroup$ This just barely answers the question. You should add more deatil about how the input voltage is used in the amplification process. $\endgroup$
    – Brandon Enright
    Commented Feb 22, 2015 at 18:37

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