# What does a transistor amplify?

hi guys it’s been days i am searching and reading articles and books to find out what is the factor that a transistor amplify? which leg of transistor does provides this amplified factor?

i know what is bias

i know bjt and fet

i know pnp and npn

i know transistor can be used as switches

i know the bel guy and the other guy before him that created transistors

i know transistor controls current through input voltage signal applying on base

the only thing i can not understand is the factor that gets amplified and gets bigger in the transistor i don’t know which leg of transistor provides this? i did some test and saw led turns on with just bit of light through only my fingers with no power supply at all with a transistor and i assume its current that gets amplified thank you

• It's beneficial to the reader, to include information that is relevant to your direct question. The first and and last paragraphs are the only useful bits to your question. While its great that you know what bias means, it doesn't help the reader a whole lot to answer your question. Keep it in mind. – efox29 May 18 '16 at 7:53
• What may confuse you is the work "amplify". The transistor itself does not amplify anything by itself. But it allows you essentially to switch a higher power path by driving a low power one. – Rev1.0 May 18 '16 at 7:56
• Here is a similar discussion which - perhaps - can give you some additional information about how the BJT is controlled (voltage or current): electronics.stackexchange.com/questions/234933/… – LvW May 18 '16 at 13:31
• No offense, but you don't "know" most of these things. You may have heard of the concepts, but you can't "know pnp and npn" and still ask a question like this... – pipe May 22 '16 at 7:08

There is no "leg of a transistor" which provides amplification. A transistor behaves in a certain way and that when used in a circuit while the transistor is biased can provide signal amplification.

The most important property of a transistor is that it has power amplification. You already see that when a transistor is used as a switch, a microcontroller's output is too weak to drive a relay for example but if you use a transistor that transistor provides the amplification you need to drive the relay. You only need a little power (which the microcontroller provides) to drive a load requiring more power (the relay).

A bipolar junction transistor (BJT) amplifies the base-emitter current into the collector-emitter current, so it's a current-controlled current-source.
It can come in NPN or PNP variants, for which the current directions are different.

A field-effect transistor (FET) amplifies the gate-source voltage into the source-drain current, so it's a voltage-controlled current source.
It can come in N-type or P-type variants, and again, the directions in which they work are different.

Lets assume you have a BJT NPN whose Emitter is connected to GND and positive power supply is connected to the collector then
the current into collector will by a constant factor $\beta$ (e.g. 100) times the current into base.

That factor $\beta$ will however vary quite a lot between one transistor and another, even if they are the same type; even for one particular transistor this "constant" will not be constant because it depends on temperature. That's why often additional measures have to be taken to get a constant amplification (e.g. negative feedback).

A transistor is a current controlled device. This means that you have the freedom to alter the current in any transistor circuit. I assume that you already know that the transistor can be configured into three different modes namely the common emitter, common collector and common base. For my explanation, I am going to consider the common emitter configuration because the other two modes are not essentially amplifiers.

In a common emitter configuration, the input in the base, collector is the output and the emitter is grounded (common to both input and output circuit). Now we have decided the mode of operation. Next we need to determine the region of operation by providing a DC bias to it. Since the transistor has three terminals, 4 different combination of biasing are possible. However, we prefer to bias it such that the emitter is forward biased and collector is in reverse bias. This ensures that the transistor is driven into the active region of operation.

Now after all these designing are complete, when we apply a small alternating signal to the base of the transistor (input in this case), the signal gets multiplied with the amplification factor that is decided by the transistor's current gain and the way in which it is biased.