How is an transistor used as a amplifier?

Answer 1

Voltage is applied between the collector and emitter. A signal is applied between the base and emitter. The input signal will control how much the transistor turns on and the larger current flowing across the Collector/Emitter will be the same, but larger, than the input. Therefore amplified.
To keep the transistor switched on and to prevent the input signal switching it off, the transistor has to be biased on. This is usually done with a network of resistors on the base, raising the voltage to keep it conducting.

Answer 2

In forward active mode, transistor can amplify current and/or voltage signals. So here, it can be used as an amplifier.

To understand this read as follows...

There are four mode of operations for a transisitor.

1. Forward active mode.

2. Reverse active mode.

3. Cut Off mode.

4. Saturation Mode.

To understand the modes, lets consider a pnp/npn transistor. In forward active mode, Emitter-Base Junction is forward biased & Collectror-Base Junction is reverse Biased.

In Reverse active mode, Emitter-Base Junction is reverse biased & Collector Base junction is forward biased.

In Cut off mode, both the junctions are reverse biased.

In Satuiration mode, both the junctions are forward biased.

Answer 3

It amplifies current and voltage from other transistors and outputs go to inputs respectively and the gain increases and eventually to the speaker output, or whatever its used for.