The amplifiers are circuits that are used to increase (to amplify) the value of the input signal (usually very small), thus you can obtain an output signal with much greater amplitude than the original one.
Amplifier Symbol
Sometimes the amplification can cause a distortion of the signal on the output of the amplifier since there is a very large amplification.
We must take into account that an amplifier can not have on its output a higher level of voltage, than the power supply has.
For Example: If an amplifier has a 12 Volts power supply, the output signal can not have a voltage greater than this.
Note: To increase the output power of an amplifier, we have to increase the amount of current it can supply (for a fixed voltage power source). Remember that:
P (power) = V (voltage) x I (current)
If we can not increase the voltage, we must increase the current.
The common amplifiers are made of bipolar transistor, but there are others that use operational amplifiers, electronic valves, FETs, etc..
In the case shown in the diagrams, the output signal (see the lower diagram) is greater than the input signal (upper diagram), but additionally the signal is inverted.
This case occurs in amplifiers, that are called inverter amplifier.
