Just as an amplifier is a device that provides amplification , the process is defined as an increase in AMPLITUDE. Amplitude is a term that describes the size of a signal.
In terms of a.c., amplitude usually refers to the amount of
voltage or current. A 5-volt peak-to-peak a.c.signal would be larger in
amplitude than a 4-volt peak-to-peak a.c. signal.
"SIGNAL" is a general term used to refer to any a.c. or d.c. of interest in a circuit; e.g., input signal and output signal. A signal can be large or small, ac. or d.c., a sine wave or nonsinusoidal, or even nonelectrical such as sound or light.
"Signal" is a very general term and, therefore, not very descriptive by itself, but it does sound more technical than the word "thing". It is not very impressive to refer to the "input thing" or the "thing that comes out of this circuit.
" Perhaps the concept of the relationship of amplifier-amplification-amplitude will be clearer if you consider a parallel situation (an analogy).
A magnifying glass is a magnifier. As such, it provides magnification which is an increase in the magnitude (size) of an object. This relationship of magnifier-magnification-magnitude is the same as the relationship of amplifier-amplification-amplitude.
The analogy is true in one other aspect as well. The magnifier does not change the object that is being magnified; it is only the image that is larger, not the object itself.
With the amplifier, the output signal differs in amplitude from the input signal, but the input signal still exists unchanged. So, the object (input signal) and the magnifier (amplifier) control the image (output signal).
An amplifier can be defined as a device that enables an input signal to control an output signal. The output signal will have some (or all) of the characteristics of the input signal but will generally be larger than the input signal in terms of voltage, current, or power.
For more on the concepts and theory click on
any of the tutorial links below.
Classification of amplifiers starting with Voltage and Power amps
Frequency response of amplifiers
The four classes of amplifiers
Stages of Amplifier Coupling
Direct Coupled Transistor Amplifier
RC Coupling in a Transistor Amplifier
Impedance coupling in a Transistor Amplifier
Transformer Coupling of a Transistor Amplifier