The circuits this section is concerned with are amplifiers. Amplifiers are devices that provide amplification. That doesn't explain much, but it does describe an amplifier if you know what amplification is and what it is used for.
What Is Amplification?
Just as an amplifier is a device that provides amplification, amplification is the process of providing an increase in amplitude. Amplitude is a term that describes the size of a signal. In terms of AC, amplitude usually refers to the amount of voltage or current. A 5-volt peak-to-peak AC signal would be larger in amplitude than a 4-volt peak-to-peak AC signal. "Signal" is a general term used to refer to any AC or DC of interest in a circuit; e.g., input signal and output signal. A signal can be large or small, AC or DC, 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.
Uses of Amplification
Most electronic devices use amplifiers to provide various amounts of signal amplification. Since most signals are originally too small to control or drive the desired device, some amplification is needed.
For example, the audio signal taken from a record is too small to drive a speaker, so amplification is needed. The signal will be amplified several times between the needle of the record player and the speaker. Each time the signal is amplified it is said to go through a stage of amplification. The audio amplifier shown connected between the turntable and speaker system in the figure below contains several stages of amplification.
Notice the triangle used in the figure above to represent the amplifier. This triangle is the standard block diagram symbol for an amplifier.
Another example of the use of an amplifier is shown in the figure below. In a radio receiver, the signal picked up by the antenna is too weak (small) to be used as it is. This signal must be amplified before it is sent to the detector. (The detector separates the audio signal from the frequency that was sent by the transmitter.)
The audio signal from the detector will then be amplified to make it large enough to drive the speaker of the radio.
Almost every electronic device contains at least one stage of amplification, so you will be seeing amplifiers in many devices that you work on.