Analog circuits are electronics systems with analog signals with any continuously variable signal. While operating on an analog signal, an analog circuit changes the signal in some manner. It can be designed to amplify, attenuate, provide isolation, distort, or modify the signal in some other way. It can be used to convert the original signal into some other format such as a digital signal. Analog circuits may also modify signals in unintended ways such as adding noise or distortion.
There are two types of analog circuits: passive and active. Passive analog circuits consume no external electrical power while active analog circuits use an electrical power source to achieve the designer's goals. An example of a passive analog circuit is a passive filter that limits the amplitude at some frequencies versus others. A similar example of an active analog circuit is an active filter. It does a similar job only it uses an amplifier to accomplish the same task.