To figure out digital circuits analog circuits, you must first figure out what a digital signal is and what an analog signal is. The natural signals that can be found everywhere are analog signals. Analog signals are continuous in time and in value, and are drawn as a continuous curve that completely “simulates” the natural signal. Digital signals are discontinuous in time and in value. The digital signal is “sampled”, the value of the digital signal can only change at the sampling point. The digital signal is “quantized”, the value of the digital signal can only be taken to some discontinuous fixed values, such as 0,1,2,3, not to the middle 1.1, 1.2, 1.428571…… digital signal used in digital circuits Generally, only 0 and 1 can be taken.
Then it’s simple. A circuit that uses and processes analog signals is an analog circuit; a circuit that uses and processes digital signals is a digital circuit. Circuits that process both analog and digital signals (such as digital-to-analog converters and numerical control oscillators) are called mixed digital-to-analog circuits, but they are generally classified as analog circuits if they are forced to be dichotomous. MOSFETs, BJTs, and even vacuum tubes can be built on both digital and analog circuits. Integrated circuits in the analog and digital circuits are integrated in the same chip, they use the same basic components. The majority of common circuits are generally digital circuits, because large-scale digital circuits are much easier to design than large-scale analog circuits, so analog circuit computers were eliminated a long time ago. Nowadays, analog circuits are generally focused on input and output and power modules, such as wireless/wireline transceivers, clock generation circuits, bandgap reference sources, etc.. And arithmetic circuits are basically all done by digital circuits.