Physically, an electronic circuit can be as small as a pin point or cover many miles. They are constructed by connecting electronic components together with conductors, which allow electricity to flow between the components. Integrated circuits are small circuits constructed from a piece of semiconductor housed in a protective package. While larger circuits may be built by assembling electronic components onto a printed circuit board (PCB), which is used to mechanically support and electrically connect the components. Integrated circuits are typically used as components in larger circuits built onto PCBs. When components are connected using wire as the conductor, the circuit may be extended to cover or connect a large area.
Analog electronic circuits are those in which signals may vary continuously with time to correspond to the information being represented. Electronic equipment like voltage amplifiers, power amplifiers, tuning circuits, and radios are largely analog (with the exception of their control sections, which may be digital, especially in modern units).
There are two main types of analog circuits: series and parallel. A string of Christmas lights is a good example of a series circuit: if one goes out, they all do. In a parallel circuit, each bulb is connected to the power source separately, so if one goes out the rest still remain shining.
The basic components of analog circuits are resistors, capacitors, inductors, memristors and transistors. They may be thought of as having active independent power sources and dependent power sources. An alternative model is to take independent power sources and induction as basic electronic units; this allows modeling frequency dependent negative resistors, gyrators, negative impedance converters, and dependent sources as secondary electronic components.
In digital electronic circuits, electric signals take on discrete values, which are not dependent upon time, to represent logical and numeric values. These values represent the information that is being processed. The transistor is one of the primary components used in digital circuits, and combinations of these can be used to create logic gates. These logic gates may then be used in combination to create a desired output from an input.
Larger circuits may contain several complex components, such as FPGAs or microprocessors. These along with several other components may be interconnected to create a large circuit that operates on large amount of data.
Examples of electronic equipment which use digital circuits include digital wristwatches, calculators, PDAs, and computers.
Mixed-signal or hybrid circuits contain elements of both analog and digital circuits. Examples include comparators, timers, PLLs, ADCs (analog-to-digital converters), and DACs (digital-to-analog converters).
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