<P> In a series circuit, the same current passes through a series of components . A string of Christmas lights is a good example of a series circuit: if one goes out, they all do . </P> <P> In a parallel circuit, all the components are connected to the same voltage, and the current divides between the various components according to their resistance . </P> <P> The basic components of analog circuits are wires, resistors, capacitors, inductors, diodes, and transistors . (In 2012 it was demonstrated that memristors can be added to the list of available components .) Analog circuits are very commonly represented in schematic diagrams, in which wires are shown as lines, and each component has a unique symbol . Analog circuit analysis employs Kirchhoff's circuit laws: all the currents at a node (a place where wires meet), and the voltage around a closed loop of wires is 0 . Wires are usually treated as ideal zero - voltage interconnections; any resistance or reactance is captured by explicitly adding a parasitic element, such as a discrete resistor or inductor . Active components such as transistors are often treated as controlled current or voltage sources: for example, a field - effect transistor can be modeled as a current source from the source to the drain, with the current controlled by the gate - source voltage . </P> <P> When the circuit size is comparable to a wavelength of the relevant signal frequency, a more sophisticated approach must be used . Wires are treated as transmission lines, with (hopefully) constant characteristic impedance, and the impedances at the start and end determine transmitted and reflected waves on the line . Such considerations typically become important for circuit boards at frequencies above a GHz; integrated circuits are smaller and can be treated as lumped elements for frequencies less than 10GHz or so . </P>

Three basic features all electric circuits must have
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