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So far we've only talked about dc signals in circuit analysis. These constant signals do not change in time. However, signals must change with time to carry information. The series of videos will cover three basic, time-varying signals used in linear circuits as well as being used to create more complex signal.
An electrical signal such as a current i(t) and voltage v(t) is a function of time called a waveform. So far, most of the circuit analysis has been associated with constant or dc waveforms.
Upper case letters are used for dc waveforms, e.g. Vo or Io, and lower case letters are for time-varying signals, like v(t) and i(t). However, it is implicit that the lower-case notation for variables v and i are functions of time or simply waveforms.