# Linear Circuits

March 14th Fact-of-the-DayAny circuit that
has a ratio of output-signal to input-signal (*called a transfer
function*) that is not a function of any input signal characteristic,
such as, for example, frequency, phase or amplitude, is by definition
a linear circuit. Circuits that meet that requirement are unable to alter
the shape of an input wave in any way. Therefore, comparison of input
and output waveshapes is the prime test for circuit linearity. Note, however,
that the shape of a wave is unrelated to its constant amplitude or to
any constant time-shift. Therefore, a circuit can change the amplitude
of a signal and still be linear, so long as the amplitude difference does
not depend on any input signal characteristic. Likewise, a circuit can
advance or retard an output signal with respect to an input signal and
still be linear, so long as the input/output signal timing does not depend
on any characteristic of the input signal. *©2005 Martek International
All rights reserved.*

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