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Design >> Analog Design >> AC analysis
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Message started by tmahi_420 on Jun 26^th, 2008, 11:51am

Title: AC analysis
Post by tmahi_420 on Jun 26^th, 2008, 11:51am

Can any one explain why do we need to do small signal analysis?
what is the purpose of doing ac analysis?.
Thanks in advance.

Title: Re: AC analysis
Post by thechopper on Jun 26^th, 2008, 7:21pm

Hi

A very basic reason for running ac analysis on a circuit is that is the only way to analyze a circuit with active components in a linear way. Ac analysis (or small signal analysis) basically consider a quiescent operating point for the circuit (given by the bias current and voltages at each node); and from such operating point several parameters are derived for each active component. Those parameters are used for modeling such active component (like transistors) as a linear (and in most cases time invariant) system. Consequently mathematical techniques for analyzing linear systems can be used.

Regards
Tosei

Title: Re: AC analysis
Post by buddypoor on Jun 27^th, 2008, 1:26am

Although Tosei gave already a comprehensive and complete answer, here is a short practical view:

All formulas to be used for gain, input/output impedances, ..... are based on the assumption that the system is linear (resp. linearized in a certain bias point).
Example: A PLL is a highly non-linear system. However, for PLL design it is important and neccesary to use a linearized (that means: idealized) system. Only in this case the knowledge and all the rules of the linear control theory (loop gain, stabilty,....) are applicable.