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I am trying to understand Small Signal Analysis (SSA) for Diodes and BJT transistors through different textbooks but I have some issues.

Is AC Analysis the same as SSA? I have seen hints that imply this but most textbooks say that SSA is about small signals so what I understand is that it is for both AC and DC signals as long as they are small enough. But if that is the case then why in SSA we have to short DC voltage sources?

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Yes, when we say "AC analysis" we mean a small-signal analysis of the variations around an operating point due to an AC stimulus.

This meaning has been reinforced over the past 40ish years by the widely used SPICE simulation program, where a punch card or line in an input file that begins with .AC is used to initiate a small-signal AC analysis.

As you say, we can also do a small signal analysis to determine the effect of a small dc perturbation about an operating point. With that in mind, the term "small-signal analysis" is somewhat broader than "AC analysis".

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  • \$\begingroup\$ Thank you for your answer. In the SSA how can we determine a small DC when we eliminate the DC sources? \$\endgroup\$
    – Adam
    Commented Apr 11, 2016 at 0:22
  • \$\begingroup\$ Even in a DC steady-state analysis (I've never seen this abbreviated as "SSA" before), you'd eliminate the DC sources, find the perturbation effect, and then add this back to the operating-point output to get the total output. This is essentially the superposition principle at work. \$\endgroup\$
    – The Photon
    Commented Apr 11, 2016 at 0:28
  • \$\begingroup\$ I just used that abbreviation SSA so to avoid writing Small Signal Analysis every time. Thanks though! \$\endgroup\$
    – Adam
    Commented Apr 11, 2016 at 0:33

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