loose-electron wrote on Oct 23^rd, 2011, 7:33pm:


I do not completely agree with loose-electron.
*At first, the term "ac analysis" is much older than computer-based simulations. It is a set of rules used to calculate the frequency characteristics of passive and/or active networks.
Example: The transfer function of a simple passive RC lowpass 1/(1+sRC) is based on these rules: In this case, the impedance Xc=1/sC. This expression - and, thus, the transfer function - applies for sinusoidal signals (that is the meaning of "ac") only!.
*Secondly, both expressions are not "similar" and "do not overlap".
They cannot be compared because they express different principles, which are however closely related.

- A small signal analysis assumes that all relations between the state variables (currents and voltages within a circuit) are linear.
That means: Linearizing all functions around the dc values (bias point).
This leads to the (funny) result that you can use kilovolts as input voltages during small-signal analyses. Such a small-signal analysis can be performed for any kind of input sgnal (squarewave, for example).

- The ac analysis uses this principle and calculates all frequency-dependent properties of a network with sinusoidal excitation - also based on linear equations only (which certainly is always an approximation - with acceptable deviations in most cases).

Thank you
   

Lex wrote on Oct 24^th, 2011, 2:25am:


They are both a linear estimation methodology.

Looking at a few of my older textbooks from the era prior to simulators, shows that it was referred to as "Network Analysis" in 1976 - In another book from 1986 the term "transfer function" gets used. The term "AC analysis" started getting used after it was applied to the function of spectral analysis using simulators.

The term "phasor analysis" also gets used in some earlier texts.