loose-electron wrote on Oct 23rd, 2011, 7:33pm:AC analysis tends to mean looking at the frequency response of a system.
Small signal tends to mean looking at the signal processing characterisitics while ignoring DC bias issues.
That is not precise, but the term AC analysis comes from simulator origins, and small signal term comes from hand analysis methods.
thats the history of it...
They overlap and are somewhat similar.
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