.AC — Perform an Small Signal AC Analysis Linearized About the DC Operating Point

The small signal (linear) AC analysis of LTspice computes the AC complex node voltages as a function of frequency. First, the DC operating point of the circuit is found. Next, linearized small signal models for all of the nonlinear devices in the circuit are found for this operating point. Finally, using independent voltage and current sources as the driving signal, the resultant linearized circuit is solved in the frequency domain over the specified range of frequencies.

This mode of analysis is useful for filters, networks, stability analysis, and noise considerations. An example is available by selecting Help > Open Examples > Educational > butter.asc

Syntax:

.ac <oct, dec, lin> <Nsteps> <StartFreq> <EndFreq>

The frequency is swept between frequencies <StartFreq> and <EndFreq>. The number of steps is defined with the keyword oct, dec, or lin and <Nsteps> according to the following table:

Keyword	Nsteps
oct	No. of steps per octave
dec	No. of steps per decade
lin	Total number of linearly spaced steps between StartFreq and EndFreq

.ac list <FirstFreq> [<NextFreq> [<NextFreq> ...]]

The AC analysis is performed at the listed frequencies. The list can contain only one entry, which is useful in combination with .step. It allows you to plot complex data as a function of a stepped parameter.

.ac file=<Filename>

The list of frequencies is read from the file <Filename>. This file can contain comments like a regular netlist. The numbers must be separated by white-space or commas, for example:

* example list
1µHz, 1k
100
1e3 1meg