The small-signal (AC) analysis is performed around the operating point
calculated using the OP analysis and it is exactly the same as the
manual small-signal analysis. Since the circuit is linearized for this
analysis, any *distorsion, saturation or intermodulation that would
occur in the real circuit is not considered by the analysis*. The
operating point is calculated automatically even if the OP analysis is
not specified.

The AC analysis is evaluated in a range of frequencies that must be specified in the analysis line. Only sources that contain the AC keyword are considered as inputs (vs in our example):

```
ngspice 217 -> ac dec 10 1kHz 1GHz
Doing analysis at TEMP = 27.000000 and TNOM = 27.000000
No. of Data Rows : 61
ngspice 218 -> plot abs(v(out))
ngspice 219 -> plot phase(v(out))
ngspice 220 -> plot db(v(out)/v(in))
ngspice 221 ->
```

The analysis is performed by sweeping the frequency in logarithmic
scale (`dec`, alternatively `lin`), 10 points per decade, starting
at 1 kHz and ending at 1 GHz. Keep in mind that voltages/currents in
AC analysis are complex phasors, thus you should plot magnitude and
phase separately. Note that by default the phase is given in radians:
multiply by 180/pi to obtain degrees. As with other analysis types,
remember to precede a line with a dot if included in a netlist.