Small-Signal Analysis (AC)ΒΆ

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.

Magnitude of output voltage Phase of output voltage Switch gain in dB

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Transient Analysis (tran)

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