AC analysis and Bode plots
Plot frequency response and find the cutoff frequency of a filter.
AC analysis (.ac) sweeps frequency and reports gain and phase. It linearises the circuit around its operating point, so it answers the question: how does this circuit respond to different frequencies?
- Right-click the voltage source, choose Advanced, and set the small-signal AC amplitude to 1 (or type AC 1 in the value box).
- Replace the .tran directive with .ac dec 100 1 1Meg (100 points per decade from 1 Hz to 1 MHz).
- Run and plot V(OUT). LTspice shows magnitude in dB and phase on a log frequency axis: a Bode plot.
- Attach a cursor and find where gain has fallen by 3 dB. That is the cutoff frequency fc.
Theory says fc = 1 / (2 x pi x R x C). For 1 kohm and 1 uF that is about 159 Hz. At fc the phase is -45 degrees. Try the calculator to check other values.
Above fc a first-order filter rolls off at 20 dB per decade. Measure it: compare the gain at 1 kHz and 10 kHz.