AC Small-Signal Analysis

AC Small-Signal Analysis (Premium) #

AC analysis (.ac) computes your circuit’s frequency response — gain and phase versus frequency. It’s essential for filters, amplifier bandwidth, and stability. AC analysis is a Premium feature.

How it works #

  1. Breadpad finds the DC operating point.
  2. It linearizes the circuit around that point.
  3. It applies a small test signal across a range of frequencies and measures the magnitude and phase at each probe.

Your circuit needs an AC source. In a netlist this looks like:

V1 in 0 DC 0 AC 1

The AC 1 is the 1 V test signal.

Configuration #

  • Start / stop frequency — the range to sweep (e.g. 1 Hz to 1 MHz).
  • Points — resolution of the sweep.
  • Spacing — decade, octave, or linear.

Reading the result #

Magnitude (Bode) plot — gain in dB (20·log₁₀(Vout/Vin)) vs. frequency:

  • 0 dB = unity gain; −3 dB = the cutoff (half-power) frequency.
  • −20 dB/decade = first-order rolloff; −40 dB/decade = second-order.

Phase plot — phase shift in degrees vs. frequency. Watch for −180° where the gain crosses 0 dB (a stability warning).

Example: RC low-pass #

V1 in 0 DC 0 AC 1
R1 in out 1k
C1 out 0 1u
.ac dec 10 0.1 100k

Cutoff fc = 1/(2πRC) ≈ 159 Hz, rolling off at −20 dB/decade.

See also #