LTSpice AC Analysis Tutorial? - Page 1
Forum section: electronics / projects

author: edpalmer42
author_rank: Super Contributor
timestamp: June 16, 2025, 04:11:35 am
content: Can anyone suggest a tutorial on doing AC stability analysis on a power supply in LTSpice?I've designed a linear power supply circuit that has a few unique features. The circuit works, but I'd like to do an AC analysis to expose any subtle sources of instability. My application requires a really quiet circuit and any hint of instability would cause problems.I've found info on AC stability analysis for amplifiers, but not much on how to analyze a power supply.Any help would be appreciated.

author: moffy
author_rank: Super Contributor
timestamp: June 16, 2025, 05:30:08 am
content: A step in the load current and the response in the supply will normally betray any tendancy for instability. You could also try a step on the input voltage to see what the output does.

author: iMo
author_rank: Super Contributor
timestamp: June 16, 2025, 07:52:02 am
content: There are many tutorials on-line. Simply search for "FRA analysis LTspice".The FRA analyzer creates a Bode diagram for your linear power supply by inserting the "FRA" component into its feedback loop. Configuring the FRA device is easy (again - you may find several tutorials easily, there are examples in LTspice as well, afaik).

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 16, 2025, 06:13:34 pm
content: I would only bother with the FRA stuff if you are specifically interested in some nonlinear behavior that wouldn't show up in a small signal AC analysis.Regardless of whether the circuit is an amplifier or power supply, you just have to identify a feedback path, inject some signal into it, and then calculate loop gain from that. Depending on the circuit complexity, there may be multiple feedback paths that need testing. If you attach your asc file we can probably offer more specific help.

author: MathWizard
author_rank: Super Contributor
timestamp: June 17, 2025, 07:10:35 am
content: Isn't there also an FFT analysis command, maybe for noise. How is that different to the .AC frequency response again ?I've barely used the FFT command, or noise analysis, so far, I guess I'm not deep enough into for noise to matter.

author: iMo
author_rank: Super Contributor
timestamp: June 17, 2025, 07:26:55 am
content: The AC and FRA analyses are different.With AC it a) decomposes the schematics into a simplified model (small signal model with linear parts only, no semicinductors, no sources, etc), and b) it calculates the AC response (it could be done analytically).FRA works like "a real Bode measurement" with the real circuit switched on, it injects a real disturbance signal into circuit's feedback loop and it measures amplitude/gain and phase in the original circuit with all components "working".In past there was a method published by ADI how to do the "FRA" - you had to execute some commands (see below) in order to get the same results. Apparently they created the .FRA command based on that math in order to simplify the thing.

author: iMo
author_rank: Super Contributor
timestamp: June 17, 2025, 07:33:56 am
content: Quote from: MathWizard on June 17, 2025, 07:10:35 amFFT is not used in AC, afaik. The noise analysis works provided your parts used in the schematics have got the noise parameters defined (which is rarely so).

author: iMo
author_rank: Super Contributor
timestamp: June 17, 2025, 09:19:51 am
content: Quote from: iMo on June 17, 2025, 07:26:55 amAn example (a quick hack only) how the small signal model for the AC analysis is created by LTspice (it is created internally and there is none netlist visible for it, afaik).The gm of the voltage controlled current source and the resistances around are derived from the .OP analysis (a linearization at the "working point of the transistor") it does as a first step of the decomposition, imho..

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 17, 2025, 12:13:46 pm
content: Quote from: MathWizard on June 17, 2025, 07:10:35 amfft is only used with transient simulations (.tran). tran outputs time domain waveforms. You can use the fft to convert them to frequency-domain waveforms. However, this is not a good way to estimate frequency response from transient simulations. That is what .fra is for..ac directly gives you frequency domain results (as bode plots by default). No need for fft..noise also directly gives frequency domain results (as noise spectral density by default).