# Ohmforge AC Sweep (Bode plot)

> Frequency response of a circuit: Bode magnitude/phase, −3 dB corner and unity gain. Linearises the circuit at its DC operating point and solves the complex MNA equations across a logarithmic frequency sweep. Returns magnitude in dB and unwrapped phase of an output node (optionally relative to an input node), the peak gain, lower/upper −3 dB frequencies, the unity-gain frequency, a Bode chart and CSV.

Part of usefulweb.site (https://usefulweb.site/). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://ohmforgeacsweep.usefulweb.site/ (interactive tool at https://ohmforgeacsweep.usefulweb.site/tool)
- JSON API: POST https://ohmforgeacsweep.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://ohmforgeacsweep.usefulweb.site/api/v1/tool)
- OpenAPI: https://ohmforgeacsweep.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://ohmforgeacsweep.usefulweb.site/mcp - tool `ohmforge_ac_sweep`

## Inputs
- netlist (textarea): SPICE netlist - Subset: R C L V I D Q M E G K, .model, SIN/PULSE/PWL sources, engineering suffixes (k, m, u, n, p, meg). Node 0 is ground. Leave empty when you pass an Ohmforge project instead.
- project (textarea): Ohmforge project JSON (optional) - Alternative to the netlist: a .ohmforge.json project as saved by the app (format "ohmforge", version 1).
- output (text, required): Output node
- input (text): Input source (gets AC = 1) - Name of the V or I source to excite (e.g. V1). Empty = keep the AC magnitudes from the netlist.
- inputNode (text): Reference node (optional) - Report V(out)/V(ref) instead of V(out).
- fstart (number): Start frequency - Empty = .ac line or 1 Hz.
- fstop (number): Stop frequency - Empty = .ac line or 10 MHz.
- pointsPerDecade (integer): Points per decade - Empty = .ac line or 20.

## Method
- The DC operating point is solved first; every nonlinear device is replaced by its small-signal conductances at that point (diode gd, BJT gm/gπ/go, MOSFET gm/gds, op-amp output slope).
- Capacitors become admittances jωC, inductors impedances jωL in their branch equations, coupled inductors jωM; the complex system is solved by dense LU with partial pivoting at every frequency.
- Magnitude is 20·log10|H|, phase is unwrapped to be continuous. The −3 dB points are found 3.01 dB below the peak by logarithmic interpolation; unity gain is the 0 dB crossing.

## Limits
- Teaching-grade simulation: device models are simplified (Ebers–Moll BJT, level-1 MOSFET, behavioural op-amps and 555) and part ratings are typical values. Verify real designs on the bench and against manufacturer data before building anything connected to mains voltage.