Spice Bench Craft

SPICE measurement bench craft

Author the bench from one parameter dictionary, establish the operating point, then measure. The measurement-practice notes these sections are taken from are in references/BENCH_NOTES.md; read that file if you need the original wording.

The operating point comes first

An amplifier with 60-100 dB of DC gain multiplies any input offset by 1,000-100,000. Driving both inputs from ideal DC sources at the same potential does not put the output at mid-rail: the amplifier's own input-referred offset (even tens of microvolts) drives the output to a supply rail, and the gain, bandwidth and phase you then measure are those of a saturated transistor stack, not of the amplifier in its linear region. Symptoms: DC gain tens of dB lower than expected, an .op output voltage within ~100 mV of either rail, device operating regions showing triode/cutoff where saturation was intended.

Always check v(out) in the .op result before trusting an AC sweep. If it is not near the intended quiescent level, the measurement is invalid.

DC servo for open-loop AC analysis

The standard bench closes the loop at DC only, with elements too large to matter in the measured band:

* loop closed at DC through a huge inductor; AC injected differentially
LFB  out  inn  1T        ; 1 tera-henry: short at DC, open at any AC freq
CFB  inn  0    1T        ; 1 tera-farad: blocks DC, grounds inn for AC
VIP  inp  0    DC {CM} AC 0.5
* inn receives -0.5 of the AC drive THROUGH the cap if driven; or drive
* single-ended and read v(out)/v(inp-inn) — both are open-loop above
* the (vanishingly low) servo corner at 1/(2*pi*sqrt(LC)).

At DC the feedback forces the output to the level that zeroes the input differential, so the amplifier settles at its own correct operating point. Above a few microhertz the loop is open and v(out)/v(inp-inn) is the true open-loop transfer function.

Reading the result

• Gain: magnitude at the lowest swept frequency (sweep from well below the dominant pole).
• Unity crossing: use the first 0 dB crossing; verify there is only one, or evaluate phase at every crossing, because a later crossing can carry an unstable mode that a first-crossing readout misses.
• Phase: unwrap before computing margin; wrapped (modulo-360) phase can show a margin that is not there.
• Supply current: measure the supply source current at the .op point, not a sum of device currents.

Sanity checks for measurement benches

1. .op first, AC second: output within the linear region?
2. Does measured DC gain roughly match gm*ro expectations (within an order of magnitude)? A 25 dB shortfall usually means the bench is wrong, not the amplifier.
3. Re-run one point at 2x sweep density: if gain or PM change, the difference comes from the sweep density or interpolation, not from the circuit.

Render benches from one parameter dictionary

Keep DUT pin order and run conditions in data, not copied through dozens of decks. A useful dictionary has DUT_INCLUDE, DUT_INSTANCE, VDD, VCM, CL, TEMP, and analysis limits such as FSTART, FSTOP, or TSTOP. Render the @NAME@ host placeholders below, preserve SPICE {PARAM} braces, and reject output containing an unresolved @NAME@. Keep the title first and .end last. Choose include paths (absolute or deck-relative) explicitly.

Operating-point and supply-current archetype

Use the same servo as the AC bench so the .op result is the exact bias point the AC run will linearize around. Read V(out), device regions, and I(VDD); source-current sign follows the source orientation.

* generated operating-point bench
.include @DUT_INCLUDE@
.param VDDV=@VDD@ VCMV=@VCM@ CLV=@CL@
.temp @TEMP@
VDD  vdd 0 {VDDV}
VIP  inp 0 DC {VCMV}
@DUT_INSTANCE@
CL   out 0 {CLV}
LFB  out inn 1T
CFB  inn 0 1T
.op
* LTspice: .options logopinfo
* ngspice: .save V(out) I(VDD) @m1[gm] @m1[gds] @m1[id]
.end

Open-loop AC archetype

Keep the DUT instance, load, temperature, and servo identical to the .op deck. Single-ended AC 1 makes V(out)/V(inp,inn) the unambiguous loop gain; use the differential denominator even when it is numerically one.

* generated open-loop AC bench
.include @DUT_INCLUDE@
.param VDDV=@VDD@ VCMV=@VCM@ CLV=@CL@
.temp @TEMP@
VDD  vdd 0 {VDDV}
VIP  inp 0 DC {VCMV} AC 1
@DUT_INSTANCE@
CL   out 0 {CLV}
LFB  out inn 1T
CFB  inn 0 1T
.ac dec @POINTS_PER_DECADE@ @FSTART@ @FSTOP@
.save V(out) V(inp) V(inn) I(VDD)
.end

For balanced drive, give VIP AC 0.5 0, replace CFB inn 0 1T with CFB inn ndrive 1T, and add VIM ndrive 0 DC 0 AC 0.5 180; keep the inductor as the only DC feedback path. Always compute gain and phase from V(out)/V(inp,inn), unwrap phase, inspect every 0 dB crossing, and repeat at twice the point density.

Closed-loop transient and load-step archetype

Use ordinary resistive feedback for a closed-loop transient bench; the DC-only servo belongs to open-loop characterization. Parameterize both the command step and the load step so one renderer covers settling and load regulation.

* generated closed-loop transient bench
.include @DUT_INCLUDE@
.param VDDV=@VDD@ VLO=@VIN_LO@ VHI=@VIN_HI@ CLV=@CL@
.temp @TEMP@
VDD   vdd 0 {VDDV}
VIN   inp 0 PULSE({VLO} {VHI} @TDELAY@ @TRISE@ @TFALL@ @TON@ @PERIOD@)
@DUT_INSTANCE@
RFB   out inn @RFB@
RG    inn 0 @RG@
CL    out 0 {CLV}
ILOAD out 0 PULSE(@ILOAD_LO@ @ILOAD_HI@ @TDELAY@ @TRISE@ @TFALL@ @TON@ @PERIOD@)
.tran 0 @TSTOP@ 0 @MAXSTEP@
.save V(inp) V(out) I(VDD) I(ILOAD)
.end

Before extracting slew or settling, confirm the initial .op is linear and the requested step does not turn the measurement into an overload test.

ngspice batch-output practice

This server invokes ngspice with batch mode and a raw output (-b -r), which suppresses top-level .meas. Move measurements into a .control block as the dot-less interactive meas command; a dotted .meas inside .control is not valid.

.control
run
meas ac gain_10 find vdb(out) at=10
meas ac unity when vdb(out)=0 cross=1
.endc

Prefer saved traces plus the server's analysis tools when they express the metric. If using wrdata, its columns repeat the scale vector for every dumped vector: dumping v(out) and i(VDD) yields scale, v(out), scale, i(VDD), not one shared scale followed by both values. Parse repeated scale/value groups, and remember that wrdata writes only the text table; add an explicit write too if later server analysis needs a rawfile.

Routing: pick one interface per session

Pick the interface once, based on what this session can do, and stay with it.

• You can run code (shell + Python): do all circuit work through from ltspice_mcp.api import Api. Look up arguments instead of guessing: api.reference() lists the six ops, api.reference("<op>") gives the full argument tree with enums and one example; help(api.<op>) shows the same. Use run_experiments for any LTspice run, for declared sweep/corner/Monte-Carlo matrices, and for anything that must outlive the call (jobs waits or cancels). Use edit_schematic/verify_circuit for .asc work; they do orthogonal routing and collision checks, so never hand-write .asc files. analyze_results reads any rawfile, including one you ran yourself (raw_path=). Keep intermediate results in your interpreter; print the crossing or the worst corner, not the full table.
• You cannot run code: use the six MCP tools available to you and nothing else.
• One exception, and say so when you use it: a hand-run ngspice one-off is fine only when you pass its raw straight to analyze_results(raw_path=...).