SPICE analysis guide

SPICE AC analysis and Bode plots for circuit design

Set up a SPICE AC sweep, choose the small-signal source, plot gain and phase, place Bode markers, and avoid confusing AC analysis with transient simulation.

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Reviewed: 2026-07-29

Analysis

small-signal linear AC

Output

magnitude + phase

Sweep

linear / octave / decade

Common plot

Bode response

Know what AC analysis calculates

SPICE AC analysis linearizes nonlinear devices around the DC operating point and solves the small-signal response over frequency. It does not simulate a large time-varying sine wave at every frequency. This makes an AC sweep efficient for filters, amplifiers, feedback networks, and impedance studies, but only while the small-signal operating-point assumption matches the design question.

A valid DC operating point is calculated first
Nonlinear devices are replaced by small-signal models
Each requested frequency is solved in the frequency domain
Large-signal distortion and startup behavior require other analyses

Define the source and sweep explicitly

Give the independent source a small-signal AC magnitude, commonly 1 V for convenient transfer-function reading. Choose start and stop frequencies that include behavior below and above every important pole, zero, resonance, or cutoff. A decade sweep is usually appropriate for a Bode plot because it provides uniform points per logarithmic decade.

Set the AC source magnitude and phase
Choose linear, octave, or decade sweep intentionally
Include enough frequency range to reveal asymptotes
Use sufficient points to resolve narrow resonances

Plot a transfer function, not only a node voltage

If the source magnitude is exactly 1 V, plotting the output voltage can numerically resemble gain, but an explicit ratio V(out)/V(in) is clearer and survives source changes. Plot 20 log10 of magnitude for gain in dB and plot phase in degrees. For a two-port RF network, S-parameter analysis may be more appropriate because it treats traveling waves and port matching directly.

Plot V(out)/V(in) for voltage transfer
Inspect both magnitude and phase
Mark cutoff, crossover, peak, and phase-margin frequencies
Use S-parameters when port reflection and power waves matter

Validate the Bode result before tuning

Check that the circuit reached the intended bias point, source and load impedances are correct, device models are valid over the sweep, and marker values agree with hand estimates or limiting cases. Then tune one design variable at a time and compare traces under identical sweep settings. A plausible-looking curve is not evidence that the setup is correct.

Review DC bias before trusting the AC plot
Confirm loading and reference nodes
Compare low- and high-frequency limits with expectations
Preserve sweep settings when comparing component values

Primary sources

Verify the RF principles

Definitions and matching guidance are grounded in the engineering documentation below. Always validate a simulated match with realistic models and measurements.

Related workflows

Continue the RF design path

Move between matching theory, a practical Tersa workflow, S-parameter analysis, and Smith chart inspection.

Low-pass filter workflow

Apply an AC or RF sweep to a practical filter.

LTspice alternative

Compare Tersa with a mature desktop SPICE workflow.

S11 and S21 explained

Know when an RF port analysis is the better model.

FAQ

What is the difference between AC and transient analysis?

AC analysis is a small-signal frequency-domain calculation around a DC operating point. Transient analysis solves circuit behavior over time and can represent startup, clipping, switching, and other large-signal effects.

Why use an AC magnitude of 1 V?

It makes node voltage numerically convenient for reading voltage gain, but an explicit V(out)/V(in) transfer expression is still clearer and more robust.

How many points per decade should I use?

Use enough points to resolve the narrowest feature that matters. A smooth passive filter may need fewer points than a high-Q resonance or sharp feedback crossover.

When should I use S-parameter analysis instead?

Use S-parameters when incident and reflected power waves, port matching, return loss, insertion loss, or a defined RF reference impedance are central to the result.