A circuit simulator can answer different questions about the same circuit depending on which analysis you run. The three most common are DC, transient, and AC. Choosing the right one is about matching the analysis to the question you are actually asking.
DC analysis — the settled operating point
DC analysis (also called the operating point, or bias point) finds the steady state: the node voltages and branch currents after everything has settled and nothing is changing. Time does not appear at all.
- Capacitors are treated as open circuits (fully charged, no current).
- Inductors are treated as short circuits (no voltage across them).
- Use it for: resistor networks, dividers, bias points, LED currents — anything where you want the final numbers.
Transient analysis — behaviour over time
Transient analysis solves the circuit repeatedly over a sequence of small time steps, so it captures how voltages and currents evolve. This is the only analysis that shows a capacitor charging or an oscillator running.
Vc(t) = Vs × (1 − e^(−t / RC))
- Reactive components (capacitors, inductors) are modelled with their time-dependent behaviour.
- Initial conditions matter — for example, a capacitor's starting voltage.
- Use it for: charge/discharge curves, timing circuits, switching, oscillators, any waveform in time.
AC analysis — response versus frequency
AC (small-signal) analysis sweeps a sinusoidal source across a range of frequencies and reports how the circuit responds at each one — the gain and phase shift. It linearises the circuit around its DC operating point, so it answers "how does this behave as a function of frequency?" rather than "what happens over time?"
- Output is typically a Bode plot: magnitude and phase versus frequency.
- Reactive components have frequency-dependent impedance, so filters reveal their cutoff and rolloff.
- Use it for: filters, amplifier bandwidth, resonance — anything described in the frequency domain.