Understanding AC Small Signal Analysis

The AC small signal analysis portion of Star-Hspice computes (see AC Small Signal Analysis Flow) AC output variables as a function of frequency. Star-Hspice first solves for the DC operating point conditions, which are used to develop linearized, small-signal models for all nonlinear devices in the circuit.

Figure 12-1: AC Small Signal Analysis Flow

Capacitor and inductor values are converted to their corresponding admittances:

	 			for capacitors

and

				for inductors

Star-Hspice allows resistors to have different DC and AC values. If AC = <value> is specified in a resistor statement, the operating point is calculated using the DC value of resistance, but the AC resistance value is used in the AC analysis. This is convenient when analyzing operational amplifiers, since the operating point computation can be performed on the unity gain configuration using a low value for the feedback resistance. The AC analysis then can be performed on the open loop configuration by using a very large value for the AC resistance.

AC analysis of bipolar transistors is based on the small-signal equivalent circuit, as described in Using BJT Model Equations (NPN and PNP). MOSFET AC equivalent circuit models are described in Introducing MOSFETs.

The AC analysis statement permits sweeping values for:

Additionally, as part of the small signal analysis tools, Star-Hspice provides:

Star-Hspice Manual - Release 2001.2 - June 2001