關於大信號和小信號的差別

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4 O& w& B* n" w9 m7 G, b+ @0 {9 h/ TDC versus AC
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This section highlights the importance of an accurate pre-determined DC compact model.\cite{dc!!!}

For compact model developers, extraction of the DC parameters always needs to be carried out before the extraction of AC ones.For devices measurement, DC measurement has to be carried out before AC measurement.
1 y" n1 [2 i2 C- G) `+ SFor modern IC designers, DC analysis needs to be carried out before AC analysis.\cite[4.6.4]{smith!} For SPICE-like simulators, DC simulation also needs to be swept first and then follows by AC simulation.\cite{nlz!}In other words, before performing all other simulations such as Transient, S-parameter, AC, DC simulation stands as the initial first step.

From the device-level perspectives,
a set of nonlinear differential equations which describes the terminal characteristics of DUT are written in a compact model.The equations are solved by SPICE-like simulators.
The procedure of establishing equations, describing device's electrical characteristics, in a compact model and have them solved by SPICE-like simulators is termed as: Device Characterization. The fully Characterizations are treated by two independent steps: (1)DC Characterization or DC parameter extraction(2)AC Characterization or AC parameter extraction.Characterizing a nonlinear electronics component always begins with DC characterization and follows by the AC characterization.Because the AC model is originated from linearization of the DC model at an indicated operating point.
Accordingly, accuracy is highly required in DC characterization because of its priority in the procedure of parameter extraction.

From the circuit-level perspectives,Circuit analysis refers to solving a circuit with KVL and KCL.To be more specific, it means solving out nodes voltages and branches currents of each element in a circuit.As the source of stimulus can be systematically separated into DC sources and AC sources,
- s) N+ ^- U# P5 y7 y3 J0 B, c" A0 [The unknown/solutions of the circuit are also separated into DC part and AC part. Analyzing a circuit is treated in two independent meansA)DC analysis (B)AC analysis
3 N. T6 m% a3 k! ZThe separation between DC analysis and AC analysis greatly simplified a complex circuit.DC analysis is being carried out before AC analysis.DC analysis determines the Q-point, including each node voltage and branch current.
7 i- P/ V1 q$ O5 r( A- wAC analysis gives the frequency response, including bandwidth and gain.Before performing AC Analysis, the DC operating point needed to be calculated from DC analysis first.
0 @% F* l  I- i2 Y; p- f& t) q$ v4 wThis is to construct a linear small-signal model for the nonlinear component. So, the small signal (AC) response is highly dependent upon the preset (DC) bias condition.
DC simulation in analog SPICE-like simulator, aims for computing the equilibrium points, which are the calculated DC node voltages and DC branch currents in a circuit.
9 B* w# t0 z) @( QThey are the DC solutions of the DC equivalent equation/circuit. A circuit will only reach its equilibrium if its stimulus is off and the independent sources are remained constantly employed.

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There is an important reason why a given electronic circuit always needs to be reduced into a DC equivalent circuit (large-signal model) and followed by an AC equivalent one (small-signal model). It has to do with the presence of an active component in the circuit. The active component is a nonlinear element. So, it will have to be linearized. The employment of active elements, like transistors, make the circuit a nonlinear algebra system. Normally, the nonlinear equation can only be solved by means of iterative methods, such as Newton-Raphson algorithm \cite{nlz!}. This algorithm transforms the solutions of the nonlinear equation into a sequence of linear equation.
Likely, in linear electronic circuits, the superposition principle skillful separates all the electrical characteristics, voltages and currents, into the DC part and the AC part. The AC signals are superimposed on the DC signals.The two independent quantities, AC quantities and DC quantities, can be determined by means of two independent means in circuit analyses. Firstly, DC analysis is carried out in order to determine DC bias points, which are the DC quantities $V_{E}$ $V_{B}$ $V_{C}$ $I_{E}$ $I_{B}$ $I_
/ C: p: q/ `! m; G{C}$.The DC analysis are performed when the original circuit is simplified into the equivalent DC circuit. Where all the passive elements are remained in the circuit, but all the reactive elements such as: capacitors and inductors are removed.Secondly, the AC analysis is performed on the equivalent AC circuit, which is the so called: the small-signal equivalent circuit.The equivalent AC circuit is executed by removing all the DC sources and replacing transistors with the small-signal models.