DC analysis needs to be prior to AC analysis

DaShiaSun@FB

請各位前輩高手指教
; _3 F9 U, k6 e2 y4 J我的論述: 講述為何要先做 "直流分析"完才能做"交流分析"


9 U  Q# K/ }0 R( Z9 Q6 u5 D9 N5 BAnother discussion here is to highlight the priority demand of an accurately static(DC) compact model.\cite{dc!!!}
3 D! a$ A( `5 N5 j7 _9 uFrom the device-level's prospectives, a set of equations that can describe the terminal characteristics of DUT in a compact model is the so called: Device Characterization. The fully Characterizations are treated by two means: (1)DC Characterization or DC model (2)AC Characterization or AC model. Characterizing a nonlinear electronics component always begins with the characterization of its DC behaviors and follows by the AC one.

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- ^" x. z: G* {& w4 N; HFrom the circuit-level's prospectives,
8 w9 Z6 N% V6 a+ FCircuit analysis refers to solving a circuit with KVL and KCL.
  N5 l8 C* H# s. gTo 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, the unknowns and the solutions of the circuit
' H5 a4 u8 @/ e  W% |( Lare also separated into DC part and AC part. Therefore, we have DC analysis and AC analysis in analyzing circuit.
The separation between DC analysis and AC analysis greatly simplified a complex circuit.
DC analysis is carried out before AC analysis.\cite[4.6.4]{smith!}
( L4 a* I6 s* u* ^3 Z9 DDC analysis determines the Q-point, including  each node voltage and branch current.
AC analysis gives the frequency response, including bandwidth and gain.
1 d9 K7 W' {& A: ?# M# U" ZBefore doing AC Analysis, the DC operating point needed to be calculated from DC analysis first
in order to construct a linear, small-signal model for the nonlinear component.  
So, the small signal  (AC) response is highly dependent upon the presetting (DC) bias condition .

DC analysis in analog SPICE-like simulator,  aims for computing the equilibrium points, which are the calculated DC node voltages  plus DC branch currents in a circuit.They are the DC solutions of the DC equivalent equation/circuit. A circuit will only reach its equilibrium if its stimulus is off and only the independent sources are remain constantly employed.
There is an important reason why a given electronics circuit always need to be reduced into a DC equivalent circuit and followed by an AC equivalent one. It has to do with the present of an active component in the circuit. The active components, such as 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 electronics circuits, the superposition principle skillful separate the electrical characteristics, voltages and currents, into the DC part and the AC part. From another point of view, 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 methods in circuit analyses. Firstly, DC analysis is carried out in order to determine DC bias points, which are the DC quantities $V_{E}$ $V_{B}$ $V_{C}$  $I_{E}$ $I_{B}$ $I_{C}$.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.
* {$ n1 G# ^# `' L' o# w 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.
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