標題: 為何先做DC分析 才接著做AC分析 [打印本頁] 作者: DaShiaSun@FB 時間: 2016-12-1 01:26 AM 標題: 為何先做DC分析 才接著做AC分析 這是小弟寫的文章 編講義用的 歡迎高手同行找碴: 3 r ?# }; P2 q+ D! B0 x- D 2 |( U" k9 d- G6 m7 s! S8 ~This section highlights the importance of an accurate pre-determined DC compact model.\cite{dc!!!}7 C" T* h) N* ^
For compact model developers, extraction of the DC parameters always needs to be carried out before the extraction of AC ones. 6 l* w5 o& G: U6 k9 I0 @For devices measurement, DC measurement has to be carried out before AC measurement. 4 n! u7 t2 `. {0 F" T! VFor modern IC designers, DC analysis needs to be carried out before AC analysis.\cite[4.6.4]{smith!} $ L0 s, j0 r& `* B, \- vFor SPICE-like simulators, DC simulation also needs to be swept first and then follows by AC simulation.\cite{nlz!}' l* F$ z+ y1 n) A- h
From the device-level perspectives, i4 ~& y6 E* r2 @6 X
a set of equations which describe the terminal characteristics of DUT are written in a compact model.( j: D" X9 u/ Q7 {, B, V! ?
The equations are solved by SPICE-like simulators.; H# M |$ N* q( }
The procedure of establishing equations, describing device's electrical characteristics, in a compact model 0 J m" L |8 Dand have them solved by SPICE-like simulators is termed as: Device Characterization. 0 Z0 o2 b9 V0 x9 I, [' @# sThe fully Characterizations are treated by two independent steps: % y4 |0 b+ r+ Q' v(1)DC Characterization or DC parameters extraction) e+ b5 U( T( g2 U" i
(2)AC Characterization or AC parameters extraction 4 E/ w1 \& x0 c; O% gCharacterizing a nonlinear electronics component always begins with6 v7 ]8 v1 X8 U9 z
DC characterization and follows by the AC characterization.# D. {) }$ z& s8 t7 I
Because the AC model is origined from the linearization of the DC model at an indicated operating point. " H& J/ n9 V9 r6 l. NAccordingly, accuracy is highly required in DC characterization because of its piority in the procedure of parameter extraction .9 ^8 }; r4 t2 M! r" M1 c
|- }; s3 y/ f! e# Y" R/ |From the circuit-level perspectives,,5 ], X# j9 p1 V# a
Circuit analysis refers to solving a circuit with KVL and KCL. ( M% t0 o# c- D7 Y( O* _9 N* K0 qTo be more specific, it means solving out nodes voltages and branches currents of each element in a circuit., j5 p9 J4 q" @3 k( B
As the source of stimulus can be systematically separated into DC sources and AC sources,6 U, w0 m6 S% C" }9 Q
the unknown/solutions of the circuit are also separated into DC part and AC part.) h8 M) Q7 s& z6 X/ w+ U8 J2 d
Analyzing a circuit is treated in two independent meansA)DC analysis (B)AC analysis$ {. I7 w e o" r3 i, I
The separation between DC analysis and AC analysis greatly simplified a complex circuit.$ r+ |" m; ?5 p. q3 Y+ [2 x
DC analysis is being carried out before AC analysis. 0 s I0 Q5 B/ B% \# mDC analysis determines the Q-point, including each node voltage and branch current.# W. E& M1 ~) G- H2 u/ k
AC analysis gives the frequency response, including bandwidth and gain. 7 i0 [5 |$ y+ [2 i3 u0 b5 ]Before performing AC Analysis, the DC operating point needed to be calculated from DC analysis first. " p% C$ T7 \8 ^* u: IThis is to construct a linear small-signal model for the nonlinear component. a/ c: w* o K2 m; u- dSo, the small signal (AC) response is highly dependent upon the presetting (DC) bias condition .( z6 s4 U4 |' `6 N3 e1 E
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DC simulation in analog SPICE-like simulator, aims for computing the equilibrium points,( A: m! q) r% y: m6 u% m. |9 \% G
which are the calculated DC node voltages and DC branch currents in a circuit. 5 h f# H/ f# S7 I ]6 X. ^They are the DC solutions of the DC equivalent equation/circuit.# x# J# r3 K$ I( M& B4 V0 Z
A circuit will only reach its equilibrium if its stimulus is off" x% Q$ K! G" L+ T
and the independent sources are remain constantly employed.: w2 v+ X/ s" F1 F' I
: l& \5 J4 ^2 R$ \ * X! P' J; Q$ S. Q2 TThere is an important reason why a given electronics circuit always: ?2 M2 m& P0 p9 ~
need to be reduced into a DC equivalent circuit (large-signal model) 3 t2 l" ` i4 J) j" Pand followed by an AC equivalent one(small-signal model). 1 M2 [9 K: {& E( `5 g+ ^; GIt has to do with the present of an active component in the circuit. ' Z. k+ ~: ?3 C) ~0 c" F$ XThe active component is a nonlinear element.So, it will have to be linearized. : ^( ?7 F1 v1 x( B7 p/ V: jThe employment of active elements, like transistors, make the circuit a nonlinear algebra system.* I9 n; c& r/ H" q% J6 ?8 U- U% ?3 ~
Normally, the nonlinear equation can only be solved by means of iterative methods, * T7 }% i& Y, M! nsuch as Newton-Raphson algorithm \cite{nlz!}. This algorithm transforms the solutions 4 R9 _! K$ U" W3 k* Kof the nonlinear equation into a sequence of linear equation. 2 {$ j: k# u' \+ |. a3 M) H% ~' I' [, |/ [; |& M& z
" z" h! ~0 W& @7 e * H& K5 b( D! H! _2 s- U $ V$ A: }& `6 A, i- l' w8 S作者: DaShiaSun@FB 時間: 2016-12-1 01:27 AM
接著寫.... {. _6 Z! j9 N) p3 ]( R/ d
Likely, in linear electronic circuits, the superposition principle skillful separates all the electrical characteristics, voltages and currents, ) `$ ?3 j( o9 y" }into the DC part and the AC part. The AC signals are superimposed on the DC signals. ( \: F, W! ~* E, D5 y+ yThe two independent quantities, AC quantities and DC quantities,( D. ?0 ?" C# {' f
can be determined by means of two independent means in circuit analyses. ) Z# [9 _3 z5 e! p% PFirstly, DC analysis is carried out in order to determine DC bias points, which are the DC quantities $V_{E}$ $V_{B}$ $V_{C}$ $I_{E}$ $I_{B}$ $I_ - U+ N* \5 i' n" s8 h7 r. C F{C}$.The DC analysis are performed when the original circuit is simplified into the equivalent DC circuit. Where all the passive elements are : b/ N. x% p: e( f3 @% w7 ]# Y+ yremained in the circuit, but all the reactive elements such as: capacitors and inductors are removed.- I% U: D. p; Y) o. |
Secondly, the AC analysis is performed on the equivalent AC circuit, which is the so called: the small-signal equivalent circuit.& m3 t3 W+ H/ }9 p+ g+ t( }$ j
The equivalent AC circuit is executed by removing all the DC sources and replacing transistors with the small-signal models.
A)DC analysis (B)AC analysis$ {. I7 w e o" r3 i, I