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:7 r2 y# Q: S' \$ ?& N$ z
4 ^/ O7 R, w6 ?$ tThis section highlights the importance of an accurate pre-determined DC compact model.\cite{dc!!!}, }; a& S, ?" J' b
For compact model developers, extraction of the DC parameters always needs to be carried out before the extraction of AC ones. / q) ?5 D$ a, p* Y7 j- t. PFor devices measurement, DC measurement has to be carried out before AC measurement.4 J; r1 K+ \1 A8 k- N& y$ e/ s. a Y
For modern IC designers, DC analysis needs to be carried out before AC analysis.\cite[4.6.4]{smith!}5 |7 F: G8 s: C3 I7 \, x% N3 J3 O
For SPICE-like simulators, DC simulation also needs to be swept first and then follows by AC simulation.\cite{nlz!} & F4 E5 f2 i: O' i" E, K7 ]& \From the device-level perspectives, - |9 t s, Q( T0 ]9 `" Fa set of equations which describe the terminal characteristics of DUT are written in a compact model.0 l: k/ x; s0 K
The equations are solved by SPICE-like simulators.. E) y7 l1 L r/ j9 s
The procedure of establishing equations, describing device's electrical characteristics, in a compact model . D! @" p; S9 {1 q$ q) T0 nand have them solved by SPICE-like simulators is termed as: Device Characterization.6 J" }! [8 c9 o5 W! S1 j5 D" D
The fully Characterizations are treated by two independent steps: 4 ^( z, {) e8 F8 e* x" o8 v(1)DC Characterization or DC parameters extraction : S" l4 g. }0 [9 `4 Y% C, j. c(2)AC Characterization or AC parameters extraction - F! M F! j! a$ [! g$ y4 {Characterizing a nonlinear electronics component always begins with " t/ ?9 a. Z1 T2 r4 Z# A" M/ p" |DC characterization and follows by the AC characterization.2 Z4 i/ c" P& K) h, v1 V
Because the AC model is origined from the linearization of the DC model at an indicated operating point. ( i! J- V' O' M* _, K& lAccordingly, accuracy is highly required in DC characterization because of its piority in the procedure of parameter extraction . * z9 `6 E. O7 F* e& s! ^# P2 v. t: P0 E. h
From the circuit-level perspectives,, 1 j3 E- S0 h" k, n; |Circuit analysis refers to solving a circuit with KVL and KCL.: F" p. ]2 l* t& ?' p7 U h. S! j
To be more specific, it means solving out nodes voltages and branches currents of each element in a circuit.0 a0 I" M \( ?( `3 E2 N
As the source of stimulus can be systematically separated into DC sources and AC sources, B$ u# m+ M' {0 P$ Q! V( l3 y
the unknown/solutions of the circuit are also separated into DC part and AC part.9 l$ r8 S8 @* R, N V
Analyzing a circuit is treated in two independent meansA)DC analysis (B)AC analysis 5 Y7 g9 ?5 A6 z2 ^0 |, @The separation between DC analysis and AC analysis greatly simplified a complex circuit.! C& ^, e' Y) K! q
DC analysis is being carried out before AC analysis.( V# B8 q8 B5 g$ N: H
DC analysis determines the Q-point, including each node voltage and branch current. , V. _5 @' q4 {$ m! N2 n* {AC analysis gives the frequency response, including bandwidth and gain.3 b0 v: p' ?0 a& Z( \
Before performing AC Analysis, the DC operating point needed to be calculated from DC analysis first.- h/ P' Q: ]6 Y& j9 D" p
This is to construct a linear small-signal model for the nonlinear component.2 X2 x% @: ~7 a! c
So, the small signal (AC) response is highly dependent upon the presetting (DC) bias condition ., P5 o* _$ k9 }
/ U7 O& R5 F P& `# I* Z! D3 o& _DC simulation in analog SPICE-like simulator, aims for computing the equilibrium points, 3 y, g8 L) W8 Nwhich are the calculated DC node voltages and DC branch currents in a circuit.* U, [( e7 ^- d# n2 I
They are the DC solutions of the DC equivalent equation/circuit.4 J: O) B7 X( q8 Y( Y7 N
A circuit will only reach its equilibrium if its stimulus is off# b4 z2 I' S4 K7 k( w/ R5 G0 i* ]" }( n
and the independent sources are remain constantly employed. + O1 {) F8 F8 G( i# m- m% q3 X8 \* H, M2 z w
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There is an important reason why a given electronics circuit always 0 j: \/ B) T3 A" I. S% vneed to be reduced into a DC equivalent circuit (large-signal model) 3 Q; J* ]( q# ?5 X! J( t" gand followed by an AC equivalent one(small-signal model).. X, x, Q& j- _8 x
It has to do with the present of an active component in the circuit.6 e* d8 i* x$ Q @4 n2 u1 T
The active component is a nonlinear element.So, it will have to be linearized. 9 T2 z6 t1 p4 c6 |5 X8 ~- i. FThe employment of active elements, like transistors, make the circuit a nonlinear algebra system.+ q% g' S C, E" i% i
Normally, the nonlinear equation can only be solved by means of iterative methods,7 |/ a$ q/ i) w8 T7 @& R
such as Newton-Raphson algorithm \cite{nlz!}. This algorithm transforms the solutions 7 S/ j0 z8 Y+ l& _, f5 c0 d, ^- Tof the nonlinear equation into a sequence of linear equation.+ A6 f/ A+ \; P7 w) s
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4 O" s' R/ _1 f3 w' J* G作者: DaShiaSun@FB 時間: 2016-12-1 01:27 AM
接著寫.... 5 ?; D' S2 D( M; eLikely, in linear electronic circuits, the superposition principle skillful separates all the electrical characteristics, voltages and currents,/ u* |: c! i* P' q
into the DC part and the AC part. The AC signals are superimposed on the DC signals.# M4 `6 X# ^. m2 Z
The two independent quantities, AC quantities and DC quantities, 3 V5 Q/ A$ N1 Qcan be determined by means of two independent means in circuit analyses.; D, Q! H& |- c9 |; b
Firstly, DC analysis is carried out in order to determine DC bias points, which are the DC quantities $V_{E}$ $V_{B}$ $V_{C}$ $I_{E}$ $I_{B}$ $I_, Y+ Z. k* e- {2 g" u/ b: ^
{C}$.The DC analysis are performed when the original circuit is simplified into the equivalent DC circuit. Where all the passive elements are }7 P. L6 K# `9 g3 b$ X, K
remained in the circuit, but all the reactive elements such as: capacitors and inductors are removed. . r& K& t: T3 q2 k9 \0 B/ u3 P3 |Secondly, the AC analysis is performed on the equivalent AC circuit, which is the so called: the small-signal equivalent circuit.* `9 u! B! J9 k! a9 M
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