為何先做DC分析 才接著做AC分析

DaShiaSun@FB

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' U  ~1 @1 H/ Y3 I  DThis section highlights the importance of an accurate pre-determined DC compact model.\cite{dc!!!}
) T) _1 H6 b+ F( w( o% [; Y, SFor compact model developers, extraction of the DC parameters always needs to be carried out before the extraction of AC ones.
  U+ [9 S9 @0 L) `9 M6 GFor devices measurement, DC measurement has to be carried out before AC measurement.
# z& d) q+ z/ U4 k1 D4 {, d. H3 dFor 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!}
From the device-level perspectives,
a set of equations which describe the terminal characteristics of DUT are written in a compact model.
The equations are solved by SPICE-like simulators.
8 t% H9 w# [% n7 b: m( @# RThe 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 parameters extraction
4 g  n/ o# Z) b, ?(2)AC Characterization or AC parameters extraction
Characterizing a nonlinear electronics component always begins with
DC characterization and follows by the AC characterization.
) D+ C; B+ ^9 l* Y6 i+ ?Because the AC model is origined from the linearization of the DC model at an indicated operating point.
! k) n/ f$ G5 L5 {7 nAccordingly, accuracy is highly required in DC characterization because of its piority in the procedure of parameter extraction .

From the circuit-level perspectives,,
  W1 Q# F! F. s( D1 f3 Q! Y  H; ICircuit analysis refers to solving a circuit with KVL and KCL.
$ P; i0 ~9 i* jTo be more specific, it means solving out nodes voltages and branches currents of each element in a circuit.
( B+ l6 V3 T! c" Y, U" l3 ~' P+ oAs the source of stimulus can be systematically separated into DC sources and AC sources,
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
: J. v2 e& ]7 x, F0 h, }  O# YThe separation between DC analysis and AC analysis greatly simplified a complex circuit.
0 K2 i  Y9 k& L- M7 r, D4 c, e1 kDC analysis is being carried out before AC analysis.
DC analysis determines the Q-point, including each node voltage and branch current.
AC analysis gives the frequency response, including bandwidth and gain.
# {. m, t4 l2 W  f4 bBefore performing AC Analysis, the DC operating point needed to be calculated from DC analysis first.
" G7 G8 i: U) i0 VThis is 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 .
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( ~1 _0 d- h5 i2 P3 g1 O% QDC 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.
; n5 _# f$ Z/ K7 \, [They are the DC solutions of the DC equivalent equation/circuit.
A circuit will only reach its equilibrium if its stimulus is off
! {' p/ C( k/ ^and the independent sources are remain constantly employed.
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There is an important reason why a given electronics circuit always
need 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 present of an active component in the circuit.
The active component is a nonlinear element.So, it will have to be linearized.
% E, R1 n- K$ N( F- Z5 oThe 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.

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DaShiaSun@FB

接著寫....
& o. Q1 b8 |6 SLikely, in linear electronic circuits, the superposition principle skillful separates all the electrical characteristics, voltages and currents,
" W( `  F" Q6 J' t) V1 @into the DC part and the AC part. The AC signals are superimposed on the DC signals.
* C) h9 e# |1 sThe two independent quantities, AC quantities and DC quantities,
can be determined by means of two independent means in circuit analyses.
3 i& L& p0 V2 g1 d2 V& Q2 U, t6 ZFirstly, 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
' I" l: X  i6 Q: K1 f' _; bremained in the circuit, but all the reactive elements such as: capacitors and inductors are removed.
, a7 q3 q2 M3 b0 L. nSecondly, 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.