Small-signal model: Difference between revisions

Revision as of 17:14, 3 December 2019

Revision as of 17:21, 17 May 2020

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==Variable notation==

* Large-signal DC quantities (also known as <i>bias</i>), which must be constant values with respect to time, are denoted by uppercase letters with uppercase subscripts. For example, the DC input bias voltage of a transistor would be denoted <math>V_\mathrm{IN}</math>. To give a specific example, one might say that <math>V_\mathrm{IN} = 5</math>.

* Small-signal AC quantities, which must have zero average value, are denoted using lowercase letters with lowercase subscripts. (Small signals typically used for modeling are sinusoidal, or "AC" signals.) For example, the input signal of a transistor would be denoted as <math>v_\mathrm{in}</math>. To give a specific example, one might say that <math>v_\mathrm{in}(t) = 0.2\cos (2\pi t)</math>.

* Total quantities, combining both small-signal and large-signal quantities, are denoted using lower case letters and uppercase subscripts. For example, the total input voltage to the aforementioned transistor would be denoted as <math>v_\mathrm{IN}(t)</math>. The small-signal model of the total signal is then the sum of the DC component and the small-signal component of the total signal, or in algebraic notation, <math>v_\mathrm{IN}(t)=V_\mathrm{IN}+v_\mathrm{in}(t)</math>. To continue the specific examples above, <math>v_\mathrm{IN}(t)=5 + 0.2\cos (2\pi t)</math>

==PN junction diodes==

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==Differences between small signal and large signal==

A large signal is any signal having enough magnitude to reveal a circuit's non-linear behavior. The signal may be a DC signal (or an AC signal ator indeed, any signal. How large a pointsignal needs to be (in timemagnitude) before thatit is oneconsidered ora more<i>large orderssignal</i> ofdepends magnitudeon largerthe thancircuit theand smallcontext signalin andwhich the signal is being used. to analyseIn asome circuit containinghighly non-linear componentscircuits andpractically calculateall ansignals operatingneed pointto (bias)be considered ofas theselarge componentssignals.

A small signal is an AC signal (more technically, a signal having zero average value) superimposed on a bias signal (or superimposed on a DC constant signal). This resolution of a signal into two components allows the technique of superposition to be used to simplify further analysis. (If superposition applies in the context.)

A small signal is an AC signal superimposed on a circuit containing a large signal.

In analysis of the small signal's contribution to the circuit, the non-linear components, arewhich modeledwould asbe linearthe DC components, are analyzed separately taking into account non-linearity.

==See also==