Small-signal model: Difference between revisions

Revision as of 17:21, 17 May 2020

Revision as of 18:18, 17 May 2020

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==Variable notation==

* 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 specificFor example, one might say that <math>V_\mathrm{IN} = 5</math>.

* Small-signal 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 specificFor 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>. ToFor continue the specific examples aboveexample, <math>v_\mathrm{IN}(t)=5 + 0.2\cos (2\pi t)</math>

==PN junction diodes==

{{main|Diode modelling#Small-signal modelling}}

The (large-signal) Shockley equation for a diode can be linear about the bias point or quiescent point (sometimes called [[Q-point]]) to find the small-signal [[Electrical conductance|conductance]], capacitance and resistance of the diode. This procedure is described in more detail under [[diode modelling#Small-signal modeling|diode modeling]], which provides an example of the linear procedure followed in all small-signal models of semiconductor devices.

==Differences between small signal and large signal==

A large signal is any signal having enough magnitude to reveal a circuit's non-linearnonlinear behavior. The signal may be a DC signal or an AC signal or indeed, any signal. How large a signal needs to be (in magnitude) before it is considered a <i>''large signal</i>'' depends on the circuit and context in which the signal is being used. In some highly non-linearnonlinear circuits practically all signals need to be considered as large signals.

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.)

In analysis of the small signal's contribution to the circuit, the non-linearnonlinear components, which would be the DC components, are analyzed separately taking into account non-linearitynonlinearity.

==See also==

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*[[Hybrid-pi model]]

*[[Early effect]]

*[[SPICE]] -– Simulation Program with Integrated Circuit Emphasis, a general purpose analog electronic circuit simulator capable of solving small signal models.

==References==