- Small-signal model
**Small-signal modeling**is a common analysis method used inelectrical engineering to describenonlinear device s in terms oflinear equations . This linearization is done by first calculating (possibly by aniterative process if the circuit is complex) the DC bias point (that is, thevoltage /current levels present when no signal is applied), and then forming linear approximations about this point.**Motivation**Electronic circuits generally involve small time-varying signals carried over a constant bias. This suggests using a method akin to approximation by

differentials to analyze relatively small perturbations about thebias point .Any nonlinear device which can be described quantitatively using a formula can then be 'linearized' about a bias point by taking partial derivatives of the formula with respect to all governing variables. These partial derivatives can be associated with physical quantities (such as

capacitance , resistance andinductance ), and a circuit diagram relating them can be formulated.Small-signal models exist fordiode s,field-effect transistor s (FET) andbipolar transistors , notably thehybrid-pi model and varioustwo-port network s.**Notation*** Large-signal DC quantities are denoted by uppercase letters with uppercase subscripts. For example, the DC input bias voltage of a transistor would be denoted $V\_\{IN\}$.

* Small-signal quantities are denoted using lowercase letters with lowercase subscripts. For example, the input signal of a transistor would be denoted as $v\_\{in\}$.

* 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 $v\_\{IN\}(t)=V\_\{IN\}(t)+v\_\{in\}(t)$.

**Example: PN junction diodes**The (large-signal) Shockley equation for a diode can be linearized about the bias point or quiescent point (sometimes called

Q-point ) to find the small-signalconductance , capacitance and resistance of the diode. This procedure is described in more detail under diode modeling, which provides an example of the linearization procedure followed in all small-signal models of semiconductor devices.**ee also***

Diode modelling

*Hybrid-pi model

*Early effect

*SPICE ... (Simulation Program with Integrated Circuit Emphasis) is a general purpose analog electronic circuit simulator capable of solving small signal models.**References**

*Wikimedia Foundation.
2010.*