Small-signal model: Difference between revisions

Revision as of 08:59, 5 February 2017

Revision as of 17:37, 14 September 2017

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In contrast, many of the components that make up ''electronic'' circuits, such as [[diode]]s, [[transistor]]s, [[integrated circuit]]s, and [[vacuum tube]]s are [[linear circuit|nonlinear]]; that is the current through them is not proportional to the voltage, and the output of [[two-port network|two-port]] devices like transistors is not proportional to their input. The relationship between current and voltage in them is given by a curved line on a graph, their [[Current-voltage characteristic|characteristic curve]] (I-V curve) . In general these circuits don't have simple mathematical solutions. To calculate the current and voltage in them generally requires either [[graphical method]]s or simulation on computers using [[electronic circuit simulation]] programs like [[SPICE]].

However in some electronic circuits such as [[radio receiver]]s, telecommunications, sensors, instrumentation and [[signal processing]] circuits, the AC signals are "small" compared to the DC voltages and currents in the circuit. In these, [[perturbation theory]] can be used to derive an approximate [[equivalent circuit|AC equivalent circuit]] which is linear, allowing the AC behavior of the circuit to be calculated easily. In these circuits a steady [[direct current|DC]] current or voltage from the power supply, called a ''[[bias (electrical engineering)|bias]]'', is applied to each nonlinear component such as a transistor and vacuum tube to set its operating point, and the time-varying [[alternating current|AC]] current or voltage which represents the [[signal (electrical engineering)|signal]] to be processed is added to it. The point on the graph representing the bias current and voltage is called the ''[[quiescent point]]'' (Q point). In the above circuits the AC signal is small compared to the bias, representing a small perturbation of the DC voltage or current in the circuit about the Q point. If the characteristic curve of the device is sufficiently flat over the region occupied by the signal, using a [[Taylor series]] expansion the nonlinear function can be approximated near the bias point by its first order [[partial derivative]]. These partial derivatives represent the incremental [[capacitance]], [[electrical resistance|resistance]] [[inductance]] and [[gain (electronics)|gain]] seen by the signal, and can be used to create a linear [[equivalent circuit]] giving the response of the real circuit to a small AC signal. This is called the "small-signal model".

The small signal model is dependent on the DC bias currents and voltages in the circuit (the [[Q point]]). Changing the bias moves the operating point up or down on the curves, thus changing the equivalent small-signal AC resistance, gain, etc. seen by the signal.

Any nonlinear component whose characteristics are given by a [[continuity (mathematics)|continuous]], smooth ([[differentiability|differentiabledifferential]]) curve can be approximated by the linear small-signal model. Small-signal models exist for [[Vacuum tube|electron tube]]s, [[diode]]s, [[field-effect transistor]]s (FET) and [[Bipolar junction transistor|bipolar transistors]], notably the [[hybrid-pi model]] and various [[two-port network]]s. Manufacturers often list the small-signal characteristics of such components at "typical" bias values on their data sheets.

==Variable notation==