Small signal equivalent circuit diagram

The behavior of electronic components or circuits is difficult or impossible to calculate because of their usually non- linear properties. A small-signal equivalent circuit has the goal to come for a limited area of application to a description allowing a simple mathematical treatment.

Basics

A small signal (for example an alternating current ) is a sufficiently small area of ​​the overall signal possible on the component . The range extends around a constant or quiescent quantity (for example a direct current ). A working point ( Q-point or bias point ) in the vicinity of which the component is to be operated is defined with this quiescent variable. In a sufficiently small signal range, the component behaves linearly to a certain extent. The equivalent circuit diagram contains components with linear behavior.

When changing the input variable by , the output variable changes by . If it is sufficiently small , it becomes proportional to with the increase in the tangent as the proportionality factor ${\displaystyle \Delta x}$ ${\displaystyle \Delta y}$ ${\displaystyle \Delta x}$ ${\displaystyle \Delta y}$ ${\displaystyle \Delta x}$

The mathematical basis for the linear approximation is provided by the fact that every continuously curved characteristic curve in the area around an operating point can be approximated by the tangent at the operating point . A shift in the operating point changes the rise in the tangent and thus also the small-signal behavior and the characteristic values ​​of the equivalent circuit diagram. The definition of the operating point requires knowledge of the large signal behavior, either graphically from the family of characteristics or mathematically from the non-linear equations of the component.

Components of a small-signal equivalent circuit diagram

As linear components are known

• ideal current and voltage sources ,
• the basic passive components ohmic resistance , coil and capacitor .

The equivalent circuit contains such components. Components that are used to set the operating point and the non-linear components that are to be replaced are not part of the equivalent circuit diagram.

Depending on the requirement for the quality of the approximation or depending on the width of the small-signal range, there may be several equivalent circuit diagrams with different complexity, in which non-linear diodes are then again included.

Examples

Circuit with JFET, which works as a constant current source for a consumer , and its equivalent circuit diagram that can be used for a part of the FET characteristic ${\displaystyle R_{V}}$

• Small signal model of the diode
• Small signal equivalent circuit diagram of the bipolar transistor
• A junction field effect transistor (JFET) behaves approximately linearly in part of its output characteristic. Its equivalent circuit diagram for small signals can therefore be used in a very wide range, even with variable DC voltage.

See also

• Differential resistance

literature

• Erwin Böhmer: Elements of Applied Electronics , Vieweg + Teubner, 2009
• Klaus Bystron, Johannes Borgmeyer: Fundamentals of Technical Electronics , Hanser, 1990