Editing Network analysis (electrical circuits) (section)

====Small signal equivalent circuit====
{{main|Small-signal model}}
This method can be used where the deviation of the input and output signals in a network stay within a substantially linear portion of the non-linear devices transfer function, or else are so small that the curve of the transfer function can be considered linear.  Under a set of these specific conditions, the non-linear device can be represented by an equivalent linear network.  It must be remembered that this equivalent circuit is entirely notional and only valid for the small signal deviations.  It is entirely inapplicable to the dc biasing of the device.

For a simple two-terminal device, the small signal equivalent circuit may be no more than two components.  A resistance equal to the slope of the v/i curve at the operating point (called the dynamic resistance), and tangent to the curve.  A generator, because this tangent will not, in general, pass through the origin.  With more terminals, more complicated equivalent circuits are required.

A popular form of specifying the small signal equivalent circuit amongst transistor manufacturers is to use the two-port network parameters known as [[two-port network#Hybrid parameters (h-parameters)|[h] parameters]].  These are a matrix of four parameters as with the [z] parameters but in the case of the [h] parameters they are a hybrid mixture of impedances, admittances, current gains and voltage gains.  In this model the three terminal transistor is considered to be a two port network, one of its terminals being common to both ports.  The [h] parameters are quite different depending on which terminal is chosen as the common one.  The most important parameter for transistors is usually the forward current gain, h<sub>21</sub>, in the common emitter configuration.  This is designated h<sub>fe</sub> on data sheets.

The small signal equivalent circuit in terms of two-port parameters leads to the concept of dependent generators.  That is, the value of a voltage or current generator depends linearly on a voltage or current elsewhere in the circuit.  For instance the [z] parameter model leads to dependent voltage generators as shown in this diagram;

[[Image:Z-equivalent two port.png|thumbnail|none|400px|[z] parameter equivalent circuit showing dependent voltage generators]]

There will always be dependent generators in a two-port parameter equivalent circuit.  This applies to the [h] parameters as well as to the [z] and any other kind.  These dependencies must be preserved when developing the equations in a larger linear network analysis.