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===Small-signal linear models===
[[Small-signal model|Small-signal]] or [[Linear system|linear]] models are used to evaluate [[BIBO stability|stability]], [[Gain (electronics)|gain]], [[Electronic noise|noise]] and [[Bandwidth (signal processing)|bandwidth]], both in the conceptual stages of circuit design (to decide between alternative design ideas before computer simulation is warranted) and using computers. A small-signal model is generated by taking derivatives of the current-voltage curves about a bias point or [[Q-point]]. As long as the signal is small relative to the nonlinearity of the device, the derivatives do not vary significantly, and can be treated as standard linear circuit elements.
A big advantage of small signal models is they can be solved directly, while large signal nonlinear models are generally solved iteratively, with possible [[Numerical_ordinary_differential_equations#Analysis|convergence or stability]] issues. By simplification to a linear model, the whole apparatus for solving linear equations becomes available, for example, [[simultaneous equations]], [[determinant]]s, and [[Matrix (mathematics)|matrix theory]] (often studied as part of [[linear algebra]]), especially [[Cramer's rule]]. Another advantage is that a linear model is easier to think about, and helps to organize thought.
====Small-signal parameters====
A transistor’s parameters represent its electrical properties. Engineers employ transistor parameters in production-line testing and in circuit design. A group of a transistor’s parameters sufficient to predict circuit [[Gain (electronics)|gain]], input [[Electrical impedance|impedance]], and output [[Electrical impedance|impedance]] are components in its [[small-signal model]].
A number of different [[two-port network]] parameter sets may be used to model a transistor. These include:
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