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What Is a Large-Signal Model?

Geisha A. Legazpi
Geisha A. Legazpi

A large-signal model is a representation used in the analysis of electric circuits using voltages and currents that are considered above the low-signal category. The main reason for having a low- and large-signal model is that the behavior circuits, specifically the semiconductors, depend on the relative amplitudes of the signals involved. The large-signal model also reveals the characteristics of circuits when the signal levels are near the maximum allowable levels for devices. Transistor models make use of the large-signal model to predict performance and characteristics during times when maximum signal levels are fed and maximum output is being drawn. The mechanisms for reducing distortion and noise output at highest signal levels are designed based on the large-signal nonlinear models.

The forward voltage drop in a diode is the voltage across the diode when the cathode is negative and the anode is positive. In diode modeling, the small-signal model takes into account, for instance, the 0.7-volt (V) forward voltage drop across the silicon diode and the 0.3 V forward drop across the germanium diode. In the large-signal model, approaching the maximum allowable forward currents in a typical diode will increase the actual forward voltage drop considerably.

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In the reverse bias, a diode has a positive cathode and a negative anode. There is little conduction in both the small- and large-signal models for the reverse-biased diode. In the reverse bias mode, the diode is treated in almost the same way whether in the small- or large-signal model. The difference in the large-signal model for a reverse-biased diode is the reverse breakdown voltage where a diode will fail permanently if the diode is allowed to absorb power, producing an irreversible damage to the positive-negative (P-N) junction of the diode, a junction between a positive (P)-type and a negative (N)-type semiconductor.

For large-signal modeling, almost all the characteristics of the active device will change. When more power is dissipated, the temperature increases usually leading to an increase in gain as well as leakage currents for most transistors. With proper design, active devices are able to automatically control any chance of a state called runaway. For instance, in thermal runaway, the bias currents that maintain the static operating characteristics of an active device may progress into an extreme situation where more and more power is being absorbed by the active device. This type of condition is avoided by proper additional resistors in the active device terminals that compensate for changes, much like a negative feedback mechanism.

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