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I'm in an electronics class and I feel like this portion of the lecture was covered in a vague manner. I'm not really sure what the purpose of doing either a large or small signal analysis is, and what the difference is as applied to transistors.

Thanks in advance!

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This is going to sound somewhat like a circular argument, but here goes: You need to do large-signal analysis whenever you need to account for the nonlinear effects of how a transistor operates. For example, when you are calculating bias points, you need to account for the relatively fixed voltage drop across the B-E junction.

However, when dealing with "small" signals, where "small" means that over the range of the signal voltage or current, the characteristics of the transistor do not change significantly, you can use a much simpler, linearized model of the transistor that simplifies most of the calculations. But you need to be aware of when this model no longer applies, such as when the transistor begins to approach saturation or cutoff. When that happens, you need to go back to the large-signal model in order to get an accurate analysis of the circuit's behavior.

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    \$\begingroup\$ +1 Worth adding that the typical large-signal model is inadequate for predicting some small signal effects such as voltage gain of a transistor, just as the small signal model is inadequate for bias point calculations. If you need to take both into account, then simulation may be the only practical method. \$\endgroup\$ Jan 26, 2016 at 18:07

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