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Input IMPEDANCE of Large and small signal models

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raul260291

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Is input impedance same for both large signal and small signal models. My friends say that they are different, but I didn't get it.
I think that they are the same for both the models.
My theory: While calculating the input resistance in the lab, small signal model, we usually divide the voltage and current waveforms, which also consists of bias voltage and bias current and Hence they are the same.
Is the above realization correct??
 

chuckey

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If you keep within the linear portion of the input range they should be the same. If however you over drive an amplifier with junction transistors they input current will rise dramatically so one half of the input waveform can be driving in a lot of current and the other half , having cut the transistor off driving in no current at all. The question is what is the input impedance then?
Frank
 

LvW

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If you keep within the linear portion of the input range they should be the same. If however you over drive an amplifier with junction transistors they input current will rise dramatically so one half of the input waveform can be driving in a lot of current and the other half , having cut the transistor off driving in no current at all. The question is what is the input impedance then?
Frank
What happens if we try to divide two waveforms (voltage and current) that are of different shape?
 

raul260291

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DSC08737.JPG
According to the above example we get Two impedance in valid region.
 

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