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Transition frequency (Related to Frequency Response).

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I started reading the Frequency Responce chapter in my book, and in the section where they analyzed Short Circuit Current Gain of the CE amplifier, it was said that:
"The quasi-static model is not valid at frequencies higher than the point at which the magnitude of the current gain is equal to one".

Could you please explain to me what is the quasi-static model which represents a CE amplifier, and why is it valid in freq. below the transition frequency? (in which the current gain equals to one).

Thank you.


The current gain or "beta" of a Common Emitter (CE) amplifier is always measured with dc currents.

However, if you drive the base circuit with an ever increasing frequency, the ac current gain falls as the frequency increases. Eventually the current gain will fall to one, where the change in collector current then equals the change in base current. That is known as the "Transition Frequency", or Ft of that particular transistor.

Beyond that frequency is ceases to amplify ac signals, and becomes pretty useless.

Any modelling that assumes the transistor can amplify becomes invalid, and any circuit will fail to work as intended.
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