Question

In: Electrical Engineering

draw appropriate diagrams and explain in detail, why in common emitter amplifier we should have the...

draw appropriate diagrams and explain in detail, why in common emitter amplifier we should have the ac signal between base and emitter to be much less than the thermal voltage

Expert Solution

By analyzing the IC/VBE curve:

Q point is the DC bias point, that defines AC characteristics like , gm and the operation (saturation, linear, cut-off).

In a common emitter amplifier, input voltage is applied to the base, which has an exponential relation with collector current.

Negglecting the -1 term as an approximation:

The base emmiter voltage should not generate variation to the Q point, if the input voltage has as much amplitude as VT:

Current would increase by 2.71x -> 271% the bias current ICQ, thus the point Q has moved.

The variation of current with the variation of the VBE voltage (input voltage of a common emitter):

Note current variation grows exponentially with the relation of Input voltage and Thermal voltage, writing Vi as a proportion x of VT

To maintain a current variation under 10% and maintain Q unchanged:

times smaller than VT

Manojponduru answered 2 years ago

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