The BJT as an Amplifier homework question

[RoyHobbs808]

A 50mV signal is applied to the base of a properly biased transistor with r'e=10ohms and RC=560ohms. Determine the signal voltage at the collector.
In my text book for this section there are formulas
Vb=Ier'e
Vc=IcRC
Vc=IeRC

Do I take 50mV/560ohms to get Ic then take that result and multiply by 560ohms to get VC? Not sure where the r'e=10ohms comes into play.
Any help would be much appreciated.
Thanks,
Roy

 

[Arouse1973]

Hi Roy

Is this signal a sine wave? re do they mean RE (Emitter Resistor), re is the intrinsic emitter resistor and is 25 mV/ IC. Do you have a circuit diagram.

Thanks
Adam

 

[Colin Mitchell]

Basically you can only get a gain of 56 from the circuit.
Applying 50mV to the base really only means the collector voltage can only increase or decrease by a factor of 56 and this allows a max change of 2800mV, if the circuit will allow this rise or fall.
It is the ratio of emitter resistor to collector resistor (load) that limits the gain of the circuit.

 

[LvW]

Roy - for calculation of the signal output voltage you need to know the gain.
It shouldn`t be aproblem for you to find the gain formula for a common-emitter amplifier without emitter degeneration (because you didn`t mention any external emitter resistor RE).
It`s really a simple formula that contains RC and the quantity r`e which is identical to 1/gm (gm: transconductance of the transistor).

 

[RoyHobbs808]

Thanks for the help. I found the formula that I needed to find out the gain.
Thanks everyone for the help.
Roy