# common emitter amplifier

Discussion in 'Homework Help' started by jackich, Apr 1, 2013.

1. ### jackich Thread Starter New Member

Apr 1, 2013
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0
i got a common emitter amplifier question need to calculate the voltage gain.
the info that have are β=99, base current=35uA
by calculated, IEQ=3.5mA
how should i calculate the voltage gain?
i cannot calculate by using the formula Av=-RL/Re.

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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I don't see any collector resistor in your diagram. Are you sure that you have a correct diagram?
Do you know anything about BJT small signal model?

3. ### mlog Member

Feb 11, 2012
276
36
It looks like a common collector to me.

4. ### WBahn Moderator

Mar 31, 2012
23,856
7,382
To calculate a voltage gain you must first define what voltages the gain refers to. You give no indication of this. What voltage is Vin and what voltage is Vout.

You say that this is a common emitter amplifier, yet in the small signal model for this circuit the emitter is not at common, but rather the collector is. Are you sure this isn't an emitter-follower amplifier question?

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5. ### jackich Thread Starter New Member

Apr 1, 2013
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i see. i though it was a common emitter.
it is a common collector....
actually how to differ the common?

6. ### jackich Thread Starter New Member

Apr 1, 2013
9
0
the question really didn't state any voltage for me.
since this is common collector, so it should use RE/(RE+re) to calculate the voltage gain.
i still stuck on it, cant get RE although re can be calculate.

7. ### WBahn Moderator

Mar 31, 2012
23,856
7,382
If the question didn't indicate which voltage is the input and which voltage is the output, how are you supposed to calculate a voltage gain?

It sounds like you are trying to do analysis using the "pick an equation and throw against the circuit" approach.

You want to know WHERE each of the equations you use comes from so that you will be better prepared to known WHEN to use it. Also, not all circuits you will encounter fall neatly into the tiny subset of circuits that are covered by a handful of plug-and-chug equations.

Answer the question first asked by Jony130: Are you familiar with small signal equivalent circuits?

8. ### jackich Thread Starter New Member

Apr 1, 2013
9
0
yes, i knew it.

9. ### WBahn Moderator

Mar 31, 2012
23,856
7,382
Okay, so draw the small signal circuit and usually there is one terminal of the transistor that is tied to the common small signal ground shared by the input and output circuits. That's what is being referred to.