# Help with analyze a simple analog cirucit

Discussion in 'Homework Help' started by eladta, Apr 20, 2013.

Apr 20, 2013
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Hi all,
I have a pretty simple circuit but i'm not sure how to analyze it. Can anyone give me a hand ?

Thanks !

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,667
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Do you know how ideal op amp work? And you could start your analysis with assumption that Vin =1V.

Apr 20, 2013
41
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Yes i do. this is a negative feedback op amp, so i can asume v-=v+
that doesn't make sense to me cause if the BJT is on the emitter potential (which is close to Vout) equals to Vin
Am i wrong ?
What i need is the transfer func. (ie. Vout/Vin).

4. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198

The op amp will adjust its output voltage so that the - input equals the + input.
So, what effect does the bjt have?

Apr 20, 2013
41
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if the bjt is on than the emitter potential is close to 5v (Vc) right ?
in addition we have - input equals the + input as u said. so Vin = 5v always ?!
Clearly i'm not realizing it right but what ?

6. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
In this circuit the bjt is operating as a voltage follower, not as a switch.
Have you studied voltage followers ?

Apr 20, 2013
41
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i did but i don't remember it well

Apr 20, 2013
41
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can u tell me the principal of it ?

9. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
Emitter voltage = base voltage minus base to emitter voltage drop.

Apr 20, 2013
41
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thanks. so if i'm reading it right the circuit is a buffer. why do we need the bjt for ?

11. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
Exactly.. What does the buffer do ?

12. ### WBahn Moderator

Mar 31, 2012
23,730
7,295
At what point do you become responsible for relearning things that you have been taught but have forgotten?

It sounds like you have a very weak grasp of the fundamental concepts. If that's the case, they you really need to take a step back and gain that grounding, because it's only going to get worse from here and become harder and harder to make progress.