# Hard transistor question - Pleease anwer

Discussion in 'Homework Help' started by anb2020, Jan 2, 2013.

Jan 2, 2013
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2. ### bountyhunter Well-Known Member

Sep 7, 2009
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I think that question is gibberish.

How does the term "amplification" even apply to an op amp using positive feedback?

3. ### anb2020 Thread Starter New Member

Jan 2, 2013
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I dunno ..

Last edited: Jan 2, 2013
4. ### anb2020 Thread Starter New Member

Jan 2, 2013
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and yeah ,, pleeease try to solve it ..

5. ### Brownout Well-Known Member

Jan 10, 2012
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If I solve it, who gets the bonus points?

6. ### anb2020 Thread Starter New Member

Jan 2, 2013
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YOU WILL GET IT XD
Seriously, can you solve it?

7. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
I can give you some hints. The output will swing until the "+" input is equal to the "-" input. Treat the input impleadances as being infinite ( input currents = 0 ) Use simple circuit analysis to solve. That's all you get from me.

8. ### anb2020 Thread Starter New Member

Jan 2, 2013
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Cmoon the prof said you can ask anyone .. but you have to understand it ..

9. ### anb2020 Thread Starter New Member

Jan 2, 2013
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and yeah .. I know that information ..
High impedance and short circuit assumptions ..
It's okay if you don't know how to solve it : )

Last edited: Jan 2, 2013
10. ### anb2020 Thread Starter New Member

Jan 2, 2013
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and it's an amplifier not a transistor *facepalm*

11. ### bountyhunter Well-Known Member

Sep 7, 2009
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I guess I don't understand it either.

The output is hard wired to the positive input (POSITIVE FEEDBACK): if the inverting input moves a mV below the + input, the output will start to swing high and drag the + input with it "locking" the output and + input against the positive rail.

If the inverting input was pulled a mV above the + input, same thing the other way: the output slams against the negative rail.

This is positive feedback.

Again I would ask: how does that relate to "amplification"?

It's acting like a comparator, not an amplifier.

12. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
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"Would the figure in circuit 1 works... (has some amplification relationship with the input)"

1. Yes, the figure would "works", based on that definiton of "works".

2. Amplification gain is based on Rinfinity/R where Rinfinity is the missing gain control resistor, so the gain is infinity assuming a perfect opamp.

13. ### WBahn Moderator

Mar 31, 2012
17,715
4,788
Guys, the question is pretty clearly asking the student to determine whether

Vout = A*Vin

or perhaps

Vout = A*Vin + B

applies to this circuit.

Now, since the OP is limited to the knowledge they have obtained up through mechatronics, and since I have no idea what they have been exposed to up to that point (I'm assuming "mechatronics" refers to some specific part of their curriculum) there is no way to really guide the OP toward a suitable answer.

If the OP has been exposed to the notion that the basic opamp model is

Vout = Av(Vp-Vn)

where Av is the open loop gain of the opamp and Vp and Vn are the voltages at the positive and negative inputs and that the opamp output saturates at some point, then it is pretty easy to answer the question.

14. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
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Hence my first post way back where I said the problem was gibberish.

I suspect the person who wrote the problem reversed the input polarities.

As drawn, it's a comparator with positive feedback.

15. ### WBahn Moderator

Mar 31, 2012
17,715
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I suspect the problem was written just the way the person intended. I think the point is for the student to discover the impact of having positive feedback and it might well be a way to drive home the point that you can't just ignore the polarity of the input connections. It's hard to say because we don't know if they have gone through comparator circuits or if, perhaps, this is part of the lead-in to comparator circuits.

If I had to grade this problem, there are a few things I would be looking for the student to recognize. Depending on the explanation they gave -- and what it reveals about the perspective from which the circuit is being viewed -- I could accept either Yes or No as a reasonable answer. If they just said Yes or No, however, they would not get a lot of credit (especially since the question specifically calls for them to back up their answer).

justtrying likes this.
16. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
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I think you are being overly generous. Posting a circuit like this and asking questions about "amplification" teaches nothing, the term does not even apply in the conventional way. IMHO, the question is gibberish and it serves no purpose and teaches nothing.

But the question intentionally implies something false. Seems pretty clear the person who wrote the question screwed up.

My point is: when a teacher gave me a question that made no sense, I went to that teacher and made him explain it so that either I would then understand what he meant or he realized he had made a mistake.

Last edited: Jan 3, 2013
17. ### anb2020 Thread Starter New Member

Jan 2, 2013
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thanks so much guys ..
No .. We didn't study comparator circuits but I think he just mentioned the name as an example of circuits that can be made using an Op-amp ..

This is a simple problem that I solved .. It sums up everything I know about op-amp
http://i.imgur.com/7cUJm.jpg

Would I be able to solve the question with my knowledge?

and please speak simple language .. It's just an introductory course ..

Lol ,, the teacher said nooo questions ..
and no it is not a mistake .. He means the question ..

Last edited: Jan 3, 2013
18. ### WBahn Moderator

Mar 31, 2012
17,715
4,788
There doesn't appear to be any feedback (no connection between Vout and anything feeding back to the op-amp inputs).

Probably not. The assumption of a circuit short between the two opamp inputs assumes that the op-amp is in its active region (i.e., not saturated or "railed").

A good answer to the question should be based on a circuit analysis that assumes finite op-amp gain. First, assume the opamp is in its active region, then show what constraints this places on the signals at the input, then discuss what happens if the inputs are outside this region, then discuss what happens to this region as the op-amp gain gets arbitrarily large.

and please speak simple language .. It's just an introductory course ..

Lol ,, the teacher said nooo questions ..
and no it is not a mistake .. He means the question ..[/QUOTE]

19. ### justtrying Active Member

Mar 9, 2011
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Do you think you may have copied the circuit of the second problem wrong? It makes very little sense.

I think there is great value in the original question as it forces you to think how the op-amp operates, and it is possible to answer it knowing only the basics - concept of the feedback and the op-amp model, but you need to understand those first.

p.s. also note the question does not say there is amplification but asks the student to reason whether there is one or not.

Last edited: Jan 4, 2013
20. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
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Define "Amplification"? Does a comparator output Voltage with essentially 100% distortion count? What are the input and output impedances?

Op Amp Circuits provide Voltage Gain. Amplification is power (Current & Voltage) increase.

Knowing those parameters, in addition to what has been stated above, can you answer the question?