# Getting V

Discussion in 'Homework Help' started by mattysoxfan, May 6, 2013.

1. ### mattysoxfan Thread Starter New Member

May 3, 2013
10
0
One more circuit that i have. Which has two questions. I understand question B and how it is done. As for question A there is something I'm not getting. The answer given is 8 volts. It asks what the voltage should be for the input in order to get a voltage of 0 between A and B.

Second question asks what the voltage at C will be. And if i am correct, it should be 7.3 volts. there is a voltage drop of .7 from base to emitter.

I'm sorry as far as attempts, I'm calculating all the resistor values, the 10 volts on top. no idea how to get a value of 8 volts for the input. usually in the problems the input is given. teacher is throwing me a curve ball.

http://smg.photobucket.com/user/SS3Gotenks/media/ScreenShot2013-05-06at30417PM.png.html

2. ### WBahn Moderator

Mar 31, 2012
23,214
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Technically, there is not enough information to answer the question because you aren't given resistor values for the two collector resistors. In particular, you are given nothing explicit indicating that they are even equal in value to each other.

But, assuming they are, your first question you should be able to answer by symmetry arguments. If the voltage difference between A and B is zero what does that say about the current in the two collector resistors and, hence, about the current in the two transistors? What does this then say about the voltages at the bases of the two transistors?

The answer to the second question is correct and for the correct reasoning.

screen1988 likes this.
3. ### mattysoxfan Thread Starter New Member

May 3, 2013
10
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Well if there is no voltage. then there wouldn't be any current. i would need voltage to have current in the first place.

4. ### WBahn Moderator

Mar 31, 2012
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You are wrong on two accounts. First, you do not have to have a voltage to have a current. Think inductor. Second, we are talking about the voltage difference between A and B -- even with a non-zero voltage between them there would not be any current because there is no path between them for it to flow through.

You are abusing Ohm's Law. Ohm's Law is very specific and related the voltage across a resistor to the current through THAT resistor and the resistance of THAT resistor. Don't read into it stuff that isn't there.

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5. ### mattysoxfan Thread Starter New Member

May 3, 2013
10
0
I'll have to remember that one. So is the point between A and B considered an open? This circuit is grouded twice. With the input being 8. There is a voltage source up top being 10. There is a voltage drop at point C. The only thing that comes to thought is there being a voltage gain somewhere.

6. ### WBahn Moderator

Mar 31, 2012
23,214
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Yes and no. There is no direct path between them, but there are paths between them. So depending on how you look at it you have have superposed currents that go "between" A and B. But from most practical stanpoints they are isolated from each other.

Please. The input is not 8, it is 8V. Units matter. The supply voltage at the top is 10V. The tidier you are with your terminoligy the tidier you will become with your thoughts. It won't happen overnight, but it will happen.

I can't for the life of me figure out what point you are trying to make. Take a step back and try to explain what you are getting at more clearly.

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7. ### mattysoxfan Thread Starter New Member

May 3, 2013
10
0
Sorry for no units. Habit of mine. What I was simply doing was examining the circuit a bit. I was getting really getting anywhere with it to be honest. As to how to get 8V, I still don't get how to get that. With the values given. I'm missing something and I know it.

8. ### WBahn Moderator

Mar 31, 2012
23,214
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Follow the line of reasoning I presented.

Let's just stipulate that the two collector resistors are each 1kΩ. Let's call the Rca and Rcb.

Let's call the current going down through the left hand transistor Ia and the current going down through the right hand transistor Ib.

Q1) What is the voltage at A (let's call this Va) in terms of Ia, Vcc (the 10V at the top), and Rca?

Q2) What is the voltage at B (let's call this Vb) in terms of Ib, Vcc (the 10V at the top), and Rcb?

Q3) What must be true for (Va-Vb) to be zero, given that Rca=Rcb?

Q4) What does this say about the operating condition of the two transistors in terms of which one is on, which one is off, are they both on, are the both off, is the left flowing more or less current than the right? etc.

Q5) What does this say about the voltage on the base of the left transistor compared to the voltage on the base of the right transistor?

Q6) What IS the voltage on the base of the right transistor?

Q7) What is the votlage on the base of the left transistor?

Q8) What is the input voltage?

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9. ### mattysoxfan Thread Starter New Member

May 3, 2013
10
0
One of these transistors are off then. Both base voltages will be 7.3V. Or both be off. 0v for Va and VB when Rca and Rcb are at 1K ohms

10. ### WBahn Moderator

Mar 31, 2012
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If Va is 0V, then what is the current in the Rca resistor?

11. ### mattysoxfan Thread Starter New Member

May 3, 2013
10
0
10mA compared to the 10V source

12. ### WBahn Moderator

Mar 31, 2012
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Okay, so if there is 10mA of current flowing in it, how can the transistor be off? Where is the current going?

13. ### mattysoxfan Thread Starter New Member

May 3, 2013
10
0
Ahhh, makes more sense.

14. ### WBahn Moderator

Mar 31, 2012
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Okay, so with this new insight, try to answer my list of questions again. I think you are probably making progress.

15. ### screen1988 Member

Mar 7, 2013
310
4
I want to learn too. In this case, it is 8V if I ignore Ib. But how about if we take Ib into account? It seems not easy to calculate Vb to me.

16. ### WBahn Moderator

Mar 31, 2012
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It's more complicated, but it's still pretty straightforward. Of course, you have to have some beta value to work with. You then just analyze the circuit. You will find that, for any reasonable beta, the difference will be negligible in this case. A far more significant issue is the assumption that Vbe is always 0.7V. That could throw the base voltage off by a hundred millivolts or so.

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17. ### screen1988 Member

Mar 7, 2013
310
4
Yeah, I figure that out by using Thevenin equvalent and Ie = (β + 1)* Ib.
I think that in case of VAB ≠ 0 then the method is similar, right?
First, I write the relationship between Ia, Ib by using kirchhoff law for the loop including Rca, Rcb, output and get one equation.
Second, I calculate current through 10K resistor, it is equal to Ia + Ib. Then using Thevenin equivalent and get the second equation.
From two equations, I can solve it and get Ia, Ib.
=> Vc = (Ia + Ib)*10K

18. ### Ramussons Well-Known Member

May 3, 2013
747
129
Maybe too much theory is being discussed

It's quite simple.I presume that both the transistors are exactly alike.

I will term the 2 transistors as A and B.

* Base of B is always 20/25 * 10 = 8 Volts
* Implies Voltage at C = 8.0 - .7 = 7.3 Volts.
* A is cutoff (no Base Drive), B conducts , Collector Current = 7.3/10 = .73 mA.
* Voltage at A = 10; at B= less than 10.
* Voltage at A will be Voltage at B when both the transistors conduct equally. Which means the Base Drives have to be the same.

Therefore. Voltage at A = Voltage at B (Voltage A-B = 0) when Base Voltage of Transistor A = 8.0 Volts.

Ramesh

19. ### WBahn Moderator

Mar 31, 2012
23,214
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It's not a matter of too much theory being discussed, it's a matter of helping the OP learn by guiding their struggle to tackle the problem themselves. Not just working their problem for them and giving it to them on a silver platter, as you have just done. Consider that if seeing it worked were all they needed, then they wouldn't be asking in the first place because they have seen it in class and seen worked examples in their text, but something hasn't clicked. So seeing just one more problem worked for them is unlikely to make that something click. Instead, they need to fight and struggle through it because, at some point, they will see what has been blocking their path and push through it, gaining new understanding and insight in the process.

So, please, in the future try to abide by the guidelines of the forum and help GUIDE someone's struggle and refrain from just giving them the answer because, as well intentioned as it might be, it is not doing them the good you think it is.

20. ### WBahn Moderator

Mar 31, 2012
23,214
6,999
I would need to see your work on an example to determine if you are doing it right or not. Don't forget that Thevenin equivalents only apply to linear circuits, so you need to be careful applying it in a circuit involving large-signal analysis of transistors.