# Two Transistor Circuit Problems

Discussion in 'Homework Help' started by zakaria ounis, Aug 5, 2012.

1. ### zakaria ounis Thread Starter New Member

Aug 5, 2012
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Hello Every One i am New Here and i was wondering if you can help me solve this problem. the question was to evaluate voltages at all nodes and currents through all branches in the circuit assuming Beta=100

2. ### mlog Member

Feb 11, 2012
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Is voltage source V2 intentionally drawn with its plus terminal at ground?

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3. ### Yakima Member

Jan 23, 2012
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The upper voltage source is indeed incorrectly polarized for amplifier operation

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4. ### Ron H AAC Fanatic!

Apr 14, 2005
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You have 2 transistors in parallel. Q1 is operating in reverse mode, i.e., the emitter is acting as the collector, and the collector is acting as the emitter. Both transistors are saturated, but the reverse beta of Q1 will be much less than 100. Also, the c-b and c-e junctions have different areas, which means that the base currrent split of the two transistors will differ. You don't have enough information to calculate individual transistor pin currents. The base voltages will be ≈-4.3V, and the voltage at the collector of Q2 will be ≈ -4.96v, due to saturation.

Below is the result of a simulation of the operating point.
Note that I swapped the reference designators of the transistors, i.e., the reverse mode transistor in my schematic is Q2, while in yours it is Q1.

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• ###### antiparallel transistors.asc
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Last edited: Aug 6, 2012
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5. ### WBahn Moderator

Mar 31, 2012
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While I suspect that v2 is meant to be the other way, it won't matter too much.

Q1) Assume that Q2 is active, what it, roughly Vbe for Q2?
Q2) What is the emitter voltage for Q2?
Q3) Given your answers to (Q1) and (Q2), what is the base voltage for Q2?

Let's assume Q1 isn't playing much of a role, at least for now.

Q4) What is the voltage across R1?
Q5) What is the current through R1?

Q6) If Q1 isn't conducting, what is the base current of Q2?
Q7) If Q1 is in the active state (our initial assumption), what is the collector current?
Q8) If all of this current is flowing in R2, what is the collector voltage of Q2?

Q9) Given your answer to (Q8), us Q2 active? If not, what mode it is in?
Q10) Given your answer to (Q9), what is the collector voltage of Q2?
Q11) Given you answer to (Q10), what is the current in R2?

Now let's turn out attention to Q1.

Let's assume that V2 is supposed to be the other way and that the collector voltage is +5V.

Q12) Given your answers to (Q3) and (Q10), what is the base-emitter voltage of Q1?
Q13) Given your answer to (Q12), what state is the transistor in?

If V2 is hooked up correctly, then you actually have a really poor transistor in parallel with Q2 (imagine calling the collector of Q1 the emitter and calling the emitter the collector). You have no idea what the beta is for this transistor, but it is going to be a lot lower than the beta when it is used properly. If you have a really poor transistor in parallel with a really good transistor, it's hard to say what will happen. The poor one will tend to hog the base current, but the good one will stay in the active region that much longer with it's high beta.

I really do think they have V2 backwards, unless you are expected to run a sim and see the effect, but that can be tricky because a lot of basic transistor models don't model inverse mode very well.

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6. ### zakaria ounis Thread Starter New Member

Aug 5, 2012
6
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Oh, i am so sorry yeah i just drew that circuit, and yes that 5v suppose to be the other way around.

7. ### zakaria ounis Thread Starter New Member

Aug 5, 2012
6
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the upper 5 volts is should be the other way, and what if the second transistor was a pnp transistor ?

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
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So does this mean you are really looking at a class B totem pole configuration?

• ###### Totem pole classB stage.jpg
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Last edited: Aug 6, 2012
9. ### zakaria ounis Thread Starter New Member

Aug 5, 2012
6
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Here is the original circuit as given in the exercise

beta is still the same Beta=100

10. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Why did you waste everyone's time by posting the non-original circuit?

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11. ### zakaria ounis Thread Starter New Member

Aug 5, 2012
6
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in the first circuit i tried to draw it using a simulator which have the above symbols for the voltage source, but i the second one i edited the circuit with paint so it matches the one given in the recitation paper, Sorry again.

12. ### WBahn Moderator

Mar 31, 2012
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For the most part, the questions I asked in my previous post still apply.

In general, when analyzing transistor circuits (particularly at DC), make you best guess at what mode each transistor is in. Don't worry about guessing wrong. Then analyze the circuit based on your assumptions and check if the results are consistent with those assumptions. If they aren't, then you guessed wrong. Revise your guess based on your new knowledge and try again. With just a little bit of experience, you'll be able to guess correctly the first time, most of the time.

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13. ### zakaria ounis Thread Starter New Member

Aug 5, 2012
6
0
I see Now.., i remember one of my friends sad that in circuits like this, one of the transistors is in cut-off mode while the other in active mode. so based on what you told me i should analyze the circuit based on that assumption and then do the other way around ?

14. ### WBahn Moderator

Mar 31, 2012
18,085
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Depends on what "like this" means. There are lots of two-transistor circuits in which both transistors are both active at the same time. But for many circuits, the one-on, one-off is a good starting point if you can't decide a better one. But always remember that, at the end of the day, you may need to try all possible combinations (though usually you can quickly rule out some, or even most, of them).