# indicate where should be NPN and PNP transistors?

Discussion in 'Electronics Homework Help' started by za_slovom, Apr 20, 2017.

1. ### za_slovom

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Apr 20, 2017
Hi everyone. Please help me to understand this problem. I have attached the problem itself and my version of solution. I think I made some mistakes, maybe not. I could not draw output signals for current. I will be glad and thankful if someone will show me the right way of solving this.

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2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Well, they could be either PNP or NPN in those circuits.

However, if you know what the transfer function looks like, you can determine which one it is.

But hey, I was only considering common emitter and emitter follower (common collector). It's possible that a common base configuration is used too.

Unless there is more information presented than we can see, I think the are several possible answers.

3. ### Ratch

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Mar 10, 2013
Looks to me like you did the problem correctly. You specified the correct type of transistor (NPN,PNP) for the voltages given, and showed the signal voltage inverted for the common emitter configuration, and no inversion for the common collector configuration. The current present in the resistors will be proportional to the voltage across them.

Ratch

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
I agree that the solution given seems to answer the question, but what eliminates an answer where NPN & PNP are reversed (with appropriate connections and output)?

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Mar 10, 2013

Ratch

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
That also implies an answer where the transistor was placed completely weirdly would also be correct if one could predict the output.

I'm not really comfortable with that -- 12 possible correct answers for each part?

7. ### Ratch

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Mar 10, 2013
Each diagram can show two correct transistor placements. I don't see what is weird about the emitter being drawn on top.

Ratch

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Emitter on top was not what I considered weird.

How about all the other permutations? e.g.

A (wrong) common emitter.

The output remains at approx 0.6V no matter what the input is (within the range of the supply rails as shown). Replacing this with an NPN transistor gives you a more correct common base, but with an invalid input voltage range. Again the output remains constant.

The question does not seem to require a "correct" circuit, but it does require that you can describe the output correctly.

HOWEVER... Partially cut off are a pair of transistors above and tot he right of the question. If there are only two, and they must be used in the orientations shown, then the use of the word "should" in the question now makes sense. In this case there is only one unique correct answer.

IF that is correct, then I agree that the answer given is correct. The only reservation I have is that the slope of the transition on one circuit should be steeper than that of the other, however it probably doesn't matter a great deal since the question does not ask that they be placed on the same axies and the input is only loosely defined.