Confirmation of T/F questions concerning Diodes-Transistors

[matzis]

Homework Statement

Hello everyone,

I would like to ask some True/False questions and I would like the correct option. I need this for exams in my College, to be sure I got the right answers. Sorry for any mistakes in terminology, English is not my native language

So here are the questions:
1) Does the type "Ιd=Ιs[exp(V/nVT)-1]" applies to both Inverted and Correct polarization of a diode?
2) Are thermal losings on a 2 diode type transformer greater than a bridge-type transformer (same DC output)
3) In a transistor, does Ic depends on Vbe?
4) To make Ic 10 times bigger, do you need to increase Vbe by 60mv?
5) Is temperature coeefficient (a) on Vbe diode of a transistor positive?
6) A bipolar transistor can be represented with two diodes on a back to back connection
7) The profit (β) of a transistor is greater if it is working on corruption region rather than on the linear rigeon
8) The point of constructing a circuit of an amplifier transistor is to achieve the greatest profit (β) possible

Thanks in advance, sorry again for mistakes in terminology, If you don't underestand a question I will try to explain it, so ask me!

Homework Equations

Ιd=Ιs[exp(V/nVT)-1]

The Attempt at a Solution

My opinion in the answers:
1) I believe the first one is True, as In my book there is a diagram that confirms that, however I am not sure this is why I wanted to confirm here

2) False, because there are not any answers in my book for this, and I could not find any specific information about this

3) False, because Vbe is 0.7 (depends on diode) and it does not change, so Ic cannot depend on that

4) False, because Vbe depends on the diode type and you cannot change that manually

5) My book doesn't include any information regarding this question but I think it's True

6) No it cannot because transistor is a different device than didoe so False

7) Transistor is designed to work in linear region so False

8) True since when we built an amplifier we want the max possible outcome (profit)

Are my answers correct?

 

[rude man]

My opinion in the answers:
3) False, because Vbe is 0.7 (depends on diode) and it does not change, so Ic cannot depend on that

No. Use your equation in (1) to disprove your answer. You can substitute i_d → i_c, i_s →i₀, V → V_be.
cf . https://ocw.mit.edu/courses/electri...rement-spring-2006/lecture-notes/19_bjt_1.pdf: Therefore, the collector current is related to the emitter current which is in turn a function of the B-E voltage.

4) False, because Vbe depends on the diode type and you cannot change that manually

No, for the same reason as in (3).

5) My book doesn't include any information regarding this question but I think it's True

No. But this is not derivable from the basic equation, so don't feel bad about getting it wrong.

8) True since when we built an amplifier we want the max possible outcome (profit).

. No. In a good circuit design beta is assumed to be infinite, just as the gain of an op amp is assumed to be infinite. Circuit operation should not depend significantly on beta.

 

[matzis]

Thank you very much @rude man for your time and answers, I really appreciate it. Also I have realized the mistakes. Thanks again, you are a life saver!

 

[Merlin3189]

Rudeman has done a good job of answering the substantive questions. I just thought I'd add my opinion on a couple of them.

6) A bipolar transistor can be represented with two diodes on a back to back connection
6) No it cannot because transistor is a different device than diode so False
Ok. A transistor certainly can't be replaced by back to back diodes. But I have seen them represented that way sometimes in the context of testing and identifying transitors.

8) The point of constructing a circuit of an amplifier transistor is to achieve the greatest profit (β) possible
8) True since when we built an amplifier we want the max possible outcome (profit)
No. We do not want the greatest gain, rather we generally want a stable, predictable and often linear gain. To achieve this negative feedback is used and a single amplifier stage generally provides a gain only a fraction of its β.

As rudeman said, "Circuit operation should not depend significantly on beta."
β varies significantly between transistors of the same type number. So a designer could not predict the gain of his amplifier without testing every transistor in it and adjusting other components to match. Commercial production would be very difficult. I
It also varies with temperature, so even if he knew the gain when he built it, it would only have that gain when used at the same temperature. Circuits designed for a certain β might cease to function at all, when their β changed with temperature and that upset the bias point.

To get a high gain linear amplifier you would use several transistor stages, each having its gain limited and probably with multistage negative feedback. The product of all the betas will be very much more than the net gain of the amplifier.
An op-amp which needs a very high overall gain, may have some maximum gain stages, but in use op-amps nearly always have large amounts of external negative feedback, otherwise they would be extremely unstable with a very tiny active range only suitable for switching operation.

I would slightly disagree with Rudeman in his comment about infinite gain. BJT beta is typically in the 10's to 100's and 1000 is quite high. In creating formulae for transistor operation we generally assume beta >>1 but I don't think we consider it infinite. Op amps, which contain several transistors and have overall gain of perhaps 10^4 to 10^6 and more, do use the infinite value approximation in their formulae.

Homework Equations

[/B]Ιd=Ιs[exp(V/nVT)-1]

 

[rude man]

Rudeman has done a good job of answering the substantive questions. I just thought I'd add my opinion on a couple of them.

If your design cannot assume a beta of infinity then it is not a good design since then by definition it is beta-dependent.