Common emmiter amp

[Crackle]

I wonder if someone can please help me to understand and work out the answer to this question.

   

Thanks
Mike

[Mark Hennessy]

Looks like they are using the current-controlled model of transistor operation, rather than the correct voltage control model. No matter...

The input current signal is given, as is the current gain of the transistor, so we know the signal variations in collector current. As we know the load resistor, we can translate that into a voltage.

Happy to expand on all of that, but I know that I often launch into long and detailed explanations that aren't always wanted or necessary, so hopefully that points you in the right direction

[ppppenguin]

Like Mark, I wasn't pleased to see them perpetuating the very poor current controlled model. It's bad mainly because the current gain of a transistor is very badly defined. The exact value must not be relied on when doing sensible design. They should also have said "hfe" or "beta" rather than gain because that's rather ambiguous.

At least this time they have specified that the base current is peak, rather than making you guess if they really mean RMS. So 10uA peak base current multiplied by gain of 100 is 1mA. Without a signal the collector will sit at 10V - 2mA x 1.5k = 10V -3V = 7V. 1mA peak signal will change the collector voltage by 1mA x 1.5k = 1.5V. This is an inverting amplifer so when the base current increases by 10uA the collector current increases hence the collector voltage will fall by 1.5V. On the other half cycle when the base current drops by 10uA peak the collector voltage will rise by 1.5V.

So the peak collector voltage will be 7V + 1.5V = 8.5V.

[Crackle]

Hi Mark, sorry I did need a little more but Jeffrey has shown me the way, thanks.

This is how the answers were given.
   

I suppose having arrived at 1mA x 1.5k = 1.5V. I would have had a chance of selecting the THEIR BEST answer.

Mike

[Kalee20]

Hmm.

1.5V is the correct answer. But it is a very artificial question, transistors having a current gain that's predictable to a few percent, are almost unheard of. And circuits which rely on that, are bad circuits.

The circuit has also the heavily-stabilised bias system that almost disappeared when silicon took over from germanium. Relatively unknown in the field, now.

There's some redundant info in the question too - 2mA collector current; 1.5kilohms; 3V dropped across load. Given two of these, the third could have been deduced! (I suppose we've got to be glad they were consistent!)

[Crackle]

They seem fond of including too much information, it seems they want to trick or confuse you.

Mike

[Murphyv310]

Looks like an early 70s C&G question that was revised later on and removed current controlled questions.
You know I was about to sign up to the foundation test then move on, the two questions posed you have asked for help with and the ambiguous way they have been put down has put me right off.

[Crackle]

Trevor, don't be put off, the Foundation exam is easy, the hardest part is learning the licence conditions.
If I could get 100% you would get 200%
These current questions I am discussing are from an Intermediate course.
The other thing to bear in mind is this is not an official RSGB course I am practising on. So the way the questions are worded may be not as they should, there are loads of typos in this course.
But it is an easy way for me to learn as I find reading from a book difficult, reading books either send me to sleep or gives me headaches. I think it is the eye movement across a small page when reading that gives me problems.
Its surprising how many countries you can talk to on 10 watts.

Mike

[radioman]

I agree that the Foundation exam is easy and that the questions can be worded strangely... like they're trying to make things more difficult than they really are.
You certainly need to read the questions and all the possible answers more than once !

Here's some links to resources on the RSGB website plus some mock exams.
https://rsgb.org/main/clubs-training/tra...resources/
https://rsgb.org/main/clubs-training/for...abus-2019/

Andy

[Doodlebug]

What is the significant difference between AC and DC?
A. Direct currents are always greater than alternating currents.
B. Direct currents take a long time to change direction.
C. Alternating currents are continually changing direction.
D. Alternating currents have a fixed polarity.

Answer B tickles me. I would answer,"One has to be patient." :-)