Bjt circuit

Thread Starter

Alexx98

Joined Nov 5, 2017
30
I didnt understand the Vout part if you can calculate for only one value given below it would be very helpful. I tried to figure this circuit by KVL but i am not sure about where to put the 1k resistance in the equation. Please help me to understand this type of bjt questions

SmartSelectImage_2017-11-05-17-59-08.png
 

Thread Starter

Alexx98

Joined Nov 5, 2017
30
Or where can i find this type of questions to understand the bjts. I did not take the basic course for KVL KCL so i am learning it by myself. I am not asking for the whole solution to this question please do not misunderstand me:)
 

dl324

Joined Mar 30, 2015
16,845
Welcome to AAC!

We don't know what you've been studying/taught. Show us how you would do the calculation for one of the required base voltages so we can see what technique you've been taught to employ and where you're having difficulty.
 

Thread Starter

Alexx98

Joined Nov 5, 2017
30
Welcome to AAC!

We don't know what you've been studying/taught. Show us how you would do the calculation for one of the required base voltages so we can see what technique you've been taught to employ and where you're having difficulty.
Thank you so much! I am new in this course and my calculations are so short to share i am at the beginning so that it would be great to learn where should i begin if you can help
 

dl324

Joined Mar 30, 2015
16,845
I am new in this course and my calculations are so short to share i am at the beginning so that it would be great to learn where should i begin if you can help
You should go to your instructor or teaching assistant so they can instruct you in how to do it the way they intended.

It's been decades since I thought about transistor theory at that level because, in the real world, we (at least I) rarely need to use models for our designs.

Since you were given reverse saturation current and temperature and were told to ignore Vbe and beta variation, there must be something specific you're supposed to learn from this problem.
 

Thread Starter

Alexx98

Joined Nov 5, 2017
30
You should go to your instructor or teaching assistant so they can instruct you in how to do it the way they intended.

It's been decades since I thought about transistor theory at that level because, in the real world, we (at least I) rarely need to use models for our designs.

Since you were given reverse saturation current and temperature and were told to ignore Vbe and beta variation, there must be something specific you're supposed to learn from this problem.
I will do it so. Thank you for your replies:):)
 

Thread Starter

Alexx98

Joined Nov 5, 2017
30
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MrAl

Joined Jun 17, 2014
11,389
Hi,

Because they give some restrictions and Beta and Vsat i would think this is just a more or less amplifier problem with an input and output, and watch for one restriction on the output voltage, that's all.
 

Thread Starter

Alexx98

Joined Nov 5, 2017
30
Hi,

Because they give some restrictions and Beta and Vsat i would think this is just a more or less amplifier problem with an input and output, and watch for one restriction on the output voltage, that's all.
Hi, we havent come to the amplifiers yet. And our instructor didnt solve questions like this before. I thought i can solve this question but i couldnt compounded it with saturation. If i say Vcc-Ic×Rc=Vout then how should i plot Vout and Vin?
 

MrAl

Joined Jun 17, 2014
11,389
Hi, we havent come to the amplifiers yet. And our instructor didnt solve questions like this before. I thought i can solve this question but i couldnt compounded it with saturation. If i say Vcc-Ic×Rc=Vout then how should i plot Vout and Vin?
Hi,

Well they are allowing you to use the current controlled current source model for the transistor, so you just have to calculate the input and output current and output voltage, then if the voltage every goes below 0.2v you simply call it 0.2v. Does that make sense to you?

[EDIT: ADDED THIS LATER:]
You also have to use the diode model:
iB=Is*e^(Vbe/Vt)

with Vt normally taken to be 0.026 for these kind of problem where the temperature is not given. Since the temperature is given however then you have to calculate Vt also.
 
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MrAl

Joined Jun 17, 2014
11,389
Looks more like a voltage-controlled current-source to me, as per the equation in post #9.
Hi,

Yes you are certainly correct and thanks for mentioning this. I read the thing wrong entirely :)
I thought i saw a base resistor and didnt notice the Is spec either. I read everything too fast.

Now that i've read it over again though, i see that this looks like they want the student to use the single diode transistor model. That would not have an equation for Ic directly as in post #9 but instead would be:
iB=Is*e^(Vbe/Vt)

and that would be an ideal diode and taking Vt=0.026 as usual. Since they dont mention any Re this makes it much simpler too.

So the base current is first calculated, then the collector current is:
Ic=iB*Beta

and then we test for Vce>0.2 to see if it is in saturation.
If Vce>0.2 then not saturated, but
if Vce<=0.2, then saturated and we dont allow the collector voltage to follow Vcc-Ic*Rc anymore.

This is nice and simple because we dont have to use any feedback at all for both collector base and emitter base.

I hope the OP understands this now. I'll go back and mod my previous post which was obviously in err :)

[LATER]
Oh wait a minute. My previous post was right, it simply did not tell the whole story. When you said it was wrong i assumed it was wrong too because i thought i saw a base resistor. But even without the base resistor, the original post is correct just that we need to add something about how to calculate the base current. Once we do that, we use the current controlled current source model as mentioned in that post.
I did add a note now to calculate the base current first though, so i am still grateful you said something about this.
 
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MrAl

Joined Jun 17, 2014
11,389
Unless the β value is just a red herring and is not needed in the calculation.

Hi,

Yes, that's true, but i did not take the minimalist view here for the following two reasons.

First, the more advanced model for a transistor has more than one diode in it. A second diode would steal current from the collector and inject it into the base which means the max Beta would fall and would be measured as just Beta then. That's a more exact analysis. We can get 'rid' of that second diode if we just assume some saturation point like here where they spec 0.2 volts. So a strong point for using Beta i think is that this is a typical way to calculate current in a transistor.

Second, with the parameters given numerically we would get a collector current that is much smaller than we normally see with small transistors if we assume Ic=Is*e^(Vbe/Vt). However, if we include the Beta given as a max Beta spec, we get a more real world collector current with a reasonable base voltage.
For example, with a base voltage of 0.7v we only get about 8ua of collector current if we dont use Beta, but if we include Beta in the calculation we get 0.8ma, which is much more consistent with what we normally see. That's because most transistors conduct significantly with a base emitter voltage of 0.7 volts. For example, for the spice model for a 2N2222A i get a collector current of 0.8ma with a base emitter voltage of just 0.604 volts. With the model i am assuming for this current problem i would get a collector current of 0.8ma with a base emitter voltage of 0.7 volts, so that's still less than a common small NPN transistor.
Maybe this is a bigger transistor? I guess we could look at some more transistors, but the above is enough to convince me that the Beta spec should be included. If not i would be surprised, but we also have to remember that these problems change from classroom to classroom where the previous class work had been illustrated or at least should have been before the problem was given to the students. In that case, it's not my fault :)

LATER:
Ok i tried a 2N3055A and i get around 140ma collector current with just 0.604v base emitter voltage, so it's even higher for that larger transistor. You could try more types if you like.
 
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