Problem with differential amplifier with BJT

[Karter]

Hi guys,


I do not know to solve this problem. If you know, please help!

For circuit from picture I need to calculate Vin. I have this known data:

Ic1=4*Ic2; Vcc=1.4V; Vref=1.1V; Vces=0.2V; Vbe=0.7V (when transistor conduct); Iee=100μA; β1=β2=100
Rb=10kΩ; Rc=1kΩ; Va1->inf. Va2->inf.

Image in attachment. And thanks in advance!

 

[(*steve*)]

Can you calculate the voltage on the emitter of Q2?

Can you calculate the total of the IC values?

Can you calculate the desired Ic for Q1?

The rest should be obvious.

 

[Karter]

This is my attempt to solve problem.

 

[(*steve*)]

Can you calculate the voltage on the emitter of Q2?

 

[Karter]

No, I can not.

 

[(*steve*)]

Do you know the voltage on the base of Q2?

Are you told what you should assume Vbe is?

Can you calculate the voltage on the emitter of Q2 now?

 

[Karter]

Well I can not calculate voltage on this transistors and I asked here. Igf I can calculate these volteages I will solve problem.

 

[(*steve*)]

Let me put it more clearly.

You are given the voltage on the base of Q2.

You are told what you should assume Vbe to be.

The voltage on the emitter of Q2 is the result of subtracting one of these from the other.

 

[Karter]

Vbe=VBE I was write that because I do not change size of letters. And to put like this VBE is not good becausa I put Vbe. That is voltage between base and emittor of transistor.

 

[(*steve*)]

OK... I'm not sure I understand your point. I'm trying (unless the phone autocorrects my posts) to use the same terminology as that used in your first post.

So, given you know what Vbe is, and the voltage on the base, can you subtract one from the other?

 

[Karter]

You mean like this

 

[(*steve*)]

That would seem right to me.

And it's obvious what Ve for Q1 is as well, right?

Put that aside for a moment.

Can you calculate what the sum of the collector currents are? It's OK to leave a couple of unknowns in there. (hint: what is the equation which defines the current through one terminal of a transistor in terms of the currents through the other two?)

 

[Karter]

Well that is a problem. I can not calculate Vin because I can not express collector current over voltages. In that part I need help.

 

[(*steve*)]

You don't need to concern yourself with the voltages.

What is the identity expressing the current flows in base, emitter, and collector?

 

[Karter]

I do not know, maybe we use exponential law. That is not for me, it is for my girlfriend, she have test tomorrow, and this problem has been on test last year. I passed this subject long time ago and I forget many things. I do programming, electronic is not my profession. And I put in previously posts what I know.

 

[(*steve*)]

Oh, COME ON.

Ie = Ic + Ib

Perhaps your girlfriend should try to do her own homework.

 

[Karter]

Ok I knew that. But I do not know how to finish this problem, To find Vin.

 

[(*steve*)]

But I do not know how to finish this problem, To find Vin.

That's where we're headed.

Now you have that equation, can you create an equation expressing the sum of the Ic in terms of Ib and Ie?