# Problem with differential amplifier with BJT

#### Karter

Mar 27, 2017
9
Hi guys,

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!

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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
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

Mar 27, 2017
9
This is my attempt to solve problem.

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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Can you calculate the voltage on the emitter of Q2?

Mar 27, 2017
9
No, I can not.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
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

Mar 27, 2017
9
Well I can not calculate voltage on this transistors and I asked here. Igf I can calculate these volteages I will solve problem.

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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
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

Mar 27, 2017
9
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*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
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

Mar 27, 2017
9
You mean like this

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
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

Mar 27, 2017
9
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*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
You don't need to concern yourself with the voltages.

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

#### Karter

Mar 27, 2017
9
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*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Oh, COME ON.

Ie = Ic + Ib

Perhaps your girlfriend should try to do her own homework.

#### Karter

Mar 27, 2017
9
Ok I knew that. But I do not know how to finish this problem, To find Vin.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
But I do not know how to finish this problem, To find Vin.

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

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