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How can I find if the transistors are OFF or ON ?

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ckck20

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then 0.7v transistor

Hi,
in the following simple schematic how can I find if the transistors
are OFF or ON(thus Ib,Ic)?For now I don't care about saturation.
We assume that the transistors are identical.(Vbe,b same).Note that VEE<0
I write the system of equations
Vin1 - Vbe - (b+1)*(Ib1+Ib2)*R - VEE=0
Vin2 - Vbe - (b+1)*(Ib1+Ib2)*R - VEE=0
But this system has a solution only if Vin1=Vin2.
This means when the schematic is completely symmetrical and in that
case Ibi=Ib2.
But what happens when Vin1 isn't equal to Vin2?
How is this solved?
Thanks.
 

Re: Which transistor is OFF?

At the bottom of the scematic you have constant current source I0=(VinDC+12V)/R.
When both voltages are equal, than both transistors have Ic=I0/2.
When one voltage on one side is slightly greater, then its transistor has slightly bigger Vbe, and it will take more current (You know Ic(Vbe) characteristics).
You can approximatelly calculate off condition, when you say that Ic of one transistor is about 100 times greater than the other, and use equations Ic(Vbe) for both transistors.
 

    ckck20

    Points: 2
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Re: Which transistor is OFF?

For easier reference let's use numbers: Vbe=0.7V b=100
Let's also assume Vin2=0.8V and we need to find the maximum value for Vin1 for which transistor 1 is OFF.
After doing what you said I ended up that Vin1max=0.8V. Therefore if Vin1<0.8V T1=OFF.
Now, if we have Vin1=0.7+0.1sin(2 pi f t) (DC+AC) according to what we found earlier T1 should be always off because Vin1 is between 0.6 and 0.8V. Right?
BUT Pspice gives me that T1=ON when sin(..)>0 =>Vin1=[0.7 , 0.8].
Where's the mistake?
 

Which transistor is OFF?

T1,T2 off-->Ve=-12V-->Vin1/2<-11.3V
T1,T2 saturate-->Ve=-4V-->Vin1/2=-1.3V
(Assume that Vsat=2V).
So it seems that your assumption is not correct.
 

Re: Which transistor is OFF?

ckck20 said:
For easier reference let's use numbers: Vbe=0.7V b=100
Let's also assume Vin2=0.8V and we need to find the maximum value for Vin1 for which transistor 1 is OFF.
After doing what you said I ended up that Vin1max=0.8V. Therefore if Vin1<0.8V T1=OFF.
Now, if we have Vin1=0.7+0.1sin(2 pi f t) (DC+AC) according to what we found earlier T1 should be always off because Vin1 is between 0.6 and 0.8V. Right?
BUT Pspice gives me that T1=ON when sin(..)>0 =>Vin1=[0.7 , 0.8].
Where's the mistake?


What's the Vbe1 and Vbe2 that Pspice gives? Probably they aren't 0.7V.

You can't assume that with a Vbe lower then 0.7V the transistors will be off.
 

Re: Which transistor is OFF?

In Pspice I enter for both transistors(edit instance model text)
(BF=100
RB=0
VJE=0.7V)

I wonder how computers work when here I've got a problem whether two transistors are working or not!! I don't even want to think the transistors in the cpu.
There has to be a mistake in my thoughts(according to spice) but to me it all seems correct. Any ideas?
 

Re: Which transistor is OFF?

Put a differential probe in each BE junction, to see the Vbe that the Pspice is calculating, and post hear.
 

Re: Which transistor is OFF?

Both transistor will turn ON if sufficient Vin will be supplied to the Base of the transistors.

In any ways if Vin1 is not equal to Vin2, their Collector currents will be different but their Emmiter currents will be equal to the sum of the two collector currents.

Their Vce will not be equal in that case also.
 

Re: Which transistor is OFF?

Here are some pics from pspice
 

Re: Which transistor is OFF?

Vout=Vc2
 

Re: Which transistor is OFF?

Vbe1 and Vbe2
Shouldn't this be constant 0.7V? How can i "force" it to be 0.7V?
 

Re: Which transistor is OFF?

ckck20 said:
Vbe1 and Vbe2
Shouldn't this be constant 0.7V? How can i "force" it to be 0.7V?


I guess that when you set in Pspice that VJE=0.7V, it means the minimal voltage to turn ON the transistor. I don't know how you can force it to be o.7V.

But why are you doing this simulation? This circuit is normally used as a differential amplifier, where the BJT's are intended to be always ON.
 

Which transistor is OFF?

Hi,

I think Vbe equal to 0.7V is the ideal mode of the transistor as it is the one used for any calculations in transistor design.

As the parameters of your transistors to be checked so that they are really equal, try to design a flip flop, if at same value of biasing currents, a race condition should not happen.
 

Re: Which transistor is OFF?

It's an exercise that the professor at the university mentioned.
If the circuit had two more resistances RE1 and RE2 after the emitter of each transistor and the wire that is going to R started between those two then it's easy to see whether the transistors work or not.
But when RE1 and RE2 are not present, I haven't been able to find a methodology to find which one is on or off.
It is intended for both bjt's to be always on,but i guess it's because we are in the learning phase, so we get to "play" to see what happens.
I intend to ask my professor, but that will happen in about a week and i can't wait that long to get an answer!!!
 

Re: Which transistor is OFF?

The exercise want an answer in DC analysis? So your equations in the begining of this thread solve the problem.

You shouldn´t simulate at Pspice, because it give an answer much more "real", including AC analysis.

If you want a equation that give an answer like Pspice´s one, you can use the same equations used for DC, but substituting ib1 and ib2 by a vb1 and vb2 dependent term (like Pixel said in the second post).
 

Re: Which transistor is OFF?

Actually the problem wanted the graph at Vout=Vc2.
But i stuck trying to do DC analysis because the system of equations in the start of the topic doesn't have a solution if vin1!=vin2.
That's why i wanted to know how to do dc analysis with different vin1,2 and i still don't know!
 

Re: Which transistor is OFF?

In the system of equations in the start of the topic, you have assumed that vbe1 = vbe2. But if it's true, the solution really is vin1=vin2. Because the highest voltage turn ON a transistor, and OFF the other.

If you want a solution more real, you can´t fix vbe. It need to be a variable, with Ic (and ib) dependent of it.
 

    ckck20

    Points: 2
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Re: Which transistor is OFF?

hi
This is just a simple differential amplifier and one of the stage that is used in building an op amp
so it would detect the difference in the voltages applied to the two inputs ie the two base s of the two transistors and the difference amplified voltage could be taken from either or both of the collectors of the two transistors yielding i n the names
unbalanced o\p
balanced o\p
resp

if you want the details ps refer the book o n
opamps by ramakant Gayakwad indian edition

bye
 

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