Hi everyone...

I need to calculate the Ic for the attached circuit, using Thevenin method!
β=167.1
Vcc = 20V
R1 = 160kΩ
R2 = 40kΩ
Re = 1kΩ

I'm calculating Vth = (40/(40+160))*20 = 4V

Then Rth = (160*40)/(160+40)=3.2kΩ

Then using input net (base net) I'm calculating Ib as:

Ib = (Vth - Vbe)/(Rth + Re(β+1)) = (4-0.7)/(3.5k+1k*(167.1+1))=19.26μA

Then using Ic = β*Ib = 167.1*19.26μA = 3.22mA but the correct result is 2.75mA... Can't figure out where am I going wrong!

 

Morning psy,
What type of BJT circuit would you say that you have posted.?

E

 

Morning psy,
What type of BJT circuit would you say that you have posted.?

E

Voltage divider biased transistor... Is this what you're asking


edited;
Ok, I got it.

I was wrong calculating Rth. It's not 3.2kΩ, but 32kΩ.

 

I have another problem... This time is with LTSpice.

I'm trying to simulate the attached circuit but it is taking for ever to finish the simulation.. Why?

 

I'm trying to simulate the attached circuit but it is taking for ever to finish the simulation.. Why?

No ground reference.!

 

No ground reference.!

OMG... But LTSpice usualy complains about it. But didn't do it this time! Bahhh... Thanks!

 

Ok, if I want to write the 2 nets (input and output), can I write then like this:

Net 1 - Input net

Vcc = R1(Ic+Ib) + Vbe + Vγ

Net 2 - Output net

Vcc = R1(Ic + Ib) + Vce + Vγ

Are these 2 equations correct?

 

Is this post #4 circuit.? and what is vy.?

 

Is this post #4 circuit.? and what is vy.?

Yes it's about post #4. Vγ is the LED voltage. I think it's called forward voltage, right? Let's consider the following values:

R1 = 2.1kΩ
Vcc = 12V
hFE = 119

We are asked to find current at R1.

So, I'm doing:

Ib = (12-0.7-2.5)/(2.1k*(1+119)) = 34.92μA
Vce = 12-2.5-2.1k*(34.92μa+34.92μA*119)=0.721V
Ic = 119*34.92μA = 4.155mA

 

Looks close enough to me.
Have you checked it using LTSpice.?

 

Looks close enough to me.
Have you checked it using LTSpice.?

Not yet checked with LTSpice...
The problem is that the answer should be 4mA.
This is a question that our tutor posts in our school's site (quizz) and that we must answer to it within a given deadline. A friend of me helped me and we have calculated a value of 4mA sharp and the quizz "told me" the 4mA values was correct.

But current across R1 is Ib + Ic. So it should be greater than 4.155mA, but the quizz said that 4mA was correct!

 

LTSpice shows
Ib=37uA
Ic=4.39mA
Vbe = 0.694V
Ve=2.09V
Vce=0.694V

Please post your calculation showing the Ic as 4.0mA

 

I can't find them in the calculator... probably the screen "run out" of memory to store the calculations history and deleted the oldest calcs I made. And I can't remember the values of R1, Vγ and hFE.

Every time we attempt to answer the quizz, these values changes!

Anyway, the most important was to know that the net equations I wrote were correct! The rest is math that the calculator does!