# BJT transistor analysis

Discussion in 'Homework Help' started by PsySc0rpi0n, Apr 30, 2015.

1. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,241
5
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!

File size:
722 bytes
Views:
7
File size:
4.7 KB
Views:
22
2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,326
592
Morning psy,
What type of BJT circuit would you say that you have posted.?

E

3. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,241
5
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.

Last edited: Apr 30, 2015
4. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,241
5
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?

File size:
733 bytes
Views:
6
File size:
75.5 KB
Views:
12
5. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,326
592
No ground reference.!

File size:
5.6 KB
Views:
17
6. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,241
5
OMG... But LTSpice usualy complains about it. But didn't do it this time! Bahhh... Thanks!

7. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,241
5
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?

8. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,326
592
Is this post #4 circuit.? and what is vy.?

9. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,241
5
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

10. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,326
592
Looks close enough to me.
Have you checked it using LTSpice.?

11. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,241
5
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!

12. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,326
592
LTSpice shows
Ib=37uA
Ic=4.39mA
Vbe = 0.694V
Ve=2.09V
Vce=0.694V