# BJT transistor

Discussion in 'Homework Help' started by PsySc0rpi0n, Apr 1, 2014.

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

Mar 4, 2014
1,191
3
I'm now about to start the analysis of the BJT transistors.

I don't understand yet how they work. I'm reading some info about it as we speak!

We will be analysing 2N2222!

Oct 2, 2009
12,657
3,461
3. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,606
396
hi Psy,
You may find LTSpice will also help with your studies with BJT.

E

File size:
878 bytes
Views:
13
• ###### AAesp02.gif
File size:
35.5 KB
Views:
41
Last edited: Apr 2, 2014
4. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
3
Ok, i need to know how to simulate the attached circuit...

File size:
895 bytes
Views:
15
5. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
3
Ahhh... I got it now...

They want a current power source to inject current thru the BJT base point... Ok. Let me do those changes in my previous circuit!

PS: Just help me on this:

Where do i have to place the probes to check the voltage drop between C and E?
And can i make 2 power sources vary at the same time at the Simulation command?

Jan 29, 2010
2,606
396

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

Mar 4, 2014
1,191
3
Teacher wants to plot Ic(Vce) and Ib must go up to 150μA in steps of 30μA and Vce must go up to 20V in steps of 0.1V

Am i doing anything wrong?

File size:
28.7 KB
Views:
45
File size:
1 KB
Views:
19
8. ### MrChips Moderator

Oct 2, 2009
12,657
3,461
You can do this by hand, tedious but doable.

Set Ib. Increase Vcc in 10 steps. Measure Icc at each Vcc.

Step up Ib.
Do this for five Ib settings.

Total 5 x 10 = 50 data points.

Plot the curves by hand or any graphing program.

9. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,606
396
hi,
Did your tutor say you must have the 100K and the 220R resistors in your circuit.??

Also why are you using DECades for the two varying parameters.?

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

Mar 4, 2014
1,191
3
So, how am i supposed to get the characteristic curve (Ic(Vce)) for this 2N2222???

Well, in the thorical part he doesn't asks spoecifically for any resistor but for the same point at the laboratorial work, he gave us a schematic that has both resistors!

And yes, i used DECade sweeps for both power sources! But i have also tried linear!

Last edited: Apr 2, 2014
11. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,995
1,117
First of all remove all resistors from you circuit. Next change your sweep type from decade to linear one. Also first source need to be V1 voltage then Ib.

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

Mar 4, 2014
1,191
3
Nice... I got it...

But i need to know why do i have to remove all resistors and why does it has to be linear and not decade, or better, what is the difference between the different type of sweeps, and lastly why does the Vce need to be the first to be sweeped????

13. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,995
1,117
In this case RB resistor has no influence on the circuit, because you use current source.
Rc resistor have influence on Ic current for low V1 voltage values. Because Ic current cannot be greater than V1/Rc. And we use Vce first because we want Vce on the X axis.

14. ### crutschow Expert

Mar 14, 2008
13,527
3,393
The Decade and Octave sweep generate the defined number of logarithmic points for each decade or octave of the variable parameter. The List sweep uses an arbitrary list for the parameter.

The type of sweep to get the display specified by the teacher is called a Linear sweep (Start point, Stop point, Step Increment).

The Source order determines which parameters are used for the X and Y axes. To display the normal characteristic curves of a transistor in Spice you need Vc first and Ib second.

15. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,606
396
hi Psy,
You should have by now realised, no resistors and use linear variable plots , in X, Y format.

Dont use a battery for the supply, use Voltage source.
E

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

Mar 4, 2014
1,191
3
Ok, thanks for explanations!

Here goes my asc file and plot for the first question!

Now, for the next question, i have to calculate Early Voltage!

File size:
791 bytes
Views:
16
File size:
27 KB
Views:
46
17. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,606
396
Looks OK.

Run this asc and then press Cntr L keys for Gain values at 5V and 10V.

Its just a demo of the .measure option.

E

File size:
905 bytes
Views:
15
18. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
3
Why do you choose 2N2222A? What is the interest of those points where you measured currents?

19. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,519
515
That looks good.

Early Voltage is not really complicated. You have five Vbe plots. Take one Vbe plot, take the section that is straight line, figure out the equation of that straight line, now extend the straight line until it intersect the Vce axis. Now do that for the other four plots of Vbe. The point where all the plots intersect Vce axis is the negative Early voltage. So just drop the minus sign and all that is left is the value of the Early voltage.

Here is illustration from my textbook.

File size:
291.3 KB
Views:
66
20. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,519
515
I included the picture that shows what I mean.