If \(\beta =30\)

* Find \(V_B, V_E, V_C\).

* If \(R_B\) changes to \(270k\Omega\), find \(V_B, V_E, V_C\).

* For what value of \(\beta\) will voltages V(B),V(E),V(C) get back to previous values.

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Here's what I got.
\(I_B=0.074977mA\)
\(I_C=2.24932mA\)
\(I_E=2.3243mA\)

\(V_B=2.0244V\)
\(V_C=-2.9268\)
\(V_E=6.65757\)

If R(B) changes to 270k:

\(I_B=0.02347mA\)
\(I_C=0.70398mA\)
\(I_E=0.72745mA\)

\(V_B=6.3358V\)
\(V_C=-7.09924\)
\(V_E=7.03588\)

Here comes the 'problem', finding Beta...
I calculated \(I_B\) using the previous (first) V(B) value.
\(I_B=0.0074977mA\)
So If V(C) gets back to the previous value, I(C) should be the same.
And \(\beta = \frac{I_B}{I_C}=300\).
But I realized I can find Beta with another method, using K-laws.
\(9-2.7kI_E-0.7-270kI_B=0\)
using:
\(I_B=\frac{I_E}{\beta +1}\)
and the value of I(C) found first, I get:
\(\beta =309\)

Can anyone tell me if any of the solutions is right or wrong and why.
Thanks in advance.

 

Do you use a simulation program to find Ib,Ic and Ie ??

 

Do you use a simulation program to find Ib,Ic and Ie ??

No.
\(I_B=\frac{9-0.7}{27k+2.7k\cdot 31}\)
Ic=Beta*Ib
Ie=(Beta+1)*Ib

=)

 

Check your original VE. It's inconsistent with your emitter current. The emitter voltage should be ~0.7 V above the base voltage.

 

For

Hfe = 30 and Rb = 27K

Ib = (9V - 0.7V)/ ( 27K + 31*2.7K) = 74.977μA

Ic = 30 *Ib = 2.249mA

Ie = 31 *Ib = 2.324mA

Ve = 9V - Ie*Re = 2.725V

Vc = Ic*Rc - 9V = -2.927V


Now for Rb = 270K and β = 30

Ib = (9V - 0.7V)/ ( 270K + 31*2.7K) = 23.466μA

Ic = 30 * Ib = 703.986μA

Ie = 31 * Ib = 727.452μA

Ve = 9 - Ie*Re = 7.035V

Vc = Ic*Rc - 9V = 7.099V

And I don't see any problem ?

 

@mlog - Yeah you're right. Ve should be 2.724V.

@Jony130 - Thanks for the confirmations, but I'm having problems with the third part of the problem.

* For what value of Beta will voltages V(B),V(E),V(C) get back to previous values.

so with Rb=270k, I have to find the value of Beta so the terminal voltages get their previous values.

I think Beta should be 300, because Rb changed from 27k to 270k.
So if Beta was 30 first, it should be 300 when Rb=270k and the terminal voltages get back to their previous values.

I tried another method (you can see in the first post) and I get Beta=309.
What am I missing ?
Is Beta 300 or 309 or none?

 

so with Rb=270k, I have to find the value of Beta so the terminal voltages get their previous values.

I think Beta should be 300, because Rb changed from 27k to 270k.
So if Beta was 30 first, it should be 300 when Rb=270k and the terminal voltages get back to their previous values.

I tried another method (you can see in the first post) and I get Beta=309.
What am I missing ?
Is Beta 300 or 309 or none?

I think that both of your answer are correct.
Simply it is impassible to get exact voltage for R = 270K.
The the reason for this is that emitter current is equal
Ie = Ic + Ib but for R=270K the Ib is ten times smeller than previous values. And this cause the "error".