# Homework Help: Simple Transistor

1. Mar 19, 2014

### Duave

.......

Last edited: Mar 19, 2014
2. Mar 19, 2014

### Jony130

Maybe this circuit will be easy for you to solve, you can even use nodal analysis.

Also notice that I1 = (Ib + I2)

Or you can replace voltage divider with his thevenin equivalent circuit.

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3. Mar 19, 2014

### Duave

Here is a revised version of the solving for VB using KVL
This is question 2(a).

Thank you for any help that you can offer.

Did I answer the question correctly and thoroughly? Can you please find any errors, and point them out to me so that I can fix them?

1. The problem statement, all variables and given/known data

2(a): Calculate VB including loading of the bias network by the BJT

2. Relevant equations

IE = IC + IB
...............................................................
VCC-(IB)(RB) -(IE)(RE)-(VBE) = 0
....................................................................................................

3. The attempt at a solution

https://scontent-b.xx.fbcdn.net/hphotos-frc1/t1.0-9/1964876_10151943479145919_1107480603_n.jpg

Is the answer correct for V(B)? If not, where are the errors located?

4. Mar 19, 2014

### Jony130

You have error already in the the second expression. Are you sure that this is correct KVL/KCL for this circuit? Can you tell me where do you see RB resistor in this circuit ?
The KCL for VB node is I1 = I2 + Ib

Or why you don't try to use thevenin's theorem and replace R1 and R2 and Vcc with his equivalent circuit (Rth and Eth)?

5. Mar 19, 2014

### Duave

Could Rb= Rb1||Rb2?

6. Mar 19, 2014

### Duave

How many loops do you have with KVL?

7. Mar 19, 2014

### Jony130

Two loops:
First loop
Vcc = I1*R1 + I2*R2
And the second one
I2*R2 = ??

Or we can use nodal equation

(15V - Vb)/130k = Vb/150k + Ib

Where Ib = ??

Yes, but what about Vcc ?

Last edited: Mar 19, 2014
8. Mar 19, 2014

### Duave

I don't understand,

Wouldn't it be :

Vcc - Ib(R1||R2) - IeRe - Vbe = 0

and
Vcc = I1*R1 + I2*R2

9. Mar 19, 2014

### Jony130

No this is incorrect because Ib current flows only through R1 resistor.
But we can use Rb = R1||R2 if you replace R1 and R2 voltage divider with his thevenin's equivalent circuit.

10. Mar 19, 2014

### Staff: Mentor

11. Mar 19, 2014

### Duave

you please just tell me the KVL for this circuit again. I cannot obtain an official answer anywhere. I've been working on this problem for four days and it's only worth two points. I don't get it, and I need it to be laid out without anymore theory, on math.

You've been a big help. Could you show me both of the KVLs for this circuit

thank you

12. Mar 19, 2014