Engineering Collector Current through BJT Circuit

[curiousguy23]

Homework Statement

Consider the following circuit
http://img109.imageshack.us/img109/80/circuit1.jpg

IL, current through R3 (unknown resistance must be found)

Homework Equations

Vth=(Vcc/Rtotal)*R1 (V tThevenin)

Rth=R1//R2 (R Thevenin)

Vbe= 0.7 volts (silicon diode)
Beta (transistor constant)=100
Vcc=10v
IL=Beta*Ib
Ie=Ib + IL

The Attempt at a Solution

I used the following Equation to try and obtain an answer.

Vcc= (Ib*Rth)+ (Ie*R4) + Vbe + Vth

Vth=5v
Rth=2.5k Ohms

Substituting gives:

10=(IL/Beta)*(2.5k) + 1000*(IL*((Beta +1)/Beta)) + 0.7 + 5

This yields an answer of IL=4.15mA.

My tutor said this is a wrong answer, the correct one is 4.3mA
What is wrong in my reasoning/Working?

 

[vk6kro]

I like your answer better.

The tutor has just taken 0.7 V from the 5 V at the center of the two 5K resistors and said this must be the voltage across the emitter resistor.

But the base current pulls this voltage down a little and that makes the difference between your answers.

 

[curiousguy23]

THanks,

I just realized what he did,


He assumed Ie=IL and Ib=0 since it is very small.