# Differential Amplifier plus CB

Discussion in 'Homework Help' started by Guitarras, Dec 10, 2010.

1. ### Guitarras Thread Starter New Member

Dec 10, 2010
14
0
Hi everyone!

I have some questions to scale the resistances of a differential amplifier with passive load. I will present the circuit:

The objective is to scale the circuit of the figure to obtain a collector current of 5 mA transistor Q3. My first step was to create a current source of 5mA. I used a diode and a BJT and tested in a simulator, it is okay.
Now my question is how to scale the resistors R1 and R2.

Things that I know:
vb = 0
vbe = vb-ve = 0.7 -> v =- 0.7

(12-vc) / R1 = ic -> VC1 = 12 - ic1 * R1 ...

Can someone tell me how I can get some value of R1 and R2 only with this information?
I don't ask for resolutions, I really prefer a explication of the operation or a method to resolve my problem than expressions with values, etc, etc because I'm not very comfortable with this.

I appreciate any clarification.

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,166
1,188
The only think we need is to use Ohms law and KVC, KVL.
For example if we assume
Ic1 = Ic2 = 2.5mA and Ic3 = 5mA
So form KCL we have:
IR2 = Ic2 + Ic3 = 7.5mA
IR1 = 2.5mA
Ib3 = 5mA/β = 5mA/100 = 50μA
And from KVL

Vb3 = 0V ----> becaues R3=R4 = 12V/ (10 * Ib3) 22KΩ
Ve3 = Vb3 + Vbe = 0.7V

R2 =( Vcc - Ve3) / (Ic2 + Ic3) = 1.5K
R2 = (Vcc - Ve3)/ Ic1
4.7K
And finally we can chose R5 for a given Vce3.
And if we assume Vce=6V then R5=1.2KΩ

3. ### Guitarras Thread Starter New Member

Dec 10, 2010
14
0
Thanks, Jony130.