how to find R2 or R1 in a voltage divider?

[cLarcy]

hi im new here ..

we have our finals coming up and our topic is Bjt frequency response (and OP-AMP).
In our quiz our prof gave us a voltage divider circuit he wanted us to solve for the value of r2 . the midband current gain is given.
I've been encountering this problem since electronics1 and i never really lerned how to solve for it...
Please can anyone help me?
The value of other resistors are given.

usually our prof gives us this kind of circuit either the value of r1 or r2 is missing

Thanks for the help will try it but what if this time you have to get the value of r1?

 

[kabiru]

Hi clarcy, can you post your schematic diagram of your question.

 

[tyassin]

Hi

You should use Thevein equivalent at the base of the transistor. Find the thevenin resistor and the thevenin voltage.

Regards

 

[abbeyromy]

Do the DC and load line analysis to find VCEQ and ICEQ using transistor characterstics.
Once you know ICEQ using current gain calculate base current.
Use thevinin at base side to get equation of IB which will only have R2 is unknown.

Refer any standard book for equations.

Suppose the device characteristics is not available then please let me know what else u have besides Vcc value and remaining resistors..

If this is a lab exercise you generate characteristics for the particular transistor, and manually plot the load line. The intersection gives you Q point values

 

[andi21]

if your prof told to you to design here is the step

First, you must choose the load line for Ic and Vce and gain for the low frequency.
usually for Amplifier class A, Vce=1/2 Vcc..
then Vbe and Hfe usually known..
Rc/RE=gain...(this is gain that you chose for low frequency)
then, Ib=Ic/Hfe

Last, to find R2 and R1 here is the formula
*I0 is a current that trough R1 and R2, usually I0=10 to 100 X Ib..
R2 =(VBE + VE)/I0=(VBE + IC RE)/I0
R1+ R2 = VCC/ I0  R1 = VCC / I0 - R2

C1=1/2πfR2
f=the low frequency
usually C1=C2

because the function of this capacitor is for coupling capasitor