# help with pre amp design

Discussion in 'Homework Help' started by aruna1, Dec 17, 2009.

1. ### aruna1 Thread Starter Active Member

Dec 20, 2008
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I have to design a multistage bjt pre amplifier with following conditions
gain 20-60dB
input impedence >= 5K ohm
output impedence <= 100 ohm
bandwidth 300Hz - 3MHz
source impedence 600 ohm

i dont know how to calculate capacitor values to match above bandwidth

can someone please give me a good tutorial link which i can learn to design above pre amplifier?

thanks

Mar 8, 2009
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3. ### aruna1 Thread Starter Active Member

Dec 20, 2008
46
0
came up with this

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Hmm, we could pick C2,C3,C4 for F=30Hz.
Xc=1/(2*PI*F*C)=0.16/(F*C)
C=0.16/(F*R)
And then C1 for 300Hz
For example
C4=0.16/(30Hz*10K)=533nF but we decide to use C4=1uF
C2=0.16/(F*(Rc+Rin2))=470nF
C3=47uF
C1=0.16/(300Hz*(Rs+Rin1))=56nF.
Then remove C5 and calculate Fg from small signal model.

$F=\frac{1 }{2*\Pi*Cin*Rs||Rin1}$

$Cin=C_{be}*\frac{r_e }{r_e+(Hfe+1)*Re1}+C_{bc}*(1-Ku)$

Ku - Voltage gain

re=26mV/Ic

gm=1/re

$C_{be}=\frac{gm }{2*\Pi*Ft}-C_{bc}$

After that calculation we must add capacitor parallel to Cbc to achieve Fg=3MHz.

Dec 20, 2008
46
0
thanks

6. ### PRS Well-Known Member

Aug 24, 2008
989
36
This looks like a typical EE student design project. I did one very much like it. I take it you know how to get the midband gain, but you are wondering how to get the 300 Hz to 3 MHz bandwidth.

The lower roll on frequency of 300 Hz is determined by your coupling capacitors and the bypass capacitors of your common emitter stages. Create a dominant pole with a coupling capacitor, and you'll get your lower roll on frequency. Remember that its impedance goes up as the frequency goes down, thereby blocking more current as the frequencygoes down. A good choice is the input capacitor of the first stage. Calculate C as 1/(2*pi*R*F) where F is 300 Hz and R is the resistance seen by that capacitor. Rs + Rb//Rin

To get the upper roll off frequency you need to consider that it is the internal capacitance s of your transistors that determine the highest frequency your circuit is capable of. Design the overall circuit with small valued resistors and small gains per stage and use high hfe (ft) transistors. Doing this you need to exceed the output specification of 3 MHz. Once this is done, if you want to limit the bandwidth to 3 MHz you need to limit the high frequency roll off point. You can do this with a capacitor in parallel with RL, the load. It's impedance goes down as the frequency goes up. So choose a capacitor such that C = 1/(2*pi*R*F) Where F is 3 MHz and R is your load resistor in parallel with the output resistance of the last stage.

Last edited: Dec 22, 2009