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  1. #1
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    Designing an amplifier with particular values

    Hi everyone,
    As part of the course i took in analog circuit i required to design an amplifier with these values:

    Rin= 52kohm
    Rout = 53 ohm
    A=35 gain in db
    F-3dblow = 400hz
    F-3dbhigh = 400khz
    M1= 40db/dec
    M2 = -20 db/dec
    Dc supply voltage are : +5 v , -5v
    Transitors are ntype BJT

    My idea was using a simple CE amplifier with 2 resistors that are parallel to the base port.
    I started assuming collector current and tried to find the circuit values that will give me the characeristic i needed.
    Every time i found the values i needed and tried to simulate a bias point in PSpice, it didnt work, the collector current was not as i assume and voltages in the circuit were wrong.

    I didnt made any progess and i just stand in 1 place and dont understand what im doing wrong.

    My questions are then:
    1- what am i doing wrong?
    2- can u give me specific rules and instruction where to start from and how to get the values i want?
    3- do you have any other idea how to design a circuit with these values?

    I search the internet and found only specific circuit desgin, it didnt helped me much.

    Thanks everyone.

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  2. #2
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    Re: Designing an amplifier with particular values

    Here's a circuit from Tietze, Schenk, Halbleiterschaltungstechnik (Electronic Circuits Handbook), 11th German edition, pg. 154. It can be probably modified to achieve the intended specs, hence you could use it as starting point. A capacitor for the high frequency corner has to be added. I already reduced R4 form original 120 to 68 ohms to achieve 35 dB gain.

    Click image for larger version. 

Name:	ts fig 2.92.PNG 
Views:	7 
Size:	10.3 KB 
ID:	157402

    There are some specification points missing like voltage swing, acceptable gain etc. tolerances. I understand that this is a simplified homework problem rather than real design, you'll apparently design with typical transistor parameters and don't care for current gain type variation or temperature effects.


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  3. #3
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    Re: Designing an amplifier with particular values

    Quote Originally Posted by FvM View Post
    Here's a circuit from Tietze, Schenk, Halbleiterschaltungstechnik (Electronic Circuits Handbook), 11th German edition, pg. 154. It can be probably modified to achieve the intended specs, hence you could use it as starting point. A capacitor for the high frequency corner has to be added. I already reduced R4 form original 120 to 68 ohms to achieve 35 dB gain.

    Click image for larger version. 

Name:	ts fig 2.92.PNG 
Views:	7 
Size:	10.3 KB 
ID:	157402

    There are some specification points missing like voltage swing, acceptable gain etc. tolerances. I understand that this is a simplified homework problem rather than real design, you'll apparently design with typical transistor parameters and don't care for current gain type variation or temperature effects.
    Indeed a good starting point, I added another CE degree to reach both input impedance and desire gain, added some capacitor to get the right angle in bode, the simulation gone perfect. Much appreciation!



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