BJT multistage Op-amp with very high gain

Discussion in 'The Projects Forum' started by Dola, Apr 4, 2010.

  1. Dola

    Thread Starter New Member

    Apr 4, 2010
    Hi all.
    It is so nice indeed for joining this giant forum , it is my first time being here :)

    I badly need your help to design op-amp. The design of the op-amp should fulfill the following minimum requirements:

    1. Voltage gain > 150000
    2. Supply voltage +-12v
    3. Input impedance > 150 MΩ
    4. Resistor (if any) < 10 KΩ
    5. The output stage should be able to handle current of more than 50 mA
    6. The bandwidth should be at least 250Khz
    8. System stability is considered and some form of short circuit should be provided.

    note : no capacitor should be used unless justified .

    thanx so much indeed.
  2. Markd77

    Senior Member

    Sep 7, 2009
    I'd suggest downloading datasheets for similar existing op amps, you will probably find one which has a schematic of the internals.
    For example the LM324 datasheet has one:
    It doesn't quite meet your specifications but hopefully you can find one that has.
    Figuring the resistor values may be tricky because they are normally not given.
  3. kkazem

    Active Member

    Jul 23, 2009
    The problem with using a manufacturers datasheet schematic is that they are not the actual schematic, they are a simplified schematic and any resistor values they show are likely what an equivalent resistor value would be, except that they are nearly always constant current sources (usually current mirrors). Try the following: a) do a google search on design of differential amplifiers, or design of operational amplifiers, or design of op-amps, and you can try to narrow your search to BJT types if you get lots of hits (and you will get lots of hits). If this is for a college course, use your textbook as it should have the theory and examples of this kind of design and probably examples very close to or even that may meet your specs as-is. You can also go to the library and look at other electrical engineering books on design of differential amps or op-amps. b) Do go to the semiconductor manufacturer's websites, but do searches for application notes on design of differential amps and/or op-amps using discrete components. c) use LTspice or PSPICE to model and test your circuit using computer analysis. If you don't have it, don't worry, it's free of charge at, just look for the link to LTSPICEIV, and there are no restrictions as you would have with the student version of PSPICE, plus the library is free. ALso, on Yahoo user groups, there is a user group for LTSPICE users that you can ask questions on if you get stuck, and the answers come quick as there are lots of users. d) IF you still have problems, you can call the semi mfgs that sell discrete BJTs, like National Semi, On Semi, Fairchild Semi, etc, and ask their application engineers for help--again, it's free help. That is their job--although I wouldn't neseccarily say that you're designing it for a class or they may not want to help.
    Good luck.
    Kamran Kazem