High gain diff amp

Discussion in 'General Electronics Chat' started by automagp68, Nov 23, 2015.

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  1. automagp68

    Thread Starter Member

    Nov 13, 2011
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    So i need to build a high gain diff amp. Only BJT,s available.

    need +- 7.5 Volt swing

    Typically i can't get a but diff amp to give me a gain of more then a 100 or so. I need 800 differential gain

    So the first thought is to cascade a few together. However, as the output voltage goes up and goes into the base of the next stage all i get is clipping.

    Say i built a single stage diff amp and then wanted to just get a bunch of gain out of it. Take the output and maybe run it into a Common emitter maybe
    How do i get the common emitter to not clip with a large input signal?

    Any other thoughts or proposed ideas on how to do this?
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Simple use diff amp as a input stage, next stage common emitter and finally stage (output stage) common collector.
    And CE stage will provide the most of a needed gain.
     
  3. automagp68

    Thread Starter Member

    Nov 13, 2011
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    Whats the point of the last stage?

    High output impedance? I only need to drive a 1k load?

    How do i keep the CE from clipping with an input voltage of a few hundred milivolts?

    Right now my diff amp is outputting around 700 mv with a 10mv input which I'm happy with i guess.s Now i need to crank up the gain.
    Will a CE take a 700mv input and no clip with +- 7.5 volt swing on a 15v supply?
    thanks for your input
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    Most amplifier datasheets contain either a simplified or complete schematic of the circuit. Read and learn from them.
     
  5. automagp68

    Thread Starter Member

    Nov 13, 2011
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    Let me also say because this is a deign for a discrete chip i can't use a bypass cap for a CE

    Any ideas on how to do this?

    Without a bypass cap i am seeing low single digit gains
    I need a 7.5 volt swing. Frequency response is unimportant as long as it works for around 50k-100k
     
  6. Veracohr

    Well-Known Member

    Jan 3, 2011
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    You could do it like an opamp: design huge open loop gain & use negative feedback to set it where you need.
     
  7. automagp68

    Thread Starter Member

    Nov 13, 2011
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    Essentially what I'm doing is building the first few stages of an low grade op amp

    However i do not have the understanding to use feedback like an op amp does

    Needs to be done without that
     
  8. DickCappels

    Moderator

    Aug 21, 2008
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