High gain from a BJT CE amp with no bypass cap

Discussion in 'General Electronics Chat' started by automagp68, Nov 24, 2015.

  1. automagp68

    Thread Starter Member

    Nov 13, 2011
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    Is this possible?

    I am restricted from using caps for an on chip device

    How do i get some gain out of a common emitter without a bypass cap

    Obviously the presence of RE in my AC model kills my gain given its multiplex by Beta in the AC model
     
  2. #12

    Expert

    Nov 30, 2010
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    The ratio of collector resistor to emitter resistor gives a gain figure. The reason for the capacitor is to make the impedance of the emitter seem to disappear at higher frequencies. Sometimes that means higher than 2 or 3 HZ. It's very negotiable with a capacitor. Very predictable without a capacitor.
     
  3. Brownout

    Well-Known Member

    Jan 10, 2012
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    By what?
     
  4. automagp68

    Thread Starter Member

    Nov 13, 2011
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    Yes i understand that. Thats why in my original post i stated that RE without a capacitor for Vo/Vin is multiplied by beta giving you a large denominator hence a small gain.

    I can't have a large difference in RC and RE giving me the typical ratio to rely on given the fact that I'm restricted to a 15V supply and need a 7.5 +- swing.

    So what do i do. Any thoughts
     
    Last edited: Nov 24, 2015
  5. automagp68

    Thread Starter Member

    Nov 13, 2011
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    The design requirements? Im sure not telling myself i can't use caps for the fun of it.
     
  6. Brownout

    Well-Known Member

    Jan 10, 2012
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    Typically, something drives design requirements. The aren't supposed to be arbitrary. Good luck.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    What RE are you referring to?
     
  8. automagp68

    Thread Starter Member

    Nov 13, 2011
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    The emitter resistor.
     
  9. automagp68

    Thread Starter Member

    Nov 13, 2011
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    These are the specifics i was given. What more do you want me to say about it?
    I was told no bypass caps do to It being ON chip
    As i said in the above post #1

    I was asking for assistance given these parameters. I wasn't asking for someone to come on and question why the parameters are what they are?????
     
  10. kyka

    New Member

    Jun 7, 2015
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    Just use an active load.
     
  11. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    You can eliminate the emitter resistor entirely, but you will need to devise some other form of thermal stabilization.
     
  12. Brownout

    Well-Known Member

    Jan 10, 2012
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    That can be done with a long tail pair. The emitter AC load is the grounded base transistor.
     
    Last edited: Nov 24, 2015
  13. automagp68

    Thread Starter Member

    Nov 13, 2011
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    Can you explain in a little more depth?

    I thought a long tailed pair was a diff amp?

    Here is what i got so far which isn't much obviously.
    The bottom bjtis just a 1mA current source as I'm sure you all have seen before

    [​IMG]


    This circuit is yielding a gain of 80 on paper and around 60 or so in the sim

    Any suggestions on where to go form here are much much appreciated.
     
  14. Brownout

    Well-Known Member

    Jan 10, 2012
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    Well, if a gain or 60 or 80 is the spec, then you're done.
     
  15. Brownout

    Well-Known Member

    Jan 10, 2012
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    r4, r3 can by much higher.
     
  16. Bordodynov

    Active Member

    May 20, 2015
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    See Gain=1500
    This gain for small signals (less than 0.1 mV). By increasing the signal going soft limit. When the signal amplitude 20 mV gain of approximately to 150.

    Joke.png

    Plot change the output amplitude and the gain on the input amplitude

    Plot1.png
     
    Last edited: Nov 25, 2015
  17. DickCappels

    Moderator

    Aug 21, 2008
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  18. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    In 6 posts the TS has not stated how much gain he needs.

    How much gain do you need, at what bandwidth?

    ak
     
  19. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    DickCappels likes this.
  20. automagp68

    Thread Starter Member

    Nov 13, 2011
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    Thanks for the help guys

    I need a gain of 800, 80dB Common mode rejection but that is flexible 7.5 +- swing across a 1k load
     
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