[BJT] Changing bandwidth without changing gain

Discussion in 'Homework Help' started by Guitarras, Mar 13, 2012.

  1. Guitarras

    Thread Starter New Member

    Dec 10, 2010
    14
    0
    Hi everybody!

    If I have a 2 stage Bjt circuit amplifier with a common emitter and then a emitter follower how can i make the bandwidth to grow without changing the gain of the circuit?

    Maybe a common base between the 2 stages (like a cascode configuration)?

    Thanks in advance.
     
  2. mlog

    Member

    Feb 11, 2012
    276
    36
    Two words: Negative feedback. Think about it and tell us your thoughts on how you might do it.
     
  3. jtrent

    New Member

    Mar 11, 2012
    26
    4
    As you probably understand gain and bandwidth have an inverse relationship. Read my full comments here.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Negative feedback generally reduces gain.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Cascode helps if you have a source impedance, which in conjunction with Miller capacitance lowers the bandwidth. A cascode stage can be used to reduce Miller capacitance. If you have a zero impedance source, Miller capacitance will not be much of an issue.
    You can reduce the Ccb of the emitter follower by bootstrapping it with another emitter follower. This will increase the bandwidth.
     
  6. mlog

    Member

    Feb 11, 2012
    276
    36
    Agreed. In my haste, I misinterpreted the question. I know that if the gain bandwidth product is fixed, then you trade off bandwidth vs. gain. I thought the question was about maintaing a fixed transistor gain (hfe or beta).
     
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