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differential amplifier

Discussion in 'Electronic Design' started by Andrew Xiang, Jul 28, 2003.

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  1. Andrew Xiang

    Andrew Xiang Guest

    If I make a differential amplifier with 2 BJTs, But I only take output
    from one collector? I know the diff gain is only half, but do I still
    get the same benefit of CMRR?

    Want to do a balanced mic preamp if it is beneficial. My wire is only
    10" , I wonder if it makes much difference?

  2. Jim Thompson

    Jim Thompson Guest

    (1) CMRR is usually limited by the performance of the stage where you
    translate from differential to single-ended.

    (2) The input current mirror behaves better than what would be
    indicated by VAF... it is, after all, usually degenerated with emitter
    resistors, which raises the output impedance.

    ...Jim Thompson
  3. Hi Jim:

    The approximation that I am familiar with for the impedance of the current
    source used with some differential amplifers is:

    rcs = rc(1+Re/re)

    where rc is a function of the Early voltage and Re is the emitter resistor
    in the current source.
  4. Jim Thompson

    Jim Thompson Guest

    Your rule of thumb seems about right... but I don't fret the
    approximations, I live in the monolithic world, I *simulate* over all
    process corners and operating temperatures ;-)

    When I need *really* stiff current mirrors I use cascodes.

    ...Jim Thompson
  5. This is correct up to the limit set by hfe. The maximum will be ro.hfe,
    for example, if Re is the collector of another transistor.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
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