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OpAmp compensation

Discussion in 'Electronic Design' started by Remco Poelstra, Nov 26, 2009.

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  1. Hi,

    I'm working on a DA converter for audio. I would like to get as close as
    possible to the theoretical 24 bits resolution. I opted for an AD1955,
    so I won't get any further than 21 bits, but that's a good start I think.
    Currently I'm working on a shunt regulator to get very low noise supply
    lines, as well as some other advantages.
    My design (http://remco.beryllium.net/regulator/schematic.pdf) didn't
    quite work in the way it's drawn and I was told it was because of the
    frequency compensation that I needed to add. With their help I got the
    regulator to work, but I don't really know why it works now and what the
    theory behind the correct operation is.
    So I would like to know more about that frequency compensation stuff.
    What's a good start? What books would you recommend?

    Thanks in advance,

    Remco Poelstra
     
  2. qrk

    qrk Guest

    A good book on opamps can be found at
    http://focus.ti.com.cn/cn/lit/an/slod006b/slod006b.pdf
    There is a chapter on compensation.
     
  3. MooseFET schreef:
    Hi,

    I choose this topology, because I would like to keep varying load
    currents out of the supply rails. With this topology there is only DC
    current on the rails, and the fluctuating currents are only in small
    loops close to the consumers (given that the regulator is placed close
    to the consumer).
    An added advantage is with the use of opamps which have no short-circuit
    protection on their outputs, they are limited to the current set by the
    current source.

    Regards,

    Remco Poelstra
     
  4. schreef:
    How can I do the same trick without the opamp? I need some kind of error
    amplifier no?
    The shunt regulators that I found all had quite high noise, at least
    when compared to ICs like the ADR440. Is there a specific IC I missed?

    Regards,

    Remco Poelstra
     
  5. MooseFET schreef:
    I've tried both with a MOSFET and BJT (shouldn't it be an NPN?). I knew
    the BJT had less noise, but at first I couldn't get it to work with the
    BJT, so I tried the MOSFET.
    I don't get the point of your last sentence; When I use a BJT, the
    voltage is still controlled indirectly, no?

    Regards,

    Remco Poelstra
     
  6. schreef:
    Sorry, I don't get your point. What do you mean with 'LTC part'?
    No, I hadn't :)
    True, but I first like to get the circuit operate correctly before doing
    the noise math and perhaps improve on that one.

    Regards,

    Remco Poelstra
     
  7. MooseFET schreef:
    I see, the PNP upside down :). Seems about time to get a good book on
    BJT's, because I don't see why this gives a low impedance path for high
    frequencies. I'll try this one though to check whether it works.
    Am I correct when I say that the opamp controls the output voltage more
    direct in this case, because there is only the constant +-0.7V offset
    from the Vbe? Can I then also conclude that the second stage has low
    gain in this configuration?

    Regards,

    Remco Poelstra
     
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