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low input impedance amplifier

Discussion in 'Electronic Design' started by [email protected], May 4, 2007.

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  1. MooseFET

    MooseFET Guest

    I agree. He may be happy with just a common base stage.
  2. Guest

    Thanks. actually trying to make a CT loaded with really low impedance
    till 50MHz. The low impedance at DC is not requirement...I tried the
    OP-CT..trying to improve it by having really small burden.....
  3. With a fast opamp, you might actually be able to load it
    with a small negative impedance, to compensate for the
    series resistance of the winding. But it probably would not
    make a measurable difference from what you would get with a
    perfect zero burden impedance.

    The fast opamp I listed with a 50 ohm feedback resistor and
    a 50 ohm output series resistor should produce a pretty low
    burden, but produce an output equivalent to what you would
    get with a perfect CT and a 50 ohm burden.
  4. Fred Bartoli

    Fred Bartoli Guest

    John Popelish a écrit :

    Negative resistance will give better LF response and plays no role at
    50MHz. At this frequency the important thing is leakage inductance.
    More precisely, from the burden voltage POV, the primary side referred
    leakage inductance is what matters.

    as you know, with CFB opamps you can't reduce the feedback impedance at
    will. I didn't checked, but onsemi used 330R in their spec. sheet so it
    must be somewhat optimal. And at unity gain, the curves show 4+dB of
    peaking and reducing the FB resistor down to 50R seems hazardous.

    Anyway, even at a low 1:10 ratio, just a tiny 1nH primary leakage
    inductance will give 31R secondary transformed impedance, showing that
    burden voltage won't be impacted at all by lowering the opamp input
    impedance beyond a few 10s ohm (easily provided by a simple common base

    Carefull primary side design is a must there.
  5. Yes, thank you for reminding me the point of a negative
    resistance CT burden. The lase application I saw was
    extending the low frequency roll off of an ELF coil antenna.

    To get anything like best performance from an opamp burden,
    the opamp would probably have to be mounted directly on the
    coil. Coming up with a core material with high permeability
    at 50 MHz would also help. Even the winding form would
    matter. It might make a measurable difference if the
    secondary came back around the core to the starting point,
    rather than going all the way around the core, forming a
    turn that was not coupled to the primary.
    I have no experience with CFB opamps, but am looking for
    circuits that allow them to shine. Do you have any feel for
    how the optimum value of the feedback resistor depends on
    the shunt to ground impedance at the inverting input?
    Does that mean that the core window should be no larger than
    necessary to contain the two windings?
  6. Fred Bartoli

    Fred Bartoli Guest

    John Popelish a écrit :
    Recently I had the opportunity of a design making use of this for a CT.
    The first estimation was extending the LF cut off from a few Hz down to
    under a 10th Hz.
    One thing you have to take care of is that copper has a 0.4% tempco, and
    a total negative resistance will obviously have a nasty behaviour so you
    have to take margins so that the resistance stays positive at the lowest
    temperature. If you want to get closer to perfect you might design in
    the same tempco for the negative resistance part, but you still have to
    ensure to stay positive.

    That sure would help.
    The inverting input impedance is essentially a 50/100R resistance (plus
    bonding impedance) and what drives the output voltage is the input
    current. All the interesting properties of CFB opamps come from this.
    For example (for a given opamp):
    - the loop gain almost depends on the FB resistor (that is without
    extrem FB networks, ie while the lower FB network impedance is
    sufficiently higher than the input impedance)
    - one interesting consequence is that you can achieve much higher BW
    with a neg input node parasitics capacitance than with VFB opamps.
    - another interesting point is that the summing node impedance is upper
    bounded by the neg input impedance.

    Not necessarily (think of a bifilar wound toroid for ex.)
    The basic rule, but that's so obvious that I don't dare mention this, is
    to provide means of reducing unlinked flux paths.
  7. Guest

    I am picturing the primary as a sinlge pass through (as is common with
    CTs), rather than a bifilar winding. In that case, it seems to me
    that the optimum coupling would involve having the window area as
    small as possible.

    I guess you could fill the unused window area with silver (or super
    conductor) to exclude flux from that space.
  8. Guest

    Could you add an additional resistance between the feedback resistor-
    current transformer node and the inverting input, to lower the total
    loop gain enough to stabilize the amp with a low value of feedback
    resistor? I have also seen some series RC from inverting input ot
    ground, to phase compensate CFB amplifiers operated with low feedback

    John Popelish
  9. Fred Bartoli

    Fred Bartoli Guest

    a écrit :
    He he, already used that. Nice trick isn't it? :)
  10. Fred Bartoli

    Fred Bartoli Guest

    a écrit :
    Sure, that would stabilize the loop, the limit case being having this
    resistor value RFB, nulling the old feedback resistor and having the CT
    connected to the output. Stable but not very efficient.
    More seriously, it can be done, but what would be the purpose? Reducing
    RFB to reduce the closed loop input impedance? I've never run the
    figures so I can't say for sure but introducing the resitance is
    lowering the opamp transimpedance on one side while you reduce the FB
    impedance on the other. There's probably a sweet spot but I suspect it
    to be with a null additionnal resistor.

    Another thing to take with care is layout, especially since CFB is
    always synonim to high BW. The dual of node capacitance parasitics for
    VFB is series inductance to the neg input. Some interesting subtleties
    can slip in there, as I discovered many years ago with my first
    encounter with CFB, which probably triggers some of the 'hot rod' comments.
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