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low input impedance amplifier

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

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  1. Guest

    Please suggest very low input impedance amplifiers, preferably
    operational ampliers.
  2. MooseFET

    MooseFET Guest

    If this is for DC operation (ASCII art):

    ! !
    ! ----!!------ !
    IN ----+---!+\ ! ! !
    ! >--+---/\/\--+--!-\ ! !
    --!-/ ! ! >------+----------
    ! ! GND---!+/

    Op-amps = TL072
    Resistors = 1.1M
    Capacitors = 50u 25V Mylar
  3. Eeyore

    Eeyore Guest

    Please suggest your application.

    Operational; amplifiers by definition all have HIGH input impedance. I strongly
    suspect you don't want/need an op-amp.

  4. Eeyore

    Eeyore Guest

    What on earth is that ?

    50u mylar ??????

  5. Tom Bruhns

    Tom Bruhns Guest

    Please suggest more design design requirements, preferably related to
    the application.
  6. err....whats wrong with a umm... feedback summing amp...for low freq, opamp
    methods will beat them all at tend to zero ohms
  7. Please suggest very low input impedance amplifiers, preferably
    Virtual earth (a.k.a., virtual ground) current to voltage converter:
  8. ---------/\/\---- !
    Resistance sets the I to V ratio, input appears "grounded" over ranges
    of operation where OpAmp can servo it.
  9. whit3rd

    whit3rd Guest

    Operational amplifiers are inverting and have high gain (at DC
    as well as at signal frequencies). Almost all idealizations
    of op amps add 'fully-differential' and 'high input impedance'
    and 'low output impedance' to this.

    Norton amplifiers (current-input-differencing) like LM3900
    have low input impedance BUT that goes with requirement to
    ensure with suitable external components that the input doesn't
    get too much current (and burn up) or go negative (and
    saturate/shut down the amplifier).

    Some op amps (like uA741) have 'input offset' pins that
    amount to a low-impedance input option.

    And an inverting-gain connected op amp (ground the (+) pin
    and use R1 from signal to (-), and R2 from output to (-) ) has
    the input impedance equal to R1; it can be anything you want.
  10. MooseFET

    MooseFET Guest

    Work out the input impaedance at Dc and you will see.
    Use 2 25uF ones in parallel if you want. The values given were based
    on the assumption it was a home work question.
  11. Guest

    Dear all,
    I mean something like transimpedance amplifier..where the input is
    current and ouput is voltage...such amplifier invariably has low input
    impedance...please suggest any commercial amplifier for it.
  12. MooseFET

    MooseFET Guest

    What do you mean by "commercial amplifier". If you mean op-amp IC
    that is one thing. If you mean a complete box that is another.
  13. Guest

    bu commercial ..i mean opamp or other amplifier aviable in IC package.
  14. Winfield

    Winfield Guest

    kristo, we all just use a feedback resistor (plus capacitor) and
    chose a suitable opamp for the problem. For example, a JFET
    opamp for low currents and high transresistance values, or a
    high-speed bipolar opamp for high current and low resistance.
    For really high speeds, in the GHz territory, such as for fibre-
    optic data receivers, its useful to have the feedback resistor
    integrated with a specialized opamp, and in fact its most useful
    to have the detector and optical connectors in the package too.
  15. MooseFET

    MooseFET Guest

    You haven't stated any frequency requirements. The circuit I posted
    way back in this thread actually would work but only for low

    Go to and download a copy of "SwitcherCad-III". It is
    a very good general purpose spice program. You can use it to verify
    your design when you get it done.

    The details of how you do a low input impedance amplifier vary a lot
    with the frequency requirements but the general idea is usually the
    same. You always have an op-amp circuit that greatly amplifies any
    voltage that may appear on the input terminal and a feedback resistor
    that passes a large current for any given voltage.

    Consider this example condition:

    1uV input ----+-------/\/\---------
    ! !
    ! !\ !
    -------! >----------
    !/ Gain = -1 million

    The output of the Gain stage will be -1V so 10mA will go through the
    100R. As far as the 1uV input is concerned the input resistance is:

    R(in) = 1uV / 10mA = 0.0001R

    For low frequencies, op-amps like the TL072 will provide the gain you
    need. You can use a two stage amplifier with two op-amps. (One would
    have to be inverting and the other not)
  16. YD

    YD Guest

    Late at night, by candle light, ""
    If it's DC or low frequency AC at reasonably high level just about any
    old thing like a 741 might do. Whatzit you need it for?

    - YD.
  17. Guest

    I want the amplifier to have low input impedance till 50MHz.
  18. MooseFET

    MooseFET Guest

    How low is low?

    At 50MHz, you are looking at a fairly fast op-amp being needed.

    You may want to look at parts like the LT1398. You aren't going to be
    able to have much gain in the amplifier and make it stable.
  19. This might be a good application for a current feedback op
    amp like this one:
  20. Fred Bartoli

    Fred Bartoli Guest

    John Popelish a écrit :
    Don't know if it's necessary to get that far, depending on what krishman
    CFB opamps have inherently low minus input impedance, typically between
    If that's not enough, then this opamp will achieve 'only' 11R at 50MHz.

    This figure could be improved with a generously biased discrete CB pair
    at the input stage of a composite opamp.

    But if the low frequency isn't DC and really low impedance is mandatory,
    a small current transformer might be more appropriate, or an opamp-CT
    hybrid if he has to get down to DC.

    As usual it'd be better if we knew what the OP wants to do in the end.
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