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    degeneration differential pair

    Hello

    I am designing opam which should read large differential input signal,
    as we know from theory that VID= Vov1-Vov2, where Vov is the overdrive voltage for both differential pare transistors.

    I tried my best to increase this difference by increasing the tail current or by reducing the transistor size but still the result is limited,

    is there better approach?

    Thank in advance

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    Re: degeneration differential pair

    "Happiness consists of wanting the right things"

    Op amps don't "read" large signals. An ideal, and even
    a run-of-the-mill op amp, will have full output swing
    on a minuscule input difference. Like, an 80dB AVOL
    amplifier with a +/-10V output range will swing across
    that range with -1 to +1mV (Vio aside).

    Don't know what you plan to do with the excess, but
    there's not much you can (unless you want less, way
    less, gain - such as operating in closed loop feedback).


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    Re: degeneration differential pair

    Hello freebird

    Actually I want to use it as instrumentation amplifier in which I read sensor signals from bridge, this signals are changing in the value of one volt.

    I am using differential deference amplifier but i called opamp.

    because I dont want to use the classical Instrumentation amplifier which consist of three op amp I am designing current sensing InAmp , and for this I am facing the trouble



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    Re: degeneration differential pair

    You can create a simple differential detector by connecting the sources to each other. Each serves as ground for the other source (in a manner of speaking).

    Click image for larger version. 

Name:	differential detector 2 sine 10V sources 2 led's.png 
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    The anti-parallel led's tell which source is greater voltage. Or else measure voltage across the resistor, which allows you to calculate Amperes.

    This is only a basic concept. You can add components to achieve filtering, level-shifting, waveform conditioning, etc.


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    Re: degeneration differential pair

    Thank you Red,

    I think adding level shifter is an good idea,,, such a circuit of source follower which shift down the input signal to VGs-Vth,

    wha about degenrative source, what the are used for?



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    Re: degeneration differential pair

    Source (or emitter) degeneration is often used for a
    tradeoff between DC gain and slew rate. But DC gain is
    not knocked down to the level you are after.

    You do know there are many perfectly fine 8-pin
    instrumentation amplifiers on the market, for about a
    buck, right? So I guess this had better be about the
    adventure and not the solution.


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    Re: degeneration differential pair

    thank you again freebird

    the thing is that my task to design the inamp

    the signals coming from the bridge are +-1V while my amplifier accept +-0.6 V,,,, is it possible to use the source follower as a level shifter ? or this idea doesnt work



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    Re: degeneration differential pair

    Let's start by distinguishing between the range where you
    want proper function, and the range where you need to
    guarantee survival.

    Then, your desired gain of the difference signal, and the
    output signal swing (range).

    Then, the tolerable loading of the input source (because
    a resistor divider (pair), throwing away 50% of signal up
    front could fix the whole thing?). Presuming that the source
    can't itself be convinced to reduce its swing, and that the
    entire range has useful information to be processed.

    I think I've seen papers on making instrumentation amps
    with less than three amplifiers but can't recall where, or the
    suggestions. Also 3-amp InAmps can have problems in low
    supply voltage circuits and high signal, headroom-wise.

    Do you consider the possibility that I/O devices and a
    higher working rail could also be a solution, post-scale
    if needed? Or is there only one, low voltage supply?


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    Re: degeneration differential pair

    i am using current feedback InAmp,,,

    do you mean that if i shift the signal i will lose the information ? may be i didnt understand this point



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    Re: degeneration differential pair

    Quote Originally Posted by Junus2012 View Post
    the signals coming from the bridge are +-1V while my amplifier accept +-0.6 V,,,, is it possible to use the source follower as a level shifter ? or this idea doesnt work
    Why don't you use your rail-to-rail amplifier fed back for a very low gain, say 2..3 , depending on your supply voltage, as dick_freebird suggested above (last line)?



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