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    driving capability of two stage opamp

    Hello,

    I have designed a two stage opamp without the output buffer. Is it possible to drive resistive loads with this block? I'm not getting the expected results when I connected the resistors at the output and fed back the portion of output.

    So, should I go for a output buffer to drive the resistive load.

    I have read in some books which states that two stage opamp without a buffer can also drive resistive loads, which created this question for me.

    Please help me here!!

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    Re: driving capability of two stage opamp

    Without an output buffer, you can only drive resistive loads in the range of MegOhms, may be down to several hundred kΩs. Depends on the current of the second stage.



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    Re: driving capability of two stage opamp

    You can "drive" everything you want. It depends what you want. Of course, you can "drive" a resistive load of 1kOhm with an output resistance of 100kOhms.
    In this case, you have build something like a voltage controlled CURRENT source that produces an output voltage (across the load), that entirely (nearly) depend on the load only.
    That means: What is your goal? A voltage amplifier that has a gain value which is independent on the load resistance? Then you need a buffer. Otherwise - see above.



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    Re: driving capability of two stage opamp

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    Hello,

    To be more specific, I have attached an image. Please find the attached and please explain me whether the opamp(two stage) i have used can drive the resistors as shown.



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    Re: driving capability of two stage opamp

    My voltage gain is near to 100db. and when I see the operating condition of my second stage transistor(in common source), it says the gds as 26u. I'm not understanding why it cannot drive the load impedance in feedback as I described in the figure.



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    Re: driving capability of two stage opamp

    please explain me whether the opamp(two stage) i have used can drive the resistors as shown
    You can answer the question yourself if you calculate open loop voltage gain and output impedance of your opamp.

    Of course the answer also depends on the resistor values. But in case of a wideband amplifier, the resistors need to be respectively low, e.g. low kohm range.

    - - - Updated - - -

    Output impedance = ?
    R1 = ?
    R2 = ?



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  7. #7
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    Re: driving capability of two stage opamp

    You know the swing at the output. Calculate I=(Vpeak-peak/Rfeed). Make sure that the second stage has at least I/2 DC current. Then you must be able to drive the Rfeed with very less distortion.



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