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    Differential Amp gain

    I have a simple Bipolar Differential Amplifier with a BJT input stage. The equivalent circuit I have drawn and attached. When I do an AC simulation of the diff amp, giving a 500m and -500m signals at the inputs, I get the output which looks like the one I have attached.
    The differential gain of the output looks around 0.4 which was about right for my circuit. But it shows a common mode gain of 5, why I am I getting such a huge common mode gain? The tail current is implemented using a MOS current source.

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    Re: Differential Amp gain

    My be you have to insert common mode feedback when simulate the amplifier!!!



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    Re: Differential Amp gain

    Quote Originally Posted by tyanata
    My be you have to insert common mode feedback when simulate the amplifier!!!
    The load of the diff amp is made by a resistor so the output common mode is precisely defined by the current times the resistance. So this would not need a common mode feedback.



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    Re: Differential Amp gain

    Hi

    My reply is a bit late (Maybe you have already figured out the answer by now)

    The outputs you are seeing are not corresponding to a common mode gain of 5.

    It's just that you get larger swings at the output when the frequency of the input is lower as compared to a higher frequency.

    As for the differential output it is wrong to predict the value just by looking at the amplitude plots. You need to plot (voutp-voutn)/(vinp-vinn). This will take care of phase as well.

    For common mode change of output you need to see (voutp+voutn). And it must be zero provided there is no un-symmetry in your ckt.

    P.S. In my post, all the voutp, voutn, vinn, and vinp are not just the mag. response. They include total info. (i.e. the phase also).



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    Re: Differential Amp gain

    did u check the input common mode which bias the bjt in active mode
    also 7µ amp is too low this may cause the gm of the transistor to be very small



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    Re: Differential Amp gain

    Hello Advaita,
    yes I did figure out the problem. By common mode gain I meant that from the transfer function you see a common mode signal which will be the average of the 2 outputs Which is 5. The input differential signal is 1 so that means the differential to common mode gain is 5. And the differential gain is as you said (Voutp-Voutn)/(Vinp-Vinn) which is 0.4, because Vinp-Vinn =1 . So yes the observations I gave in my 1st post are correct.
    The common mode gain was actually coming because of a common mode feedback circuit in the next stage that was moving the common mode voltage of the nodes. This was the 1st stage of a folded cascode amp with common mode feedback.



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    Re: Differential Amp gain

    where is output wave?



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  8. #8
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    Re: Differential Amp gain

    Quote Originally Posted by wonbef
    where is output wave?
    Its in the JPG file attached output.jpg



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