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    how to measure differential gain of a simple differential amplifier using HSPICE?

    i want to calculate/measure differential output and gain of a simple differential amplifier.i am using HSPICE and cosmos-scope.i have given +0.5v to one input and -0.5v to other input.i am varying the inputs,one from 0 to 1v and another from -1 to 0v.but i have doubt if it is correct or not.because both inputs should vary simultaneously but HSPICE generates different plot file for two vary.in one file only one is varying and other one is fixed. and in other file vice versa.and i don't know how to measure differential output and gain.

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    Re: how to measure differential gain of a simple differential amplifier using HSPICE?

    Why do you want two opposite signals at both inputs at the same time? This is not the normal working condition of a differential stage. The classical measurement is with one of the inputs grounded (assuming split supply with proper biasing for 0 volts at both inputs).



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    Re: how to measure differential gain of a simple differential amplifier using HSPICE?

    Can you please explain what do you mean by "(assuming split supply with proper biasing for 0 volts at both inputs)".Actually this is my first encounter to this kind of problem.Thanks for your reply by the way.



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    Re: how to measure differential gain of a simple differential amplifier using HSPICE?

    In my answer I have assumed a split supply that, however, is not necessary.
    Anyway, my advice is to ground one of the differential inputs for ac signals (with an appropriate capacitor) and to measure/simulate the gain related to the other input terminal. But don't forget to correctly bias the ac grounded node.


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    Re: how to measure differential gain of a simple differential amplifier using HSPICE?

    can u recommend any book(chapter) or any reading material for this.Because i am not getting how this configuration will do the work of a differential amplifier.because only one input is varying and other one is ac ground or fixed dc bias.and i want to know how to measure the output differential voltage.because there are two outputs now.and to find differential output i have to subtract each value point by point.Suggest me should i use transient/ac or dc analysis.Thank u for ur reply once again.



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    Re: how to measure differential gain of a simple differential amplifier using HSPICE?

    I think, you don't need any books or other material cause it's basic.
    Independent on the chosen analysis type you always can compute the difference between both output signals.
    *tran analysis if you are interested in one frequency only
    *ac analysis if you like to see the frequency characteristic
    *dc analysis if you like to the input resp. output voltage range that is available for (nearly) linear operation. In this case you have to evaluate the slope of the characteristic curve.



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    Re: how to measure differential gain of a simple differential amplifier using HSPICE?

    can u explain the inputs.i didn't understand how it works as a differential amplifier.Because as u said one input is ac ground.i.e dc voltage bias in parallel with a capacitor.and in other input a single frequency voltage source.Now i think for dc,ac or tran analysis this connection will be different.Can u please elaborate.Thanks again for ur answer.



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    Re: how to measure differential gain of a simple differential amplifier using HSPICE?

    Quote Originally Posted by arnabbiswas_ece View Post
    can u explain the inputs.i didn't understand how it works as a differential amplifier.Because as u said one input is ac ground.i.e dc voltage bias in parallel with a capacitor.and in other input a single frequency voltage source.Now i think for dc,ac or tran analysis this connection will be different.Can u please elaborate.Thanks again for ur answer.
    As you don't understand "how it works" with a single input:
    If one input of the diff. amplifier is ac grounded, you can treat the whole circuit as follows:
    * The first stage works as a common emitter stage with degeneration (negativ feedback due to a dynamic emitter resistance due to the input resistance of the 2nd stage)
    * The 2nd stage is a common base stage driven by the 1st stage (at the common emiiter node) which, therefore, can be regarded as a common collector stage.
    * Thus, both output nodes show an output signal.

    Therefore, you only have to connect the input source (dc, ac or tran) to the base of the 1st stage with the second input ac grounded. OK?


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