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    Differential and common mode input resistance of the fully diffferential amplifier

    Dear friends,

    I would like to ask you haw to simulate

    1. Input differential impedance of the fully differential ampliifer.
    2. Common mode input impedance of the fully differential ampliifer.

    Thank you

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    Re: Differential and common mode input resistance of the fully diffferential amplifie



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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Quote Originally Posted by pancho_hideboo View Post
    Thank you Pancho,

    That was very useful to me,

    Do you have a method please to simulate the differential input and common-mode input capacitance of the fully differential amplifier?



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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Quote Originally Posted by Junus2012 View Post
    Do you have a method please to simulate the differential input and common-mode input capacitance of the fully differential amplifier?
    Use ideal_balun in AC analysis.

    Yin_diff=Idiff/Vdiff
    Yin_com=Icom/Vcom

    Cin_diff=imag(Yin_diff)/(2*pi*freq)
    Cin_com=imag(Yin_com)/(2*pi*freq)

    This is an answer.
    There is no easy answer than this.

    Surely learn and understand linear circuit basic.
    Last edited by pancho_hideboo; 5th September 2019 at 01:04.


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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    See https://www.edaboard.com/showthread.php?372399#10
    Here I don’t use ideal_balun.


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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Quote Originally Posted by pancho_hideboo View Post
    See https://www.edaboard.com/showthread.php?372399#10
    Here I don’t use ideal_balun.
    Thank you Pancho,
    but you used skill for simulation, I usually use the schematic

    I have a question please, is the differentia output impedance of the fully differential amplifier is twice the impedance taken from each output individually? (Rout_diff = 2 Rop=2Ron)



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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Quote Originally Posted by Junus2012 View Post
    but you used skill for simulation,
    I usually use the schematic
    No.
    It is based on schematic.

    Quote Originally Posted by Junus2012 View Post
    is the differentia output impedance of the fully differential amplifier
    is twice the impedance taken from each output individually? (Rout_diff = 2 Rop=2Ron)
    Yes, as far as you can understand diffential mode correctly.

    Differential drive is different from floating drive.


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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Quote Originally Posted by pancho_hideboo View Post
    No.
    It is based on schematic.

    Yes, as far as you can understand diffential mode correctly.

    Differential drive is different from floating drive.
    Quote Originally Posted by pancho_hideboo View Post
    No.
    It is based on schematic.

    Yes, as far as you can understand diffential mode correctly.

    Differential drive is different from floating drive.

    Dear Pancho,

    I have one issue,

    I am simulating the differential output impedance of the fully differential amplifier by using voltage-controlled voltage source (VCVS) instead of using the balun, however, I am getting different result from connecting balun.

    please see my attached both test benches, I presume both test benches are equivalent, unless if I am wrong

    Click image for larger version. 

Name:	New Doc 66.jpg 
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ID:	155438



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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Quote Originally Posted by Junus2012 View Post
    I presume both test benches are equivalent
    No, they are completely different.

    Testbench using balun is correct.
    However testbench using VCVS is wrong.

    Quote Originally Posted by Junus2012 View Post
    unless if I am wrong
    Of course, you are wrong.

    Can you understand VCVS ?
    It is unidirectional.
    You can not see output of Amplifier from VCVS input.


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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Quote Originally Posted by pancho_hideboo View Post
    No, they are completely different.

    Testbench using balun is correct.
    However testbench using VCVS is wrong.

    Of course, you are wrong.

    Can you understand VCVS ?
    It is unidirectional.
    You can not see output of Amplifier from VCVS input.
    Dear Pancho,

    Thank you for correcting me,

    As you said, the VCVS is unidirectional, in which I can use to plot my output differential signal, I tried it to compare both of them in transient simulation and both are compatable. However, my mistake in the simulating of the output impedance that I can not use the VCVS to load my ampliifer because it is unidirectional so can not actually load my amplifier



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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Testbench using VCVS does not drive output of amplifier differentially.
    It does drive output of amplifier as floating.

    On the other hand, testbench using balun does drive output of amplifier differentially.

    Observe postive and negative output nodes of amplifier in Transient Analysis.
    You can see difference between differential and floating.
    Last edited by pancho_hideboo; 10th September 2019 at 14:02.


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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Quote Originally Posted by pancho_hideboo View Post
    Testbench using VCVS does not drive output of amplifier differentially.
    It does drive output of amplifier as floating.

    On the other hand, testbench using balun does drive output of amplifier differentially.

    Observe postive and negative output nodes of amplifier in Transient Analysis.
    You can see difference between differential and floating.
    Dear Pancho,

    I understand your point,
    I believe with you kind explanation that VCVS can not load my amplifier and hence cant drive the output load differentially. I was avoiding this problem by driving the load at each output individually and I use the VCVS just to see the difference in the output signal after driving the load.

    Kindly see my below images named method 1 and method 2. in Both cases I am getting an identical result and fit to the theoretical calculation. In this test benches, the feedback connection or the input circuitry is not showing

    Click image for larger version. 

Name:	New Doc 66_2.jpg 
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    That means in the method 2 I am not using the full functionality of the balun in the correct short way, I think it should look like the image below (method 3)
    Click image for larger version. 

Name:	New Doc 66_3.jpg 
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ID:	155443

    is the last method 3 is correct and the one in your mind ?, if it is yes then how much I should the value of RL and CL as compared to method 1 or 2.

    Please keep in your mind I am using a single supply operation (VDD = 3.3 V and so VCM=1.65 V)

    Thank you very much once again



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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Quote Originally Posted by Junus2012 View Post
    in Both cases I am getting an identical result and fit to the theoretical calculation.
    I don't think so.
    Method1 : Vo_diff = (Uo+-Uo-) + VCM
    Method2 : Vo_diff = (Uo+-Uo-)

    VCM is not required in method1. Surely consider direction of VCVS.

    Quote Originally Posted by Junus2012 View Post
    That means in the method 2 I am not using the full functionality of the balun in the correct short way, I think it should look like the image below (method 3)
    Connect (2*RL)//(CL/2) to d node of balun.
    Last edited by pancho_hideboo; 10th September 2019 at 16:48.



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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Dear Pancho,

    I am sencierely thankful to you kind help and your paticnce to answer my question, please forgive me is you find my questions are very week, but I am learning from you and your answeres are very useful to me.

    I will later di a simulation for you to show how results from 1 and 2 are identical, nevertheless I am leaving using VCVS toward balun.

    Concerning your answer regarding the load equivelant, I am now bit confused, therefore I have plot the three possible connection in the image below, so please you can tell me the value for each connection comparing to the method 1 that represent my reference circuit for comparesion

    Click image for larger version. 

Name:	f.jpg 
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ID:	155445

    Thank you very much once again



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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Load of method1 is floating load not differential
    Load. There is no common mode load.

    So method1 is not equivalent to both method2 and method3.
    If you ignore common mode,
    RL=5kohm, CL=20pF in method2,
    RL=10kohm, CL=10pF in method3.

    See https://www.edaboard.com/showthread.php?352410

    Quote Originally Posted by Junus2012 View Post
    I will later di a simulation for you to show how results from 1 and 2 are identical,
    nevertheless I am leaving using VCVS toward balun.
    Not necessary.
    Simply you are misunderstanding VCVS.
    Last edited by pancho_hideboo; 10th September 2019 at 18:46.


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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Quote Originally Posted by pancho_hideboo View Post
    Load of method1 is floating load not differential
    Load. There is no common mode load.

    So method1 is not equivalent to both method2 and method3.
    If you ignore common mode,
    RL=5kohm, CL=20pF in method2,
    RL=10kohm, CL=10pF in method3.

    See https://www.edaboard.com/showthread.php?352410

    Not necessary.
    Simply you are misunderstanding VCVS.
    Dear Pancho,

    Again thank you very much for your fast response,

    Now I learned from you about the load equivalent value.

    One issue to discuss with you,
    you mentioned that method 1 is not fully differential output rather it is floating load. However, see these two image below from Allen Holberg, he is referring to it as fully differential output

    Click image for larger version. 

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    Click image for larger version. 

Name:	differential_2.PNG 
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ID:	155447

    Thank you in advance



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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Consider common mode for floating load case.
    Load is infinity for common mode.


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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Dear Pancho,

    Thank you for your explanation,

    I have finished the design of signle ended amplifier, my new task is to design a fully differential amplifier wich have the same charastericitcs of the signle ended one with the same load condition,
    kindly see from the attached image below please to determince the equivalent load (same effect) of the fully differential from the single ended

    Click image for larger version. 

Name:	single-differential.jpg 
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    Thank you very much



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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Quote Originally Posted by Junus2012 View Post
    to determince the equivalent load (same effect) of the fully differential from the single ended
    What do you want to mean by “same effect” ?

    If you mean an equivalency as signal processing, RL=5kohm, CL=20pF.


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    Re: Differential and common mode input resistance of the fully diffferential amplifie

    Quote Originally Posted by pancho_hideboo View Post
    What do you want to mean by “same effect” ?

    If you mean an equivalency as signal processing, RL=5kohm, CL=20pF.
    Yes I mean an equivelant to signal processing, and you already answered it

    Many thanks to you



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