differential amplifier

Discussion in 'Homework Help' started by bug13, May 7, 2013.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    I am not sure about the following question, as the values look a bit odd.

    Q:
    Draw the circuit of a differential amplifier with a gain of 2 that has an input resistance of 10K, show pin details

    here is my drawing:
    [​IMG]
     
  2. Ron H

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    The V_in_2 input resistance is 10k. What about V_in_1?
     
  3. bug13

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    I am assuming they are looking for something R1 = R2, R3 = R4, as the question only asks for input resistance, not asks for R_Vin_1 and R_Vin_2.

    Did I answer what you are expected?
     
  4. Ron H

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    If V_in_1 and V_in_2 are two independent sources, then the input resistances in your schematic are NOT equal.

    If you have a single voltage source connected across the inputs, the input resistance is STILL not 10k.
     
    Last edited: May 7, 2013
  5. bug13

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    I am lost, can you tell me a bit more?
     
  6. t_n_k

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    So - do you require the resistance as "seen" by an input signal source connected between the differential input terminals to be 10kΩ?
     
  7. bug13

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    The question in the OP is word by word from our exercise, I am not sure, but from what I understand it, is the resistance to the ground.
     
  8. t_n_k

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    What do you mean by "resistance to ground". You suggest this is your understanding of the wording. How do you know this is what was meant by the wording?
     
  9. bug13

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    I mean the resistance see by the inputs, not the resistance between the inputs, but I don't think I truly understand the question, I will need some help.
     
  10. t_n_k

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    OK - so let's say we connect V_in_2 directly to ground and connect a non-zero ground referenced input to V_in_1. What is the input resistance on each input?
     
  11. Ron H

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    This is the circuit (attached) I would draw in response to that question.
    With two voltage sources, there are two input resistances.
     
  12. bug13

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    I got R2+R1||R4, is that what you expected?

    but isn't it become an inverting amplifier?
     
  13. bug13

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    We have talk about input impedance (of differential amplifier) in class, I am guessing the tutor want us to look the info up.

    but good to know there are two input resistances with two voltage source.

    so do you consider a high side current sensing circuit as one voltage source?
     
  14. bug13

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    Arrrh, are you trying to guild me to R_in_1 = R2+R1||R4, and R_in_2 = 0?
     
  15. t_n_k

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    Since the context is differential amplifier I would tend to make a distinction (if any) between differential mode and common mode input resistance.
     
  16. Ron H

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    This is homework. What do you think, and why?
     
  17. bug13

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    more to google: differential mode and common mode input resistance
     
  18. bug13

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    Ooops, I suppose to type "we HAVE NOT talked about this in class"

    and I think yes, a high side current sense is usually powered from the same source .
     
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