Differential amplifier math questions

Discussion in 'General Electronics Chat' started by chay, Nov 29, 2012.

  1. chay

    Thread Starter New Member

    Oct 1, 2012
    11
    0
    I've been working my way through the math of operational amplifiers, did the tutorials here, and encountered the following at electronics tutorials.

    Here's their example differential amplifier circuit:
    opamp18.gif

    Here's the math they show:
    opamp22.gif

    In particular:
    1) What do they mean by "Summing point" Va = Vb ? Is this due to the negative feedback causing Va to chase Vb?
    2) What are the steps from premise to conclusion here: "If Vb = 0, then Vout(a) = -V1(R3/R1) ?
    3) What are the steps from premise to conclusion here: "If Va = 0, then Vout(b) = V2(R4/R2+R4)(R1+R3/R1) ?

    Many thanks for any help on this.
     
  2. ramancini8

    Member

    Jul 18, 2012
    442
    118
    That is a complicated method of calculation. Ground the inverting input and calculate the gain. Ground the non-inverting input and calculate the gain. Vo= Vinv + Vnon.

    1. Yes, because of the infinite amplifier gain the inputs will always be driven to 0 volts.
    Don't understand your 2 and 3 questions.
     
    chay likes this.
  3. chay

    Thread Starter New Member

    Oct 1, 2012
    11
    0
    Thanks ramancini8. That was helpful.

    With regard to questions #2 and #3, I just hoped to understand their reasoning, how to know that their statements are true? Between "if" and "then" they're applying some assumptions that weren't clear to me.

    But now I think I understand. When they're saying "If Va = 0, then..." that's like you saying "Ground the inverting input ...."

    I think this works for me. Thanks!


    opamp18.gif

    opamp22.gif
     
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