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:


Here's the math they show:


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.

 

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.

 

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.

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) ?

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!