# 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:

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.

2. ### ramancini8 Member

Jul 18, 2012
447
119
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.

Oct 1, 2012
11
0