- #36
Abdullah Almosalami
- 49
- 15
jim hardy said:View attachment 237612
Va = 0
and Vb = Vc + (Vout - Vc) X Ri/(Ri + Rf)
Vout = Avol X (Vb - 0)
Vout = Avol X (Vc + (Vout - Vc) X Ri/(Ri + Rf))
Gain = Vout/Vc = Avol X (1 + (Vout/Vc -1) X Ri/(Ri + Rf))
looks recursive to me. Check my algebra ?
old jim
My point was not that my assumptions were correct, but that the math itself was not wrong, as in there were no algebraic errors.
On your equations, how come you can use voltage division like that? If we can't assume that no current is flowing into the positive input of the op amp, then Ri and Rf are not necessarily in series, and so the voltage division formula doesn't work, right? And so, your equations here are based on the ideal rules.
But even if it were true, then continuing your math, this is what I get:
I'm not sure what the solution to A over here means, and how there can be 2 different amplifications, so perhaps this is a paradox because of some wrong assumptions, given I didn't make algebraic errors.