# Value of RF (feedback resistor formula)

Discussion in 'Electronics Homework Help' started by qwerty22, Nov 10, 2013.

1. ### qwerty22

1
0
Nov 10, 2013
I need help as I have the answer but I don't know the formula

its an 741, with vi 1Vpp, input resistor is 10k, Value of output signal 10Vpp

I have to work out RF which is 100k, I did the usual **** 1vpp = 10k
10 vpp=10k x 10= 100k
But not the proper formula so I though this:

AV ( idk what it is) = 10v/1v = RF/10k =10

so RF = 10k*10 = 100k

Is that the formula? i need to show proper formula as working out, which i don't know,

I can't seem to find the formula online anywhere btw.

464
105
Aug 27, 2013
3. ### Laplace

1,252
184
Apr 4, 2010
Instead of trying to find the correct formula, why not derive it yourself? Just realize two facts:

1. When the 1 Vp-p input is at +1V the 10 Vp-p output is at -10V.
2. The '-' input to the opamp is a virtual ground. The '+' is tied to real ground and negative feedback keeps the '-' input at the same voltage as the '+' input (within a few microvolts).

So what you have there is a simple voltage divider where 11 volts is divided between Rin and Rf. Solve that and you have the correct formula for finding Rf.

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