Opamp calculations

I don't know if this is the right newsgroup but: I'm trying to figure out how to calculate what the Vout of an opamp will be based on Vin+ and Vin- and Gain. I've found websites that have this formula:

Vout = G(Vin+ - Vin-)

however my figures keep getting messed up... my circuit has: gain of 11 Vin+ = 5.00V Vin- = .73V and the Vout from all of this is: 7.96V

no matter what i do i can't seem to get the vout from the formula to match up with what i'm actually seeing as Vout. Is the formula wrong, or am i just going crazy?

btw i'm going from a 9v supply to a 7805 Voltage Regulator to provide a clean +5V to the Vin+ of the opamp.

The voltage applied to the power pins of the op-amp ( Vin + - ) has *no* influence on gain at all.

You need to learn some op-amp basics.

Graham

Except of course, if the op-amp is being run off 9v, then the maximum output it can generate might well be just under 8v, and would explain what he is seeing.... The op-amp calculations only apply, if you are inside the input, and output range of the device used.

Best Wishes

Any ideas where i can go to learn op-amp basics.. websites.. books.. etc?

i'm interested in learning but finding sites that are tuned to beginners is kinda confusing :|

You're far away. An OpAmp is an amplifier that amplifies the voltage between the + and - input almost infinitely. Since this is not really useable as it is, there are various possible feedback networks. Two of them are simply amplifying. Inverting and Noninverting. There, the voltage between the input terminals is always zero, at least in theory.

Rene

Pooh Bear, quit trying to confuse the newbie. Vin is "input voltage" not the power rails, they're V+ and V-.

What's wrong with bunny's circuit is that the amp is saturated at as close to the + rail as it can get.

If Vin+ = 5.00V, and Vin- = .73V, then Vin+ - Vin- = 4.27V, and the opamp can't output 46.97V.

Cheers! Rich

No.

If there was 5.73V between the + and - inputs the op-amp would swing to rail. The closed loop feedback doesn't even come into the equation.

Surely you know that feedback doesn't affect open loop gain ?

Graham

bunny said

I use this book sometimes:

IC Op-Amp Cookbook, Jung, Walter G.

These are much better:

hth

I guess when he said "gain = 11", I ass-u-me-d that he wasn't talking about open-loop gain.

Thanks, Rich