# Opamp calculations

Discussion in 'Electronic Design' started by bunny, Dec 1, 2005.

1. ### bunnyGuest

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.

2. ### Pooh BearGuest

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

3. ### Roger HamlettGuest

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

4. ### bunnyGuest

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 :|

6. ### Rene TschaggelarGuest

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

7. ### Rich GriseGuest

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

8. ### Pooh BearGuest

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

9. ### Homer.SimpsonGuest

bunny said
I use this book sometimes:

IC Op-Amp Cookbook, Jung, Walter G.

http://tinyurl.com/9jsja
That price is ridiculous though.

These are much better:
http://tinyurl.com/arm3n

hth

10. ### Rich GriseGuest

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

Thanks,
Rich

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Continue to site