Negative amplification?

[fleamonkey]

hi could anyone tell me why i get a negative gain on this circuit please. i am quite new to op amps and cannot understand why i have not got 10 times gain

 

[Nigel Goodwin]

You have the resistors the wrong way round (the 10K and 1K), so you've got a gain of 0.1 instead of 10.

I would also suggest you post diagrams as GIF or PNG, and not as large DOC files which many people won't be able to view.

 

[Hero999]

Yes, please use PNG.

I also don't have MS Word, just the free viewer program which is a bit flaky.

OpenOffice.org is good but some of the more complex Word documents don't display properly.

I've converted it to PNG but it looks a bit distorted.

What are you trying to do?

It looks like you're trying to make an instrumentation amplifier.

See the Wikipedia article for a schematic.
Instrumentation amplifier - Wikipedia, the free encyclopedia

 

[Nigel Goodwin]

What are you trying to do?

It looks like you're trying to make an instrumentation amplifier.

Presumably you didn't read the name of his attached Word file?.

 

[Hero999]

No I didn't.

What's your point? I was right, an instrumentation amplifier is commonly used to monitor a Wheatstone bridge.

 

[audioguru]

The lousy old 741 opamp has a minimum input resistance of only 300k ohms so it will severely mess up this circuit that has 1M ohm input resistors.

Why is R11 only 0.001 ohms? It should be a piece of wire because the gain of the first two opamps is only 1.

 

[Nigel Goodwin]

No I didn't.

What's your point? I was right, an instrumentation amplifier is commonly used to monitor a Wheatstone bridge.

The point is the filename tells you EXACTLY what he's doing.

 

[Hero999]

The lousy old 741 opamp has a minimum input resistance of only 300k ohms so it will severely mess up this circuit that has 1M ohm input resistors..

There's no need for the input resistors which should be removed altogether.

The bridge has an putput impedance of 14k which is then fine to drive 300k inputs.