Wheatstone bridge with 741 op amp HELP!

[fleamonkey]

hi , new to this site so hi to everyone!
i have a wheatstone bridge connected to an op amp

I have designed this circuit to measure resistance of paper. R1 is noted as a varible resistor - this is acting as my probes. It is 14k ohms as that is the base line for my paper and also to balance the bridge.
The problem i have is that every time i vary R4 the gain of the amp changes?!
Im sure i am doing something stupid somewhere.
can anyone point me in the right direction please?
thankyou

 

[audioguru]

R4 is connected parallel to RA so of course changing its value also changes the gain of the opamp.
You need an intrumentation amplifier or a differential amplifier made with 3 opamps.

 

[ericgibbs]

hi,
Download the datasheet for the AD623 instrumentation amplifier from Datasheet & Application Note Database, PDF, Circuits, Datasheets | Datasheet Archive

 

[MikeMl]

You dont need an instrumentation amplifer; a simple rail-to-rail output amplifier (which a 741 most certainly is not) configured this way will let you set the gain without effecting the offset. I centered the response curve by making the lower resistors in the bridge 14K instead of 2K, but it works with the 2K resistors, too.

In the plot, the X-axis is the sensor resistance. The slope of the output is a function of the gain resistor, without effecting the zero point.

The second sim shows that you can change the zero point without effecting gain.

 

[fleamonkey]

Thankyou all very much for your help. im now going to plug it into multisim and have a go.