I am working on a project, and am trying to amplify the voltage across wheatstone bridge. Here is the diagram:
where \$R_1 = 1,000 \Omega\$
\$R_3 = 10,000 \Omega\$
and \$R_2\$ is a potentiometer, set so that the voltage across the bridge is 10 mV
The gain of the amplifier is simply \$R_3/R_1 = 10\$, so I expect an output voltage at \$V_{out}\$ of 1V.
I do not see 1V out, I see somewhere around 1.5V, and I am certain that whatever the problem is has something to do with the fact that the voltage across my bridge doesn't stay at 10mV when connected to the amplifier. When I connect the bridge to the two leftmost op amps, and check the voltage across the bridge again, it is around 45mV.
Why is this? I've checked my amplifier circuit with classmates and rebuilt it several times, I can't tell what might be wrong. What might be causing such an issue?
Best Answer
I'm not sure why you expect 1V out, with a 10 mV input and a gain of 10. Personally, I would expect something closer to 100 mV, but in any case, 1.5 V is obviously wrong.
The LM324 should be capable of driving the output to 0V. However, what it can't do is drive the output negative. Are you sure your bridge is unbalanced in the direction that will drive the output positive — left side higher than right side?
Some opamps have the property that they invert the output when the input stage is driven too hard into saturation. I don't recall offhand if the LM324 has this problem, but it could explain what you're seeing.