Op amp lab question

[skysurf76]

I'm working on a lab for school and wanted to know what output to expect from the following setup.  The instructor told us that the minimum output gain should happen at 2k ohms.  This makes sense because the op amp is not ideal and some voltage will be generated across both input resistances as the top resistance is changed.  Right before I left tonight though we tried this part and I got some Frankenstein looking square wave with dips at the top.  Our equipment was jankity and we had to move stations which is why I'm asking.  I have to go back next week to finish the lab and I'd like to know the waveform I'm looking for in this setup.

Thanks in advance for any help.

[Andy Watson]

This makes sense because the op amp is not ideal ...

What would you expect if the "op-amp" was ideal?
The aim of this experiment appears to be to demonstrate that op-amps are not ideal and to suggest at least one possible source of this imperfection.

[danadak]

The whole point of a diff amp is to amplify differential voltage, suppress common mode
voltage. So by unbalancing the opamp signal path you will get increasing CM voltage
effects. Note you are examining the effects of AC gain, and OpAmps have finite G
vs frequency, which cause many issues. Clue is you have stray C and input C that
are a part of this experiment.

Regards, Dana.

[Z80]

An ideal opamp has infinite input impedance and therefore draws no current at it's inputs.  If this was the case then the input resistors would have no effect and the output would be zero.  In the real world this is not the case, current flows and the resitors cause a voltage drop and hence an imbalance.  Given the high gain of your amp, it will likely jump between the rails.  Your lab is investigating this current flow and seeing it's effect in a real world circuit.  Have fun with it.

[bson]

It could probably be used to measure the input leakage currents at 1kHz by adjusting the resistor until the output is nulled.

[w2aew]

It might be helpful to recognize that this is not an op amp question/lab - it is an instrumentation amp/lab question, all about Common Mode gain/rejection vs. Differential gain.

[Audioguru]

It is not an opamp. Instead it is an Instrumentation Amplifier that has 3 opamps in it and matched resistors.

[DimitriP]

I'd like to know the waveform I'm looking for in this setup.

The waveform you are looking for will vary in pk-pk value as you are changing Rin. That's it. No hidden meaning. If looking at the shape bothers you,  use a voltmeter as long as it can handle 1KHz

[skysurf76]

Thanks for all the replies.  The information is invaluable.  I had no idea there was a difference between a normal op amp and instrumentation amplifier.  This lab is the first time I have ever even heard the term instrumentation amplifier.  I also was under the impression you could neglect input resistances to amplifiers because they are so high that not enough current would flow to make a difference.  I'm glad I asked.  Its amazing how much detail is left out in some classes.

[danadak]

An excellent resource on IA's

http://www.analog.com/en/education/education-library/dh-designers-guide-to-instrumentation-amps.html


Regards, Dana.