Weird diff amp behavior?

[Sam]

hey,

i'm using a LM324 opamp configured as a diff amp. the inputs come
from opamp followers, so there shouldn't be any loading issues.

when i vary the input connected to the inverting (-) terminal
everything's fine - i see a clear change in the differential output.
but when i vary the input connected to the non-inverting (+) terminal
there's no change in the output. any ideas? (fyi, diff gain is
currently 4.7 [470s over 100s])

thanks so much!,
sam

 

[Eeyore]

i'm using a LM324 opamp configured as a diff amp. the inputs come
from opamp followers, so there shouldn't be any loading issues.

when i vary the input connected to the inverting (-) terminal
everything's fine - i see a clear change in the differential output.
but when i vary the input connected to the non-inverting (+) terminal
there's no change in the output. any ideas? (fyi, diff gain is
currently 4.7 [470s over 100s])

How about posting a circuit diagram so as to check it's right ?

Are any inputs close in voltage to the power supply voltages btw ?

Graham

 

[Sam]

Are any inputs close in voltage to the power supply voltages btw ?

Graham

yep - i forgot to use a split supply, so it was trying to drive the
output below 0 when there was no way that was possible! thanks!

-sam

 

[Eeyore]

yep - i forgot to use a split supply, so it was trying to drive the
output below 0 when there was no way that was possible! thanks!

It's nice when there's a simple answer ! ;~)

Graham