Hi everyone,
I am having a curious problem with a instrumentation amplifier built from two TL-082 JFET-input op-amps (well, actually one and a half because i only use one amplifier from the second IC) supplied from +9 volts and -9 volts. I have attached the schematic (it is just plain beautiful). It seems that, although I set it up to have a gain of 201, there is a gain of only one!! I first noticed this issue of lost gain when i attached a 10k pot to one input of the in-amp and a 3.9k Ω resistor to the other. I attached an LED to the output and observed that as I twisted the pot, the LED had a gradual descent into darkness rather than a quick comparator-like snap from positive saturation to negative saturation that a gain like 201 would suggest. So i tried a little trick; I removed the 1k resistor (Rgain), which would therefore create infinite resistance between the two inverting inputs of the first-stage buffers which, according to articles I read about in-amps, would surely result in a gain of one. I did the test again and had the same results! So, in all, I got the same results from an in-amp circuit with 201 gain as I got from an in-amp circuit with 1 gain! I also tried swapping the 1k out for a 10k, still the same. Soooo... what can i do to debug this? i checked a jillion different resources for in-amp gain equations, and they ALL agreed my circuit should have a gain of 201. What to do?
Thanks!!
-misterhamin
I am having a curious problem with a instrumentation amplifier built from two TL-082 JFET-input op-amps (well, actually one and a half because i only use one amplifier from the second IC) supplied from +9 volts and -9 volts. I have attached the schematic (it is just plain beautiful). It seems that, although I set it up to have a gain of 201, there is a gain of only one!! I first noticed this issue of lost gain when i attached a 10k pot to one input of the in-amp and a 3.9k Ω resistor to the other. I attached an LED to the output and observed that as I twisted the pot, the LED had a gradual descent into darkness rather than a quick comparator-like snap from positive saturation to negative saturation that a gain like 201 would suggest. So i tried a little trick; I removed the 1k resistor (Rgain), which would therefore create infinite resistance between the two inverting inputs of the first-stage buffers which, according to articles I read about in-amps, would surely result in a gain of one. I did the test again and had the same results! So, in all, I got the same results from an in-amp circuit with 201 gain as I got from an in-amp circuit with 1 gain! I also tried swapping the 1k out for a 10k, still the same. Soooo... what can i do to debug this? i checked a jillion different resources for in-amp gain equations, and they ALL agreed my circuit should have a gain of 201. What to do?
Thanks!!
-misterhamin
Attachments
-
25.7 KB Views: 59
Last edited: