Gain not working on an instrumentation amplifier

Discussion in 'General Electronics Chat' started by misterhamin, May 1, 2013.

  1. misterhamin

    Thread Starter New Member

    May 1, 2013
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    0
    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
     
    Last edited: May 1, 2013
  2. #12

    Expert

    Nov 30, 2010
    16,343
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    How did you connect the pot and the resistor to the input?

    The drawing is right. The mistake must be somewhere else.
     
  3. misterhamin

    Thread Starter New Member

    May 1, 2013
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    I connected the 3.9k from +Vs to the noninverting input if the in-amp (in2 in my diagram). I'm not sure what the leads of a pot are called, but I attached the "middle" one (the one that is the little arrow in the diagram) to the inverting input of the in-amp (in1), and then one of the other two to +Vs and the other of the other two to GND.

    Also, it might possibly be something with the power source (I am a newbie to dual power supplies). I am using two 9v batteries with ones (-) connected to gnd and the others (+) connected to ground. I used the first ones (+) as +Vs, and the others (-) as -Vs. In case it matters, all I seem to be using -vs for is the -vs pins on the tl082s... Was I supposed to have used -vs for anything else?

    Thanks

    -misterhamin
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Your pot can vary the -input from GND to +vs. You need to connect the +input to a voltage that is between GND and +vs.
    One way to do this is to leave your 3.9k connected as is, and add another 3.9k from the +input to GND. This will give you 4.5v on the +input, and you can vary the -input from 0 to +9V.
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    So you have +Vs on one on IN2 and a variable voltage on IN1? What sort of voltage do you expect on the output? The first stage has gain of 50, so you need only 300mV difference between the two inputs to make the output swing 15V. Maybe try connecting IN2 to the supply midpoint, change that 1k to 100k and try it out like that and try it like that. Then change the setup to whatever gain you need.
     
  6. misterhamin

    Thread Starter New Member

    May 1, 2013
    3
    0
    Hi again,

    I'm a dork. Ron H was right. I dunno why that didn't occur to me before.

    So the amp seems to be intact... but my<original> intentions for the amp seem to still my malfunctioning. I am trying I construct an EMG circuit like the one described here: http://www.ece.utah.edu/~harrison/ece3110/Lab5.pdf the only difference between my circuit and theirs (besides that I use different resistor values to get the 201 gain) is that I am using customized high-tech tin foil/duct tape electrodes in my experiment. I also added a little lotion to increase the conductivity. But when I hook it up to an oscilloscope, it showe waveforms that don't really correlate with my muscles contracting. Sometimes I see a little chance in frequency (I usually get a noisy square wave of sorts) but I think it is just my elbow pushing the electrodes tighter against my skin ( I get much bigger signal differences when I simply press the electrodes harder in my skin). This sort of thing my be behind the general scope of this forum, but I thought id share anyway. Thank you all for your help.

    -misterhamin
     
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