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9980 Views 5 Replies Latest reply: Apr 11, 2012 5:25 PM by JMAR RSS
JMAR Novice
JMAR
Currently Being Moderated

Apr 2, 2012 4:55 PM

LM741 looses gain when connected to another LM741

I'm probably in the wrong forum but am really stuck.  I have two identical LM741 circuits.  In the first circuit, I inject a 60HZ, 1Vpp signal into PIN 2.  Pin 3 has 6Vdc or 1/2 VCC.  The resistor to pin 2 is a 600 ohm.  The Gain is adjusted (pin 2 to pin 6 10k trimmer) to 4Vpp.

 

I need to input this signal into four other 741 circuits so I don't want to loose any of the signal.

 

When I attach the output of the first circuit to the input of a second IDENTICAL 741 circuit, the signal drops to 1Vpp.   How/why does the input become affected in any way?  I have to keep the input at 4Vpp to drive the other 741 circuits.

Tags: loss, op-amp, 741, gain
  • Rick designspecialist-power
    Rick
    36 posts since
    Mar 22, 2011
    Currently Being Moderated
    Apr 3, 2012 2:18 PM (in response to JMAR)
    LM741 looses gain when connected to another LM741

    First off, pictures are worth a thousand words;  http://www.digikey.com/schemeit is an a free online schematic drawing tool that could be useful if you don't have a regular CAD package handy.  There are some pieces missing from your description, such as how your signal source is connected, where the "resistor to pin 2" is coming from, and how you're taking the output measurements--a schematic could help clarify that.

     

    Assembly/layout errors are the most common problem I have with simple circuits like this: solder bridges, unintended connections, unintended non-connections, incorrect component values, etc.  Test/measurement errors are also common; forgetting to connect a ground lead, measuring the wrong point, improper loading of the signal source, and the like.  Try testing your second circuit by itself; if it doesn't work, it's likely got a issue.  If it does, then an interaction between the two is more likely.  Assuming nothing starts on fire if you do, power up the circuit, and work your way from input to output measuring the signal at each node, comparing it to what it should be according to your schematic.  Measure directly at the pins of the package whenever possible--I've found many a flawed joint this way.  At some point, you should notice that the signals start to diverge from their expected values; the place where it does so is a good place to start looking for the root of your problem.

    • JMAR Novice
      JMAR
      6 posts since
      Aug 25, 2011
      Currently Being Moderated
      Apr 3, 2012 3:19 PM (in response to Rick)
      LM741 looses gain when connected to another LM741

      Thank you Rick for the response.

       

      I made the schematic and saved it.  However, whenever I try to insert the image, internet exploder quits responding.  My internet exlorer does not like something with this site when I try to insert the image.  Therefore, I can't insert, or attache the image of the circuit to send you.

       

      I don't know how to send you the picture if I can't insert it here.   Can I send you my email address so that I can send you the picuture via EMAIL?

       

      Of course, without the image, the following discussion is rather pointless.  I really appreciate your assistance with this.  I am baffeled.

       

      Both circuits are identical.

       

      U1 = U2 = LM741

       

      If I inject a 60Hz 1Vpp Sine wave at ST1, break the connection between C2 and C4, and look at the signal right after C2, I can adjust R1 to give a gain to 4Vpp, which is what I want.

       

      With the connection between C2 and C4 still disconnected, If I inject the same signal just before C4, I can adjust R6 to give a gain to 4Vpp at ST2.

      Now, If I inject the same signal at ST1, and reconnect C2 to C4, the signal at C4 drops to 0.5 Vpp. Where does all the gain go? I can adjust R6 to get the gain back, but why does connecting these circuits together kill the signal? Why does the input of the 2nd 741 KILL the gains from the 1st 741.

       

      Perhaps the LM741 can’t handle 600 ohms. I’ve tried switching the polarity of the caps, increasing R2 and R5 to 10K and R1 and R6 to 100K, with the same problem.   One interesting note – If I inject the signal into the 1st circuit, with the 2nd circuit disconnected, and place a 10M resistor after C2, the GAIN is off the sheet. Then, as soon as I connect C4 to the 5M resistor, the gain once again drops to 0.5 Vpp.

       

      This is a STUPID mixer application. The first circuit amplifies a electret microphone, and the signal is distributed/fed to four other audio/telephone line circuits, which is why I’m using 600 ohm resistances. I don’t need 600 here, I just have them. The unity mixer at the end of the chain and other outputs that attach to the PBX, will have a 600 ohm impedance match.

       

      I’m using 741’s because that’s what we have, however, I’m certain there are better alternatives more closely matched to this application.   I don’t understand why feeding a 741 circuit from an identical 741 circuit kills all the gain.

  • TommyTech Novice
    TommyTech
    3 posts since
    Apr 11, 2012
    Currently Being Moderated
    Apr 11, 2012 1:36 PM (in response to JMAR)
    LM741 looses gain when connected to another LM741

    Are you using a single supply for the 741s? Sounds like the bias circuit may be adding a voltage divider to your circuit changing the gain calculation. Op amps also have problems with resistors with M in the value, you may have better results if you keep them in the kilo range. You may also find you can direct couple the output of the first stage into the second if the DC level is in range (offset X stage 2 gain =~ VCC/2). Try one of more of the free SPICE programs available on line to help with ballpark numbers. Remember you can get some very incorrect results with SPICE if you give it oddball numbers of incomplete data.

     

    Next step is to add AVC to the microphone preamp and maybe a VOX or noise gate to cut background noise.

  • JMAR Novice
    JMAR
    6 posts since
    Aug 25, 2011
    Currently Being Moderated
    Apr 11, 2012 5:25 PM (in response to JMAR)
    LM741 looses gain when connected to another LM741

    Thank you all for your input.  I hope I can return the favor some day.....

     

    I changed the input cap from a 1uF polarized electrolytic to 0.1uF nonpolarized tantalum, which really smoothed out the waveform.

     

    I'm using a single 15Vdc voltage source.  It uses a precision resistor divider to give 1/2 Vcc to pin 3.

     

    The mike circuit uses a 1K inverting to a 100K, which produces a perfect output waveform.  This is an electret microphone which now has the ability to cause your ears to bleed.

     

    In order to inset the signal from the microphone circuit, to the other three circuits, without signal loss (or very little), I changed the input caps from 1uf polarized electorlytic to 0.1uF nonpolarized Tantalum then, this next part is really bogus, I had to use a 100K to pin 2, then from pin 2 to pin 6, a 150K.

     

    Anything other than that either distorts the signal, or causes major signal loss as I "chain" things together.  The output (pin 6) from theother four op-amp's is though a 10k pot to ground with the wiper feeding a 741 unity gain with 10K/10K.

     

    This is an interface to a PBX at 600 ohms, and I know there are far better options than a 741.

     

    Hence, I've ordered up the following:       BA4560-ND or LM358ANFS-ND for the MICROPHONE

                                                               LME49720NA-ND for the other inputs and probably the mixer.

     

    And, just for good measure, the LM386 and OPA134PA-ND

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