Amplifying a very low signal.

Discussion in 'Analog & Mixed-Signal Design' started by A2X_, Nov 9, 2016.

  1. A2X_

    Thread Starter New Member

    Nov 9, 2016
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    Hi all,

    For a project, I'm trying to amplify a very low signal. From around 100uV to 1V. So I'm looking into making a 3 stages amplifier with gain = 100 each.

    Now I've tried with an LM741 but it saturates at such a gain (it work well with gain = 10 but it's too low and I don't want to use more than three stages... because noise + offset etc). I've been told about instrumentation amplifiers and others but I'm looking for a simple solution and parts that'll be easy to find - just a replacement for the LM741 that wouldn't saturate at G=100.

    Any suggestions?

    Thank you!
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Is this a DC signal?
    If not, what is its frequency?
    Where do you live, so I know what parts to recommend?
     
  3. A2X_

    Thread Starter New Member

    Nov 9, 2016
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    AC signal @60 hz.
     
  4. RichardO

    Late Member

    May 4, 2013
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    I think you have done a miscalculation. 1 volt divided by 100 uV is 10,000. Three stages with a gain of 100 each is 1,000,000. This is why the 741 saturates.

    Try lower gains on the three stages (about 20 each) or use two stages with gains of 100 each.
     
  5. A2X_

    Thread Starter New Member

    Nov 9, 2016
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    You're totally right, it'll be two stages of gain = 100. I tried only one stage with G=100 and it saturates so there's no way I can do that with two stages.
     
  6. RichardO

    Late Member

    May 4, 2013
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    I dont see why the amplifier saturates with a gain of 100. What power supply voltages are you using?
     
  7. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    What do you mean by "saturate"? A steady DC output at near either the positive or negative rail? A square wave that looks like a heavily clipped sine wave? Something else? Without a schematic we have no clue, and without an image of the output waveform (hand sketches are fine), we'll still be guessing 20 posts from now.

    ak
     
  8. A2X_

    Thread Starter New Member

    Nov 9, 2016
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    Well I tested the opamp with a sinewave of a 1mV amplitude. I mounted the opamp as an inverter with G = 100 and the output signal is clipped on the top. When I lower the gain, I get a perfect sinewave.

    The clipped signal looked like this: [​IMG] So I figured the amp couldn't take a gain of 100.
     
  9. A2X_

    Thread Starter New Member

    Nov 9, 2016
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  10. A2X_

    Thread Starter New Member

    Nov 9, 2016
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  11. AnalogKid

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    Last edited: Nov 9, 2016
  12. crutschow

    Expert

    Mar 14, 2008
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    You only need AC gain so you can AC couple and reduce the DC gain to avoid saturation from offsets at high gain.
    The simplest is to put a large capacitor (≥10μF) from the bottom of R1 to ground.
    That way the AC gain is unaffected but the DC gain is reduced to 1.

    Note that the 741 is a noisy op amp and may be a problem at such high gains.
    You should go to a lower noise op amp, such as an OP07.
     
  13. A2X_

    Thread Starter New Member

    Nov 9, 2016
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    Thank you Analog & Crutschow, I appreciate the help. I'll be trying a different op-amp tomorrow + add a capacitor to the schematic and I'll let you know how it went.
     
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