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Amplify Very Weak AC Signal Using Op-Amps

  1. Sep 12, 2012 #1
    Hello, I'm looking to amplify a very weak AC signal using op-amps. The problem I'm running into is that I can only operate using a +5V supply voltage, no negative voltages. My AC signal is about 5mV and I need it to be about +2.5V. The AC signal is initally DC baised and is at +3.5V, so I put a DC blocking capacitor and used a voltage divider to put the AC signal at a +2.5V virtual ground. When I apply the signal with a gain of 101 (negative feedback, non-inverting amplifer) with op-amps the output AC signal is very weak.

    Another problem is that once I put the bias voltage on it the input becomes evn weaker and isn't baised properly.
     

    Attached Files:

  2. jcsd
  3. Sep 12, 2012 #2

    uart

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    Science Advisor

    You also need to place a DC blocking capacitor in series with R18, so as to make the DC gain unity. Otherwise you're trying to amplify the 2.5 volt DC bias by 101 as well! You can see that you're output is saturating at the +DC rail.

    A couple of other points.

    - You might do better with an opamp that's spec'ed for operating on a relatively low single supply. Eg LT1006

    - As it stands your PSRR is close to non existent. You may want to look at refining the bias circuit if this this has to operate in the presence of any type of power supply noise.
     
    Last edited: Sep 12, 2012
  4. Sep 12, 2012 #3
    Yes you miss a cap in series with R18. That's the main problem.

    I can't enlarge the picture enough to read the setting. Another question is what frequency are you trying to amplifier? remember when you have gain of 101, the frequency response of the opamp is reduced by 101!!! If you are running say 20KHz, a normal 1MHz opamp will not work.
     
    Last edited: Sep 12, 2012
  5. Sep 12, 2012 #4
    uart: Thank you for the response. It worked!

    yungman: The frequency I'm trying to amplify is 20 kHz. I have a question, when you say the frequency response of the opamp is reduced by 101, do you mean for a frequency of 20 kHz I need am opamp that has to handle at least ~ 2 GHz (101 x 20 kHz)?
     
  6. Sep 12, 2012 #5
    No 101X20000=2.02MHz only. You need to find an opamp that has gain bandwidth product of over 2MHz. That is easy, you just has to be aware of this.

    The following is just my opinion:

    I usually do not make any one opamp carry gain of over 20 or 30. First I like to keep the feedback resistor low value below 20K to 30K. This is to avoid noise pickup and leakage current due to board that is not clean. Also this help keep the GBP requirement of an opamp lower. I can find cheaper opamp.

    If I do a gain of 100, I'll use a dual opamp, each gain of 10. In this case, GBP requirement is 200KHz, any el-cheapo opamp can do this. I have been working on music electronics where the frequency requirement is in this ball park. Cheap opamp like TL062 will work for you with flying color. It is about 2MHz GBP, at gain of 10, you have plenty of loop gain to keep the circuit very precise. It is dirt cheap, you'll find a single opamp and the dual opamp of the same amp cost almost the same. Keep the feed back resistor at 18K, to get gain of 10, two stages and you are home free.

    That's just me.
     
    Last edited: Sep 12, 2012
  7. Sep 14, 2012 #6
    Thank you for the reply yungman, it was very helpful!
     
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