Problem in amplifying

Discussion in 'The Projects Forum' started by Bob T., Jun 5, 2013.

  1. Bob T.

    Thread Starter Member

    Oct 22, 2012
    35
    0
    Hello eveybody.........
    I made a circuit by LM386 and it amplifies very well but i want to exactly match the original wave.
    The amplified wave looks weird at the bottom.

    In the image:
    blue - original
    yellow - amplified

    sine.png

    Thank you...
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,031
    Are we to guess at your schematic?
     
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  3. Bob T.

    Thread Starter Member

    Oct 22, 2012
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    Thank you very mutch..
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,031
    Does your signal generator share a ground connection with your amplifier? The schematic shows only one wire.

    Is this all in simulation or have you actually built something?

    What is D1 there for?
     
  5. OoglieBooglie

    New Member

    Jun 3, 2013
    21
    2
    Hm...You have asymmetrical clipping, which is kind of interesting. Perhaps it's due to the diode. I'm not really sure how the gain control works on the LM386 (I've only played around a little bit to see if I could get it to work, and have only a vague idea of it being controlled with a capacitor), but perhaps the diode is blocking current on those gain control pins. Try removing the diode, and see what happens. I've never seen a diode with any of the sample LM386 circuits in the datasheet, so I'm pretty sure it's not necessary for the chip to operate.
     
  6. Bob T.

    Thread Starter Member

    Oct 22, 2012
    35
    0
    Thank you for replying......
    D1 is a normal diode and when i removed the diode i got this graph which is not marching the original wave at all.
    pulse1.png
     
  7. Bob T.

    Thread Starter Member

    Oct 22, 2012
    35
    0
    I am trying to amplify received ultrasonic waves so you can consider the wave source sharing the same ground.
    thank you
     
  8. Shagas

    Active Member

    May 13, 2013
    802
    74
    Have you tried amplifying it with a smaller gain?

    Also maybe your ground isn't exactly inbetween +and - Vcc
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    What's the frequency and P-P amplitude of the input signal ?
    What's the supply voltage ?
    It appears there is no load on the output capacitor. Try adding a load.
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Definitely lose the diode, it has no reason to be there. May I ask why are you using a noisy power amplifier aimed at audio frequencies to amplify signals coming from an ultrasonic receiver? A standard opamp will probably work much better.
     
  11. OoglieBooglie

    New Member

    Jun 3, 2013
    21
    2
    Ah! I have an idea! Looking at it, the supply voltage looks to be 0V (ground) and +V. Apparently, the negative supply voltage is actually supposed to be negative and not simply ground voltage on a lot of amplifier chips. You may need a dual/split power supply. I learned this the hard way with an LM741 chip. Perhaps you have the same problem that I did (I never really tried an oscilloscope with it, so I can't say for certain based on the waveform).

    With my power supply problem, I basically took 2 9V batteries, put them in series, put the +V on the positive end of the combination, the -V on the negative end of the combination, and then connected the ground in between the 2 batteries.
     
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  12. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    With a properly referenced input it will work. Also the 220R resistor on the power supply will attenuate the output with larger loads.
     
  13. Bob T.

    Thread Starter Member

    Oct 22, 2012
    35
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    Thank you very much.....
    The frequency is 50 Hz and the power supply is 20V
    I will try other op-amp and i will post the result.
     
  14. Bob T.

    Thread Starter Member

    Oct 22, 2012
    35
    0
    Thanks man......

    Thanks
    Thanks Thanks
    Thanks Thanks Thanks
    Thanks Thanks Thanks Thanks
    Thanks Thanks Thanks
    Thanks Thanks
    Thanks​

    than you very much OoglieBooglie you are the man.....

    I posted the schematics with the negative supply connected to -20 V and look how the wave was amplified.
    I will put 9 volts as supply and and i will but a circuit to give by relatively negative supply .

    Thank you very much man....
    thanks for every body who posted also..:):):):):)

    New-scem.png
     
  15. Bob T.

    Thread Starter Member

    Oct 22, 2012
    35
    0
    Another problem i faced is that when i decrease the voltage supply to 10V the amplification was bad.

    thank you
     
  16. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    The LM386 is a power amp , and it doesn't behave like a typical op-amp.
    Per the datasheet it centers the output at 1/2 supply voltage.
    Operating it with +9 volts and -20 is outside it's operating specs.

    The diode needs to go. It will create asymmetrical amplification.
    Try a 10uf gain cap, and connect Pin 4 to gnd.
    And remove R2, connect V+ direct to the chip.
     
    Last edited: Jun 8, 2013
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  17. Bob T.

    Thread Starter Member

    Oct 22, 2012
    35
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    I will try this thank you
     
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