Distorted output of LM318

Discussion in 'Analog & Mixed-Signal Design' started by Aagash, Aug 8, 2017.

  1. Aagash

    Thread Starter Member

    Jun 7, 2017
    43
    2
    Hi,
    I've been trying to build the high pass (2nd order butterworth) filter using LM318 (due to high slew rate). But the O/P is highly distorted. Please solve the issue and let know the reason behind it . Thanks in advance ! Simulation picture attached.

    Vp-p = 9V
    Frequency = 68 kHZ .

    distorted ->
    IMG_20170802_175823197_HDR.jpg

    needed signal ->
    IMG_20170802_175854935.jpg

    Simulation :
    Capture.PNG
     
  2. crutschow

    Expert

    Mar 14, 2008
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    It looks like the circuit is oscillating.
    How is the circuit constructed?
    Is it on a solderless breadboard?
    Is the op amp power decoupled properly with 0.1μF ceramics?
     
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  3. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    That circuit is a low pass filter. Is it a high pass filter you want?
     
  4. LesJones

    Well-Known Member

    Jan 8, 2017
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    It looks like yiu have the inverting and non inverting inputs the wrong way round which would caues it to oscillate.

    Les.
     
    Last edited: Aug 8, 2017
  5. crutschow

    Expert

    Mar 14, 2008
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    No, they are correct.
    The op amp is configured as a unity-gain follower in a Sallen-Key filter configuration.
     
  6. RichardO

    Late Member

    May 4, 2013
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    The instructor I had in tech school liked to say: "Amplifiers oscillate and oscillators don't."

    It is hard to say which category the LM318 falls into. :eek:

    In other words. When using the LMN318 you must be very careful to use very short wiring and do very careful power supply bypassing. If you do this then the LM318 becomes an amplifier -- even on a solderless breadboard.
     
  7. Aagash

    Thread Starter Member

    Jun 7, 2017
    43
    2
    Thanks for all the replies .

    I've used the breadboard (solder-less) to place the components and the wiring is also as short as possible .

    I'll try power decoupling the lm318 to see if signal is proper.
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    If you don't need all of the super high speed of the LM318, you can add a capacitor across pins 1 and 8. Start with 10pF and add more capacitance or subtract as needed. That is, add more capacitance if it still oscillates or subtract if the amp is too slow for your needs.

    Good luck.
     
    Last edited: Aug 8, 2017
  9. crutschow

    Expert

    Mar 14, 2008
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    The version I heard was "Amplifiers oscillate and oscillators amplify".;)
     
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  10. simozz

    Member

    Jul 23, 2017
    102
    27
    Hello,
    Try to build it on a soldered breadboard. Solder-less breadboards always add parasitic components, and may this is influencing the circuit behavior.
    simozz
     
  11. DickCappels

    Moderator

    Aug 21, 2008
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    As far as I can tell you have not answered crutschow's question about power supply bypass capacitors in post #2. It is important that you place a 0.1 uf ceramic capacitor from each power supply pin to ground. Did you do that?

    I would also place 10 uf across the power supply pins.

    Do you have both positive and negative power supplies? What is your power supply voltage and what is the average voltage on the input signal?
    .
     
  12. Aagash

    Thread Starter Member

    Jun 7, 2017
    43
    2
    Now , Yes I used a bypass capacitor (ceramic) of 0.1u at both positive and negative supply , and the distortion is 70% went but still little oscillation . Lets move to very simple voltage follower .

    And also I tried a simple voltage follower circuit , O/P is not oscillating (power supply bypassed) but the output is not as required.

    Two configurations tried :

    1)
    Input -> sine wave , 68kHz , 2V (peak-peak)

    First -> without 100K resistance in series with INPUT .
    Simulation :
    Capture1.PNG

    Output : Distorted ( Not a perfect Sine wave)
    IMG_20170809_185029053.jpg


    2).

    Second config : without 100K resistance in series with INPUT .

    Input -> sine wave , 68kHz , 2V (peak-peak)

    Simulation:
    Capture2.PNG

    Output : Sine wave ( very low amplitude , ~ only 0.1 gain ) , 68kHz, 120 mV (peak-peak)
    IMG_20170809_191513638.jpg


    Its LM318 Voltage Follower circuit , in datasheet specified to not hard wire as Voltage follower directly rather use 5pF and 10K network from -ve i/p to o/p of lm318 . Even establishing this combination and the bypass power supplies (+ and -) with 0.1u , I don't get the need Output .

    Kindly if possible figure out why . Thanks !
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Now try with 10pF capactor across pins 1 and 8.
     
  14. crutschow

    Expert

    Mar 14, 2008
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    Your schematic shows 100k on the input and the capacitors in series with the supplies, both of which are incorrect.
    The resistor should be 10k and the capacitors from the power pin to ground.
     
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  15. Aagash

    Thread Starter Member

    Jun 7, 2017
    43
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    I'm Sorry , but what exactly the role of capacitor in power supply ? Is it used to pass away the ac components from Supply .
     
  16. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    The cap is there to dampen any voltage transients reaching the chip. It's common practice to place one bypass capacitor on each chip in a circuit.
     
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  17. crutschow

    Expert

    Mar 14, 2008
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    It's to bypass AC components for one chip to the other or , in this case, it's to prevent the AC components from causing instability in the chip generating the AC.
    The purpose is to keep the DC supply DC only.
     
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  18. Aagash

    Thread Starter Member

    Jun 7, 2017
    43
    2
    Thanks for all !
    Problem fixed !

    The op-amp Lm318 is working fine after power bypass with 0.1u ceramic capacitor .
     
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