LM7171 Signal Amplification Circuit

Discussion in 'Analog & Mixed-Signal Design' started by Z'YonG, Jun 10, 2018.

  1. Z'YonG

    Thread Starter Member

    Feb 2, 2017
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    Hi all, I am designing a single supply amplification circuit using LM7171 Op Amp at the moment, and having some issues that needs help. The schematic diagram is as below, and the gain is about 11 which is calculated by: Gain = 1 + Rf/R1.

    upload_2018-6-11_9-32-41.png

    The input signal looks like this, and the signal in the red circle is what I want to be amplified:
    upload_2018-6-11_9-38-1.png

    However, when I apply the supply voltage with 6V or 6.5V, the output signal looks like this:
    upload_2018-6-11_9-35-53.png

    But it is working when I supply the circuit with 5V to 5.5V, and the results is as below:
    upload_2018-6-11_9-40-23.png

    I want to make it working with 6V supply voltage, any help will be great.
    Thanks in advance!
     
  2. ebp

    Well-Known Member

    Feb 8, 2018
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    Do you use the same power supply for 5 V as you do for 6 V?

    If not:
    Instead of biasing the non-inverting input (NII) directly, make a similar voltage divider with a bypass capacitor across RDiv2 to filter any noise that might be present on the power supply. It might be necessary to use both a "bulk" capacitor of a few microfarads and a parallel 100 nF ceramic. From the filtered node, add a third resistor to the NII. Your current scheme sets the input resistance to 60 k, so perhaps 62 k from the filtered node to the NII.
     
  3. ebp

    Well-Known Member

    Feb 8, 2018
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    Also play with your scope's horizontal scale so you can (hopefully) properly see what all the "trash" is, rather than just have a screen full of aliased lit up pixels that tells you next to nothing.
     
  4. Z'YonG

    Thread Starter Member

    Feb 2, 2017
    56
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    I am using the same benchtop power supply to supply the circuit.
     
  5. Z'YonG

    Thread Starter Member

    Feb 2, 2017
    56
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    They looks like sinewave signal to me.
     
  6. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    I'm just curious....not an expert......can one power that amp that way? If one wants to single supply it......wouldn't it take a 10V gradient minimum?
     
  7. ebp

    Well-Known Member

    Feb 8, 2018
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    Power supply: is there any chance the power supply switches ranges when going form 5 to 6 volts? I've seen some bench supplies that obviously have relays that do something as the voltage setting is changed.

    What frequency is the sinewave? Any idea of where it might be originating? If you bring a finger close to the amplifier or touch the body of it (not pins, just the plastic) does anything change?

    The amp you are using has high bandwidth, extraordinary slew rate and is capable of handling considerable output current. High bandwidth amps are much more sensitive to circuit layout issues than slower op amps, though why a supply voltage threshold behavior would exist I can't readily explain. I note the datasheet recommends both bulk and high frequency decoupling capacitors. I note you are using the DIP version. Is your test circuit built on a plug-in type breadboard? If so, can you post a photo of your circuit?
     
  8. ebp

    Well-Known Member

    Feb 8, 2018
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    I just checked the input bias current spec. As expected, it is quite high, which is typical of high speed amps, which means that your bias network resistance may be too high, resulting in a large DC error at the amplifier output. Check the output pin of the amp directly with your scope with DC coupling.
     
  9. Z'YonG

    Thread Starter Member

    Feb 2, 2017
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    I am not sure about whether the power supply will switch the range or not.

    The frequency of the sinewave is around 5MHz, not quite sure where it coming from, but I have tried to change the gain of the amplification. for the gain of 11, it works with 5-5.5V, and I slow reduce the gain till 2, and it can work with slightly higher voltage, but the best I can go is 5.9V. Once I reach 6V, the sinewave appears again. Sinewave looks like below:
    upload_2018-6-11_11-21-34.png
    upload_2018-6-11_11-21-52.png
    If I continue increase the voltage, the signal becomes like below, it is clear, but the signal that I want is missing.
    upload_2018-6-11_11-24-51.png

    I have tried to touch the amplifier, nothing changed.

    I tested it on the breadboard before, and it was working, but it doesn't work anymore. I didn't change anything of the circuit. I have also noticed that sometimes when I disconnect and reconnect the GND, it works again, but not for long, the sinewave will appear again. the breadboard setup looks like below:
     
  10. Z'YonG

    Thread Starter Member

    Feb 2, 2017
    56
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    upload_2018-6-11_11-29-48.png

    I have check the output pin directly with scope with DC Coupling, the signal looks the same but with higher voltage.
    Signal directly from output pin:
    upload_2018-6-11_11-34-3.png
    Signal output from the completed circuit:
    upload_2018-6-11_11-34-55.png
     
  11. ebp

    Well-Known Member

    Feb 8, 2018
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    Try putting a decoupling capacitor directly across the top of the IC on the shortest possible leads. Your layout is quite compact, but the total path for the decoupling capacitor is long. If you have some, also add a 2.2 µF or larger tantalum capacitor across the existing decoupling cap. I think the sinewave is the amplifier oscillating, probably due to decoupling issues. I don't think it has anything directly to do with the power supply.

    Be sure the wires that connect your power supply to the breadboard are twisted together to minimize inductance
     
  12. Z'YonG

    Thread Starter Member

    Feb 2, 2017
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    I tried to add the decoupling capacitors, but the issue is still there. Any idea how I should modify the circuit? or better circuit design? this circuit will eventually need to be print on the PCB.
     
  13. eetech00

    Senior Member

    Jun 8, 2013
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    Hi

    According to the datasheet, the common mode range is +/-3.2v with a 5v supply. The input signal is biased at 1/2 supply. Might be exceeding the common mode range. Maybe the gain is too high. Or the input signal is too high, or both.

    eT
     
  14. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    How are you measuring the output? Install a 50 ohm resistor right before the output cap. Put cap to ground.
     
  15. Z'YonG

    Thread Starter Member

    Feb 2, 2017
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    that is what I thought, so I tried to add a step down resistor to drag down the input signal, and reduce the gain, but still, once it is over 6V, it is gone.
     
  16. Z'YonG

    Thread Starter Member

    Feb 2, 2017
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    I just tried that, still have the issue tho...
     
  17. eetech00

    Senior Member

    Jun 8, 2013
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    Once what is over six volts?
     
  18. Z'YonG

    Thread Starter Member

    Feb 2, 2017
    56
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    Once the supply voltage is over 6V... I don't want to reduce the supply voltage, as I am planning to use 4*AAA battery for my circuit. I am trying to find a way that amplification circuit can work for 6V, the LM7171 is able to run with much higher supply voltage.
     
  19. eetech00

    Senior Member

    Jun 8, 2013
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    What is the input signal amplitude range and frequency range?

    What is the desired output amplitude?
     
    Last edited: Jun 11, 2018
  20. ebp

    Well-Known Member

    Feb 8, 2018
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    There is something very strange going on. If the "1" marker on the left side of the scope screen marks 0 volts, the signal after the output capacitor should be very close to symmetric above and below that level. There is nothing about the sinewave that looks distorted or clipped, so the mean should be quite close to the midpoint between the limits. If the coupling capacitor DC leakage were high, this is what might be expected, but leakage in ceramic capacitors is normally extremely low. Try adding a loading resistor to ground on the output side of the output capacitor - something in the 5-10k range (the amp can drive a much lower resistance load, but I don't want to greatly increase currents in the test circuit).

    The signal after the output cap looks like it may be noisier than directly at the emitter, but again it is difficult to tell because the horizontal scale is poor and there is digital/display aliasing (always a problem with this sort of thing and the major thing I dislike about digital scopes).
     
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