Problem with op amp offset bias... again

Discussion in 'The Projects Forum' started by EpicFails, May 14, 2013.

  1. EpicFails

    Thread Starter New Member

    Sep 7, 2012
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    Hi there!

    I posted recently about an issue I thought I had with op amp input offset bias. It turned out to be a measurement error on my part... However I have come across a DC biasing / input offset problem which I can't find any answer to and am really hoping someone can suggest an answer to.

    I have been trying out an inverting gain stage running from a single supply (9V) - see schematic attached (BTW, the opamp is actually a TL072, not a TL082 as the schematic says)

    I am getting distortion of signal from this circuit when the gain is 10 and the input voltage is 100mV sine from an oscillator. I don't get distortion when the input signal is reduced to 10mV.

    At first I thought the 100mV distortion problem would be clipping at the rails, so I increased the Vcc+ from 9V to 15V but distortion still present.

    Then I tried to rule out possible loading of the op amp by connecting it to a large load (guitar amp). Distortion still there...

    I also tried decreasing the gain by changing R3 from 5K6Ω to 10KΩ. Still distortion present.

    I have also tried altering the bias point up and down, but to no avail.

    The one thing that remains a puzzle to me is that the voltage readings at the op amp input V- and at Vout is 0.2V higher than the biasing point.

    Actual recorded measurements are as follows:

    Vcc+: 8.61V
    Vcc/2: 4.05V
    Op amp V-: 4.25V
    Op amp Vout: 4.25V

    When I changed the biasing to R1=82KΩ and R2=100KΩ, the readings came back with:

    Vcc+: 8.48V (battery is older than in previous measurements)
    Vcc/2: 4.42V
    Opamp V- and Vout: 4.62V

    This is still 0.2V higher at the op amp V- and Vout!

    Now, this may be another measurement error - last time it was the DMM reading being pulled down by a high value resistor (10MΩ), but there's no such high value resistors in this circuit, so I'm at a loss...

    I should also say that I have read up on single supply op amp design and have looked at a number of suggestions from various opamp manufacturers and have tried to implement them in my circuit, but I don't know if I'm getting something fundamentally wrong.

    It's also worth pointing out that I have tried the circuit with and without C2 in the circuit.

    Anyway, sorry for the long post, but any suggestions or help with this very greatly appreciated!

    Thanks.
     
  2. ramancini8

    Member

    Jul 18, 2012
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    Read "op amps for everyone" chapt.4 to learn how to bias the circuit. The book is free on the TI web.
     
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  3. EpicFails

    Thread Starter New Member

    Sep 7, 2012
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    Will definitely have a look at that chapter. Op Amps for Everyone has been on the reading list for a while now, but didn't know that it had a chapter dedicated to single supply design. Thanks
     
  4. GopherT

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    Nov 23, 2012
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    Instead of using a simple voltage divider as your virtual ground, us an active ground. That is, connect the mid-point of your existing Vcc/2 to the non-inverting input of the un-used half of your 082. Then connect the output to the inverting input on the unused half of the 082. Then use this output as your virtual ground.

    Your passive ground is not always stable, especially at low frequencies. The capacitors help but the voltage follower active ground will be a big improvement.
     
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  5. EpicFails

    Thread Starter New Member

    Sep 7, 2012
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    GopherT, thanks for the reply and sorry for not posting back before - work been very busy.

    I will try this - I have read about this but not tried it. I think you can also get specific rail splitter op amps as well can't you? I'll try the solution with the op amp I've got first though.

    ramancini8 - I had a look at the op amps for everyone ch4 - but then looked at the single-supply circuit collection at the back of the book and the AC inverting circuit was the same as the one I'm using... I should have said that the signal is an audio signal...

    Anyway, definitely worth reading for future reference and made me remember how to do simultaneous equations!!
     
  6. EpicFails

    Thread Starter New Member

    Sep 7, 2012
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    Further update GopherT: I used the buffer you suggested for the voltage divider and this smoothed out the problem with the Vcc/2 supply. Both inputs and output at gain stage are now equal.

    However, this did not solve the distortion of the signal. Have managed to get my hands on a scope and will test with that over the weekend - see if that gives any more useful information.

    Thanks all for you help.
     
  7. Ron H

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    Apr 14, 2005
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    What kind of voltmeter are you using? I'm guessing it's an analog meter, with a needle (d'Arsonval movement). If so, your DC voltage errors are due to the meter loading the high resistance virtual ground node.
    The distortion is more puzzling. How do you know you have distortion, if you don't have a scope? Are you hearing it? If so, are you using a speaker, or low impedance (e.g., 32 ohms) headphones? TL072 cannot drive a load that is that low.
     
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  8. EpicFails

    Thread Starter New Member

    Sep 7, 2012
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    Hi Ron H. That is a good question. I am hearing it which I know is a very inexact science. That is why I want to test it with a scope. Have just got hold of one and will be doing so over the weekend.

    In terms of what I have been connecting it to - it was a guitar amp. Thought this would be ok in terms of impedance. I am now thinking that there is a chance the "distortion" is not coming from my circuit but from the guitar amp which is pretty old and may have problems of its own...

    Not having used a scope before, is there anything I should be looking for in particular? I mean in terms of the display? I will be using an oscillator to produce the signal that I feed to the circuit, so can set a sine wave to defined voltages and frequencies.

    Thanks
     
  9. Ron H

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    Well, it sounds like you are overdriving your amplifier, but I would think that extremely high volume would be an obvious indicator of this.
    I would just do what you are already doing - start with 10mV, 1kHz input, and increase the input level while monitoring the guitar amp output with the scope.
    What is the value of Cin?
    What is the value of Cout? What is the input resistance of the guitar amp?
     
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  10. GopherT

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    Nov 23, 2012
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    Improvement 1: add a pair of capacitors to the active ground output to each supply rail (one capacitor to ground and one to Vcc).

    Improvement 2: Using this active ground circuit, the load to the op-amp active ground pin cannot exceed about 30 mA (or what ever the datasheet for the 082 says for max sink/source current). If you need to handle higher load, you will need a more complex circuit with a pair of diodes, and a pair of transistors (these will actually sink/source the load). The op amp will control the Vcc/2 level and the diodes will prevent any cross-over distortion.
     
    Last edited: May 17, 2013
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  11. EpicFails

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    Sep 7, 2012
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    Ron H: I'm now pretty sure I was overdriving the amp. I just put the signal from the output of my circuit on the scope and there appears to be nothing wrong with it. I'm getting the output level that I would expect from a 100mV input...

    I don't know where to start taking readings from the output of the guitar amp.

    Cin and Cout are both 1uF.

    I'm going to do some more tests with the scope once I'm more familiar with it.

    GopherT: With improvement 1, I am already using caps from Vcc/2 to both ground and to Vcc but on the resistor voltage divider side of the op amp. With your improvement, should I move those to the output from the op amp or add two more in addition to the ones I'm already using?

    With regard improvement 2, I'm not sure I really understand this one. The load that the output from the active ground sees is the +input of the op amp for the inverting gain stage. Would this be the type of situation that would require the additional circuitry you describe?

    Thanks for your help guys.
     
  12. Ron H

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    You don't need the op amp for the active ground, because you don't have to source or sink any current to it. It won't hurt anything, but if you need the op amp for something else, you could free it up. If you do use it, don't connect any capacitors to the output. If you do, it will probably oscillate.
    You might as well use it, because it needs to be connected as a voltage follower even if you don't use it.
    I think I would use it as in the attachment. This makes your input impedance 1MegΩ, whereas with your original circuit, it is 5.6kΩ.
     
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  13. GopherT

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    Nov 23, 2012
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    Yes, I don't know what I was thinking, your method is correct.

    If you are only pushing from ne op amp to another, you are fine with Ron's design. I was just concerned that you might be adding a power amp stage and trying to use it with this weak virtual ground - it just won't work.

    Here is what I was talking about (to boost the current handling of the active ground), thanks to an old post from Bill_Marsden on this site.
     
    Last edited: May 17, 2013
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  14. Ron H

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    I think this circuit is more thermally stable.
     
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  15. GopherT

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    Agreed! .
     
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  16. EpicFails

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    Sep 7, 2012
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    Thanks for the schematic including the buffer Ron H. It's funny you should bring that up, because the inverting gain stage is just one stage in a bigger design. The overall design is for a preamp interfacing a piezo sensor, so it will definitely need a buffer, probably bigger than 1MΩ. I will post the overall circuit, which also includes a tone control stage tomorrow - I would be really grateful for any suggestions for improvements.

    As the circuit will be a preamp, there is no power amp stage - this circuit will connect to a power amp, normally a mixing desk, but thanks to Gopher T and Ron H for the suggestions about more stable virtual ground solutions.

    I could also do with some suggestions about altering the inverting stage I've posted, as what I really want to do is turn this into a variable gain stage from x10 gain down to ∞dB.

    I've been working on the basis of a solution that replaces the feedback resistor with a linear pot which connects to the op amp inverting input, op amp output and to the resistor that is currently 5k6Ω. I know this will work, but a better solution would be something like the active volume control on this site

    What I'd ideally like to do is get to a half way point between the two. In other words, have a variable active gain stage with an approximated log like the circuit in figure 1 on the page I have linked to. Is this possible?

    Finally, as the guitar amp I've been using to test these circuits clearly is not the right solution, can anyone suggest a good type of test amp I could buy to bench test these circuits? The obvious choice is a mixing desk, but I might not be able to get my hands on something like that. Any other suggestions would be great.

    Thanks as always for your help.
     
  17. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Here's a nice kit:

    http://www.parts-express.com/pe/showdetl.cfm?partnumber=320-336
     
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  18. patricktoday

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    Feb 12, 2013
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    Sorry, I didn't read the whole thread so may have missed something. If you're doing preamps and line level stuff and want to hear it, I'd recommend powered studio monitors or even the line input to a stereo. A guitar amp is a really bad choice for line level, they're meant to operate off a much lower signal level and they're designed to color the sound in a way that only sounds good with guitars.
     
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  19. EpicFails

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    Sep 7, 2012
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    tubeguy: thanks for the link - will look at that.

    patricktoday: I have some active studio monitors - don't know why I didn't realize that guitar amp would not work with line level - wasn't thinking about it properly. Will hook up the monitors with the M-audio interface I have - think that will work without computer attached...

    I have also attached the full schematic I was talking about. Would be really interested to hear what people think, particularly about the gain stage - should I use another half of a TL072 and make a full active volume control or use that half to make a stable buffered virtual ground..?

    The other consideration with this is that I don't want to use up battery power too quickly, so should I add another Tl072 half at all, or maybe even get rid of the gain stage altogether...

    Quick thing with C2 on the circuit - I included this because I've seen it included in other similar circuits, but I don't really know what purpose it serves. Can anyone tell me?

    I also read that whenever you have a gain stage with a Vcc/2 you should have a cap between it and the next stage, so should I have a coupling cap between gain stage and tone control stage?

    Any help would be great as always!

    Thanks!
     
  20. GopherT

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    Nov 23, 2012
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    C2 is there to filter (short to ground) any high frequency signals that are picked up by the input cable. As an exercise, try calculating the capacitive reactance ( ~resistance) of a 100 pF cap at 1k, 10k and 100k Hz.

    It is important to remove these frequencies, otherwise your signals will look like they have 'hair' on them on the o-scope and all that oscillation can create some heat if not removed from your output stage.
     
    Last edited: May 20, 2013
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