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Opamp Linearity

  1. Sep 15, 2009 #1
    Do opamps amplify lineary down to very low signals like a few microvolts, the OP-07 for eg.
    I have an LM324 in moving coil phono preamp and the + input in which the AC coupled signal is going to is also connected to the + supply(6v) thru a 100K resistor. Is this to bias the opamp input because of the very low signal input?
     
    Last edited: Sep 15, 2009
  2. jcsd
  3. Sep 15, 2009 #2

    berkeman

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    Opamp inputs need to have some sort of biasing, to supply their finite (but small) input currents.

    You can learn more about the subtleties of opamps in this thread:

    (see for example, circuit B in post #39...)

    .
     
  4. Sep 15, 2009 #3
    The resistor is a pull up resistor that makes sure that the input capacitor is at a defined reference voltage when there is no signal. Normally this is 0 volts, but in your set up 6V might make more sense. If there is no such resistor then the capacitor can trap a charge, and have some floating voltage that is possibly very high. The resistor has nothing to do with linearity.
     
  5. Sep 15, 2009 #4
    m718-
    Here is a website that reviews all (well, most) of the equations used in op-amp circuit design,
    http://www.opamp-electronics.com/tutorials/semi_theory_ch_008.htm [Broken]
    About 3/4 of the way through this chapter is a section on input bias current, input offset current, and compensation resistor.
    Bob S
     
    Last edited by a moderator: May 4, 2017
  6. Sep 16, 2009 #5

    vk6kro

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    Op amps are generally pretty noisy too, so it is unlikely you could use one at the microvolt level. The small signal could get swamped by the noise of the opamp.

    This is especially so for general purpose ones like LM324s and LM741s.
     
  7. Sep 16, 2009 #6
    I made about 4 different low noise transistor preamps from web schematics none of them work.
    Any schematic that had a transistor in it never worked when I made it like this one for eg.
     

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  8. Sep 16, 2009 #7
    You can get low noise op-amps that are in the range of several nanovolts per root Hz noise levels. So a 10-KHz BW might be several 100 nanovolts noise.
    Bob S
     
  9. Sep 16, 2009 #8
    Is this close to a good transistor preamp?
    and do transistor preamps have lower drift than an op amp like AD8571(.005microvolt/C)?
     
  10. Sep 16, 2009 #9
    Hi m718-
    The lowest noise-figure amplifier I ever used was a GaAs microwave amplifier with a 2 GHz BW that we cooled to about 10 kelvin.
    Bob S
     
  11. Sep 16, 2009 #10

    vk6kro

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    That "low noise amplifier" doesn't look like it would do anything because it has no biassing on the bases.

    The outputs should be out of phase but then they are joined together. Not sure what that is all about. Quite a strange arrangement.

    Seems like it would be "low noise" OK but low everything.
     
  12. Sep 16, 2009 #11

    berkeman

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    I had trouble reading the schematic as well. Not drawn well at all, at least not for traditional modern schematics. Some of my older RF textbooks draw schematics like that, though -- quite the pain to try to puzzle out.
     
  13. Sep 17, 2009 #12

    vk6kro

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    Yes, apart from being drawn strangely, I don't think it has a hope of doing anything.

    I have redrawn it in case we were missing out on a little gem here, but I don't think so.

    See attachment, but.......... DO NOT BUILD THIS.......... it is just nonsense.

    It would be so easy to get it working, but as it stands, it would do nothing.

    It was probably just someone's doodling and it was never built. Always a risk on Internet.
     

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  14. Sep 17, 2009 #13

    uart

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    I wouldn't dismiss it quite so quickly. It's certainly unconventional but it might just work. A key point with this circuit is that the 9v battery supply is floating, so it's actually possible for the positive and negative ends of the battery to be simultaneously driven at the signal frequency. This is in fact how the circuit works. Both Q1 and Q2 operate as CE stages (both emitters are AC ground) and both invert the signal from base to collector, so both ends of the battery are in fact driven in phase (with each other), and these voltage swings are coupled directly to the output.

    The bias in this circuit was the weirdest part to figure out. It's a bit like a currrent balance between the transistors and parallel resistors and it needs fairly well match transistors to work ideally. (Ideally so that the battery voltage floats to +4.5 volts at Q1's collector and -4.5 volts at Q2's collector). In general there will be some mismatch and the battery voltage will float asymmetrically, though it still might work well enough since it's only dealing with relatively small voltage swings.
     
  15. Sep 17, 2009 #14
    I got the schematic from this site: http://users.ece.gatech.edu/mleach/headamp/

    its the third and the first on that page is the original. Its supposed to have the noise equivelent of a 2.7R resistor.
    if anyone can get that to work let me know if you used the exact same schematic they show.
     
  16. Sep 17, 2009 #15

    vk6kro

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    Great bit of analysis UART. Maybe it could be got working.

    Just playing with the drawing a bit, I came up with something a bit more conventional.
    Wouldn't be too difficult to add a few components and get M718's circuit working.

    See attachment.
     

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  17. Sep 17, 2009 #16

    Averagesupernova

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    I would say the original circuit posted should work. The biasing isn't all that tricky. Consider the base/emitter junction of each transistor wired in series with each other. If the transistors are a matched pair it should work. This circuit should work very well at keeping any power supply noise from getting in the output. Granted, there shouldn't be any power supply noise from a battery. Also, I would say that it stands a better chance of working well over a wide supply voltage in the event of a drained battery. vk6kro, your redrawn schematic would be a bit easier to understand if you faced the base of the NPN towards the base of the PNP.
     
  18. Sep 17, 2009 #17

    vk6kro

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    OK about the bias arrangement. I see what you mean.

    See attached picture. Something like that?
     

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  19. Sep 17, 2009 #18

    Averagesupernova

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    Yes exactly like that. Uart hit it pretty close with the voltage on the collectors swinging in the same direction, but I prefer my description of the bias. Maybe I've misunderstood what Uart was saying. I would also tend to think that this amplifier is a very low distortion amp. Any non-linearity in the transistors would tend to cancel out as long as each transistor distorts the same as the other since they are swinging in the opposite direction (one is going closer to cuttoff while the other is going closer to saturation). I think it is a very clever circuit.
     
  20. Sep 17, 2009 #19

    vk6kro

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    I hope it works. That would be amazing.

    This is like seeing a dead dog jump up and start barking!

    The author complained that it had been used without his permission in some commercial equipment, so that sounds promising.

    Not to forget M718 though. Maybe he could take some voltage readings and give component values for his actual circuit and it could be got going?
     
  21. Sep 17, 2009 #20

    Averagesupernova

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    I'm guessing it didn't work because mismatched transistors.
     
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