single supply op amp

Discussion in 'The Projects Forum' started by Rajit28, Oct 27, 2008.

  1. Rajit28

    Thread Starter Active Member

    Oct 21, 2008
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    HOW does a single supply op amp work ?
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Any opamp will work with a single supply voltage if the inputs are biased at a voltage in between the 0V and positive supply voltage (the allowed input common-mode voltage range). The input and output might need a coupling capacitor.

    An opamp has a voltage gain of about 200,000 at DC and low frequencies.
    It has a very high input impedance and a low output impedance.
    Negative feedback is used to reduce the gain to a useable amount, increase the input impedance, reduce the output impedance, reduce distortion and increase bandwidth.
     
  3. Rajit28

    Thread Starter Active Member

    Oct 21, 2008
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    i know the basics, Anyone know anything about LM 324

    i think it is a weird op amp since we are having trouble using it?

    Supply Voltage 5V. Zin> 10 K ohms. E in = 50mVolts at F= 1 Khz

    Av= -20. How does it work any solution is fine we as a team are truly stuck.

    so far Vout is 1.38 Rf = 230K Ri = 11.5 K C= .1 Micro Farad.

    Channel 1 input 152 mV. Vref =2 V we got 2.044V. Hope this helps.
     
    Last edited: Oct 27, 2008
  4. steveb

    Senior Member

    Jul 3, 2008
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    http://instruct1.cit.cornell.edu/courses/ee476/ideas/singlesupply.pdf

    This seems to be a good article on the subject.

    I'm not sure of the exact definition of a "single-supply op-amp" (and perhaps my definition is too narrow), but when I think about single supply op-amps, the key features that comes to mind is that the output should be able to go extremely close to the negative rail (ground for single supply), or even right to ground, and the input voltages should be able go all the way to (or even below) ground without issues. Even rail-to-rail op-amps do not allow the output to go completely to the negative rail and sometimes that 10 to 50 mV difference is very important. Also, not all op-amps let the input go to the negative rail without problems.

    What if you want to make a simple non-inverting op-amp buffer with a single supply that accepts inputs to ground and drives voltage all the way to ground? Usual dual-supply op-amps (even rail-to-rail) won't do it.
     
    Last edited: Oct 27, 2008
  5. Rajit28

    Thread Starter Active Member

    Oct 21, 2008
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    good advice i think. still confused. new to op-amps.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The lousy old LM324 has trouble at frequencies higher than only 2kHz. It has crossover distortion. It is noisy.

    Please attach your schematic so we can see what is missing.
     
  7. steveb

    Senior Member

    Jul 3, 2008
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    Yeah, it can still be confusing even when you're not new to op-amps. To simplify things, we should distinguish between "single-supply op-amps" and op-amps used in "single-supply applications". Usual op-amps are really dual-supply devices, even though they can be used either with dual, or single supplies. However, there are special considerations when a dual-supply op-amp is used with a single supply, and that article seems to discuss those details. Single-supply op-amps can have better performance in single-supply applications, but these considerations really get into some nitty-gritty details.
     
  8. Rajit28

    Thread Starter Active Member

    Oct 21, 2008
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    can i send my schematic as multisim.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    If you post it in Multisim format, only those with Multisim will be able to view it.

    It's much better if you can export an image of the schematic as a graphic, preferably .PNG format. .JPG is too "lossy", and the image will be unclear.

    If Multisim won't export an image, you can do a screen print by pressing Ctrl+PrintScreen, and paste the image into MS Paint. Then crop and save in .PNG format, and attach using the "Go Advanced" and "Manage Attachments" buttons.

    Instead of the ancient and slow LM324, consider using more modern opamps.

    The LM324 uses transistors in it's output stage, so it can't get closer than about 1.5v from the positive "rail".
     
  10. Rajit28

    Thread Starter Active Member

    Oct 21, 2008
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    Instead of the ancient and slow LM324, consider using more modern opamps.

    which is the modern op amps. i heard of LM 741 and LM 118, 218 and 318.

    are these good.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Those amps are also quite old. The LM741 opamp is one of the oldest. The LM118/218/318 is far better, but requires a bipolar supply.

    Work on posting your circuit first, so that we can see what you're trying to do.
     
  12. Rajit28

    Thread Starter Active Member

    Oct 21, 2008
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    still working on circuit will post when ready.

    the analysis more important in this project.

    project due in about 2 months. any advice on new

    op amps always welcome.:confused::cool:
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Linear Technology's LT1014 is a single-supply plug-in replacement for an LM324. It has far better specifications.

    You can download a datasheet for it on their website:
    http://www.linear.com
    LT1013 is a similar opamp in dual instead of quad configuration.
    LT1006 is similar, but a single opamp.

    One caution about any opamp is to not leave inputs of unused amplifiers floating, or you may experience high frequency oscillations or heating due to the output being in saturation. Connect the output of each unused amp to the inverting input, and the noninverting input to a voltage somewhere between the rails. A simple voltage divider (pair of resistors, 5k to 100k across the power rails) will work fine.
     
    Last edited: Oct 29, 2008
  14. boff1

    Member

    Oct 15, 2008
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  15. Audioguru

    New Member

    Dec 20, 2007
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    I use MC33171 single, MC33172 dual and MC33174 quad low voltage low power opamps as replacements for lousy old LM358 dual and LM324 quad opamps.
    These Motorla (ON Semi now) opamps have the same low voltage and low power as the National Semi ones but have 9 times the bandwidth (35kHz) and no crossover distortion.
    The inputs also work at ground and the output goes very close to ground.

    I use MC34071 single, MC34072 dual and MC34074 quad low voltage opamps for low supply voltage and high speed. Their minimum supply is only 3V like the others but the output swings to maximum level to 100kHz. The power supply current is not low power but is "normal".
    The inputs also work at ground and the output goes close to ground.
     
  16. Rajit28

    Thread Starter Active Member

    Oct 21, 2008
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    op amp with high voltage recommended.

    voltage around 20 to 30 AC or DC doesn't matter.

    Best manufacturer for op amps.:confused:
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    You aren't helping us help you.

    In the 70's, there were perhaps a dozen op amps on the market.

    Nowadays, there are tens of thousands, and many varieties of each op amp.

    There is no "one best manufacturer".

    You have yet to give a complete statement of what you are attempting to do, or what you are expecting to see as an output.

    The LT1014 I suggested has an absolute maximum of 22v for supply voltage.

    The opamps that Audioguru suggest will go as high as 44v for supply voltage.

    Tell us what in the world you are attempting to do. Make a schematic, post it, explain what you have for an input, and what you are expecting for an output.
     
    Last edited: Oct 29, 2008
  18. Rajit28

    Thread Starter Active Member

    Oct 21, 2008
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    You aren't helping us help you.

    The opamps that Audioguru suggest will go as high as 44v for supply voltage.

    Tell us what in the world you are attempting to do. Make a schematic, post it, explain what you have for an input, and what you are expecting for an output.

    how come you guys are:mad:. i am trying to design a circuit

    using resistors capacitors and op amps with high voltage.

    is 44 v the highest voltage possible with op amps?:confused::cool:

    output is unknown. input is resistors and capacitors with high voltage op amps.

    any advice is always welcome.:confused:
     
  19. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I suppose your resistors-capacitors-opamps-high-voltage-thingie is some highly classified military work. Than it shall stay classified even to you.

    In Czech we have a proverb, which rougly translated says Who doesn´t listen to the advice, cannot be helped.
     
  20. beenthere

    Retired Moderator

    Apr 20, 2004
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    Look up Apex Technologies - think they have a line of power op amps that will take up to 350 volts.

    Advice - always provide as much detail about a project as you possibly can. You get more accurate help that way, and everybody benefits.
     
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