Operational Amplifier design help

Discussion in 'The Projects Forum' started by elrhazouani, Nov 22, 2011.

  1. elrhazouani

    Thread Starter New Member

    Feb 8, 2011
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    I want to design an op amp with the following specifications:

    • Allowed transistors are the 3904 (npn BJT), 3906 (pnp BJT), 2N3819 (n-channel JFET).
    • The input is a 1 mV amplitude source at 1.0 kHz riding on a DC value between -1.0V and +1.0V. The source has a 4 kW source resistance.
    • The primary goal is to provide a 2 V amplitude sinusoidal output across a 100 W load with an average value as close to 0.0V as possible.
    • No capacitors are allowed.
    • Standard resistor values must be used.
    • Supply voltage is to be provided by no more than 2(two) 9V batteries.
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The most challenging aspect of this would seem to be the need to obtain an output with zero DC content without using capacitors, given an input with such a wide range of possible DC values.

    Do you know if any other components than transistors and resistors would be permitted?
     
  3. Georacer

    Moderator

    Nov 25, 2009
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    Is this a homework question? Are you required to follow any conventions or other simplifications?

    OpAmp datasheets often have transistor schematics included.
     
  4. T.Jackson

    New Member

    Nov 22, 2011
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    ALL capacitors block DC but pass (n) degree of AC depending on what is called their 'capacitive reactance' xc = 1/ 2pi (fc)
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The OP is barred from using capacitors to block DC, so he must find some alternative.
     
  6. elrhazouani

    Thread Starter New Member

    Feb 8, 2011
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    the differential op amp does not require any capacitors. I'm thinking about starting with that and add to it a darlington pair
     
  7. Georacer

    Moderator

    Nov 25, 2009
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    @T.Jackson

    No one in this forum has the authority to put members' abilities to the test and act like some self-proclaimed examiner for E.E examinations. Each member has the responsibility of his words. What others think of him is their business. You are not to say who has the right of speech in this forum. Moderators and Administrators do.

    Furthermore, you are leading this thread offtopic with provocative comments.

    You are requested to stop any provocative and disruptive action in this site. This is your last warning before a ban of some time period takes effect.
     
    Last edited: Nov 22, 2011
  8. elrhazouani

    Thread Starter New Member

    Feb 8, 2011
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    this is the kind of amplifier I was referring to but the gain is too low around 50, and I need 2000 v/v gain:
    [​IMG]
    any suggestions will be much appreciated !
     
  9. elrhazouani

    Thread Starter New Member

    Feb 8, 2011
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  10. elrhazouani

    Thread Starter New Member

    Feb 8, 2011
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  11. Wendy

    Moderator

    Mar 24, 2008
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    That link does not work. You need to attach images instead, local hosting (which AAC offers) is always preferred.

    Click the <Go Advanced> button below your post,

    Then click <Manage Attachments>

    This will allow your to upload your image.

    .png or .gif formats preferred, as .jpg will fuzz an image badly (it is meant for photographs, not schematics).
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Here is the 1st image:
    [​IMG]

    It is now hosted on AAC.

    [eta]

    Here is the 2nd schematic:
    [​IMG]

    They are now both hosted on AAC.
     
    Last edited: Nov 23, 2011
  13. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I'm guessing the 'W' in the requirements are Ω due to context.

    Is there a limit on number of each type of transistor is used?

    The 2V amplitude output required, is that RMS or peak to peak?

    A differential amp is all I come up with for the variations in input bias without capacitors. Gain of 200 at 1kHz only? Guess, the exact frequency doesn't matter as everything must be DC coupled anyway.
     
  14. elrhazouani

    Thread Starter New Member

    Feb 8, 2011
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    there is no limit on the number of transistors. the 2V output is peak-peak. the frequency is not important.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    When I said the link didn't work, it was dead.
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    I know, it was for me as well.

    I used the URL from the 2nd image combined with the filename from the bad link to get the schematic image.
     
  17. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Quite Clever. That's how people find all the "private" photos on an account, another reason not to name them logically.

    However, I am still impressed at that prowess, most people over 40 don't know that trick.
     
  18. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Use a Sziklai/Compound pair Long Tailed Pair for input. Similar to the diagram at section 7 of ESP Sziklai Page

    That should give you the gain you need for a very low gain output stage.

    --ETA: For 0.0V output, you'll need to use a Class B or Class AB output amp.
     
    Last edited: Nov 23, 2011
  19. elrhazouani

    Thread Starter New Member

    Feb 8, 2011
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    I tried that but it is not giving the results I need.
     
  20. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    What was the behavior when it didn't work?

    Can you attach images of the schematics you have tried and haven't worked, preferably with output waveforms so what didn't work is apparent?

    Attach by clicking on the paper clip in the advanced editor, it allows you to upload images for free hosting on this server, so they are available if a hosting site exceeds bandwidth or whatnot.
     
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