Load Resistors for a Rookie

Discussion in 'General Electronics Chat' started by safortunw, Jul 12, 2012.

  1. safortunw

    Thread Starter New Member

    Jul 12, 2012
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    Hi folk,

    I have a circuit that I am trying to decide if I need a load resistor in or not.

    It contains a detector which has an amplifier in it which outputs to another op-amp circuit. The detectors output load is 47KOhms, the resistor on the front of the Op amp is 1.2MOhms, would I need to put a 47KOhm resistor to ground before this second op amp?

    I'm don't totally understand load resistors and when they are used between two circuits so if anyone can help me out here that would be much appreciated?

    Thanks in advance,

    SF
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    A schematic would make your question crystal clear for anyone to follow.

    I think you mean you have one op amp driving the input of another op amp directly. You should not need a load resistor there.
     
  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,006
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    Post your schematic for help. Click "go advanced" and then under "manage attachments" you can post the schematic.
     
  4. safortunw

    Thread Starter New Member

    Jul 12, 2012
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    0
    Mmm it's difficult to show you clearly as the circuit is; a detector > into a preamp board > into the main board with the op amps.

    Attached is the preamp board schematic and the main board schematic separately:

    So the detector feeds into the potential divider made up of the 1M on the preamp and the 33K on the main board, it then goes through filtering and amplification.

    The problem is I'm not sure when load resistors should and should not be used and how they affect the other parts of the circuit.

    Thanks for helping, much appreciated.

    SF
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    Here are the schematics. For a clearer view, go to the pdf files.

    Preamp board:

    [​IMG]

    Main Board:

    [​IMG]
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    No, you do not need additional load resistors.

    I am not sure the purpose of the 1MΩ series resistors R3 and R5 in the preamp.
    These feed into 33kΩ to ground, attenuating the signal by a factor of 33 (or 3% of the original).
     
  7. safortunw

    Thread Starter New Member

    Jul 12, 2012
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    Thanks, Mr Chips. Can you explain briefly why?

    Are load resistors only needed when there isn't enough 'load' in the circuit to allow the first piece of circuitry (in my case the detector) to dissipate or operate at full power?

    Thanks for your help!
     
  8. safortunw

    Thread Starter New Member

    Jul 12, 2012
    4
    0
    Sorry never read your full messsage there, yes the 1M resistors are part of a potential divider with the 33K resistors. This is the correct attenuation.

    I'm sure its clear by the way I've worded my questions that I don't really understand the purpose of Load resistors, can you explain why I don't require them here? Or why I would need them?

    Many thanks,

    SF
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,437
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    A load resistor is needed when you have a current source and you need to create a voltage across the load resistor.

    Typically the output of an amplifier should be a low impedance feeding into a high impedance input of the next amplifier.

    Load matching becomes more critical when you want minimum reflections, maximum power transfer and maximum signal/noise ratio.
     
    safortunw likes this.
  10. Audioguru

    New Member

    Dec 20, 2007
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    Your schematics were saved and posted as very fuzzy Jpeg files instead of very clear PNG file types. I can't see any details.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    Sorry, your PDF files are very clear but the parts are spread apart too much so the schematic is as big as my neighbourhood.
     
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