Question about differential amplifier

Discussion in 'General Electronics Chat' started by Garurumon, Mar 26, 2014.

  1. Garurumon

    Thread Starter Member

    Mar 17, 2013
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    Do I have to power my diff. amp with dual supply (e.g. V+=5 V-=-5 V), or I can use 5V and 0V instead?

    I intend to increase the voltage on wheatstone bridge.
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    Please post a diagram of the existing project.
    E
     
  3. Garurumon

    Thread Starter Member

    Mar 17, 2013
    99
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    Ok, don't judge if it's stupid idea though :D

    I'm asking about the diff amp at the bottom, and also op amp bottom right.

    And could this thing even work? And if not, why?

    EDIT:

    Btw, it's not existing, just had an idea :D
    And right op-amp would have precise trimmer instead of resistor to set the maximum gain/output voltage.
     
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  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    The Diff OPA and the 2nd OPA look OK, get the resistor values correct etc.

    The bridge drive looks a little 'iffy'.:)

    Can you describe how the circuit is going to function as an ESR tester.?
    E
     
  5. Garurumon

    Thread Starter Member

    Mar 17, 2013
    99
    1
    But what about their supply? Do I have to get a -5 supply, or I can power them from 5V / 0V?

    Well about the bridge drive, it worked nice in Multisim :D I just have to figure out a nice value for the coupling capacitor, after I calculate the bridge resistors...

    The smaller the Xc, the greater the bridge imbalance will be, I intend to adjust the output of the second op amp to be 5V when the leads are shorted, and the other values... well, I hope they'll be somehow linear, if not, I'll just measure a dozen small and precise resistors, and use the Excel to get a function how the voltage deteriorates...

    I hope the above is not nonsense :D
     
  6. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    The input to the Diff OPA will be clamped to approx +/-0.7V wrt to 0V, so the Diff will require a +/-V supply.

    Have you tried the circuit in simulation using a single supply on the Diff.??
     
  7. Garurumon

    Thread Starter Member

    Mar 17, 2013
    99
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    Oh crap. More work.

    Nope, not yet, I just tested how the AC-through-the-bridge-part will work :D

    And I'm sorry to bother, but could you explain me this part: "The input to the Diff OPA will be clamped to approx +/-0.7V wrt to 0V" ? I don't know what "clamped" means, and wiki gives me some medicine equipment for that.
     
  8. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    It means that the bridge output can never swing above or below 0.7V because of the two clamping diodes. The diodes will conduct when forward biased at about 0.7V so the effectively 'clamp' or limit the voltage range to +/-0.7v

    WRT, means 'with respect to' , which is 0V in your circuit
     
  9. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi G,
    Looking at a LTSpice simulation shows that a single supply Diff and 2nd OPA should be OK.

    What output signal change are you expecting from the different Cap ESR values.??
     
  10. Garurumon

    Thread Starter Member

    Mar 17, 2013
    99
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    Sorry for late reply, wasn't home.

    Well I do not know yet, haven't had time to do the math, and probably won't have any till tomorrow.

    I'll write here when I do!

    And tomorrow I will stick the components into the breadboard and see how it works.

    If it works, it will be one nice ESR meter ! :D
     
  11. Garurumon

    Thread Starter Member

    Mar 17, 2013
    99
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    But I'll try to make that voltage between those two points never reach 0.7V, I just gotta adjust the resistors! Low resistor values on two bottom ones!
     
  12. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    This what LTSpice shows, simple Diff OPA for the sim.
    E
     
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  13. Garurumon

    Thread Starter Member

    Mar 17, 2013
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    I did a little calculating, these values should be good for values 0.01-50 Ohm:

    R4,R5=220 Ohm
    R6=50 Ohm
    R7=100 Ohm

    With those values it should never go over 0.7V

    Is that a good range for ESR meters? Can op amps pick up signals that low?

    And yeah, it should probably be rectified somehow with 4148s to get DC at the output :$ Thanks for the simulation btw!
     
  14. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    OPA's will easily work down to mVolts

    Using a +5V supply I would use a rail 2 rail OPA's, this will give close to +V output swing.

    Post your your final circuit and we can compare Sims.
     
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