feedback op amp

Discussion in 'General Electronics Chat' started by DexterMccoy, Apr 1, 2014.

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  1. DexterMccoy

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    Feb 19, 2014
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    I see this often in circuits, it's a Reference voltage or bias voltage applied to the feedback in op amps amplifier circuits

    What is this used for? to have a voltage applied to the feedback section of op amps
     
  2. DexterMccoy

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    Feb 19, 2014
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    Here is the schematics
     
  3. DexterMccoy

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    PIC#1
    R1 has -5 volts
    R7 has +15 volts

    PIC#2
    CR4 diode has +15 volts
    CR5 diode has -15 volts

    why put a negative or positive voltage on the feedback loop on op amps circuits?
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
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    Why post schematics so the screen must be turned sideways or upside down?
     
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  5. BillB3857

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    Remove the reference and see what happens. That will explain why it is there.
     
  6. DexterMccoy

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    Are they called reference or bias?

    What is the reference voltage doing to the feedback loop? and why is it there, to do what?
     
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Depends on application.

    If op amp is used for comparator, then the voltage is reference to which the input is compared to.
     
  8. DexterMccoy

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    The reference voltage is in the feedback loop, so it's different

    What is this called when you do something like this?
     
  9. Gibson486

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    Jul 20, 2012
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    Is this audio stuff? If so, I think it has to do with getting rid of jitter.
     
  10. DexterMccoy

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    how would it get rid of jitter?
     
  11. AnalogKid

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    In your examples in the first photo, R1 and R7 are not in the feedback loops, they are parts of the input circuits. Let's go with R7:

    AR5(?) is acting as a non-inverting comparator. The - input is grounded, so that is the reference. If the voltage at pin 3 is greater than 0V, the output saturates high (approx +13.5V). If below 0V, the output saturates low (approx -13.5V). Ignoring R8 for the moment, the input comes from the output of the previous stage through a R9-R7 voltage divider to the +15. So when the AR3D output goes low enough that the R9-R7 junction goes below GND, AR5's output goes low. Use Ohms Law to solve for the AR3D output voltage that causes AR5 to transition.

    Now add in R8. R8 is positive feedback into the input voltage divider. This adds hysteresis to the circuit, and shifts the input based on the output. Without R8, if the input is transitioning very slowly, there will be a time as it croses 0V when the output will burst into oscillation. This is when the amplifier is operating open loop. But with R8 in there, the instant the output changes state for the first time it shifts the input past the open loop point so fast there there is no oscillation. To figure out how much R8 shifts the input, first calculate the Thevenin equivalent voltage and impedance, then combine this with R8 and he AR5 output voltage, approx 13.5V.

    ak

    ps. no jitter.
     
  12. DexterMccoy

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    R3 and R9 and the Rin input resistors to the Op amps

    Why did they put a Reference Voltage on the inputs?
    Op amp#1 has - negative 5 volts
    Op amp#2 has + positive 15 volts

    What kind of comparators are these?

    Most Op amps comparator i have seen don't use a Reference voltage on the input

    They use the Op amp as a differential amp
     
  13. DexterMccoy

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    Feb 19, 2014
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    R1 and R7 have a Reference DC OFFSET VOLTAGE supplied to the input of the op amps

    But why? what kind of comparator is this called when it does?

    The Reference DC offset voltage on the input changes the Threshold point/transition point
     
  14. DexterMccoy

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    What does the Reference voltages do to the comparators input?
     
  15. AnalogKid

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    You've answered your own question. Yes, the reference dc offset voltage does change the threshold point, often called the trip point.

    Why? We don't know because you haven't told us what this circuit does, what the equipment is, or anything else about the environment. And the partial schematics you have posted do not give us any of the information we need. From what I've seen, these are not simple amplifier or comparator circuit like those on data sheets and app notes.

    Any opamp can be used as a comparator, and any almost comparator can be used as an amplifier (although that is sometimes more difficult). So up to a point I can say what the circuit is doing. I can't say why without a lot more information.

    ak
     
  16. Little Ghostman

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    He isnt allowed to tell you because its Avionics related, in another post it was clearly stated he wouldnt be helped on any aircraft topics.
     
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  17. DexterMccoy

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    If it isn't an amplifier or a comparator , what is the op amp configured as?
     
  18. tshuck

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    My question is how does your employer feel about you giving away proprietary information to the world?
     
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  19. shortbus

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    My question is, how do you keep a job, when I've known of him working on the same set of boards since at least the first of the year.
     
  20. DexterMccoy

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    Feb 19, 2014
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    Well do u guys know what kind old amplifier or comparator these are?
     
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