Summing on negative input of difference amp??

Discussion in 'Homework Help' started by 02Troll, Oct 28, 2011.

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  1. 02Troll

    Thread Starter New Member

    Oct 22, 2011
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    Not really homework, as the challenge was set by my self, but looking for advice anyway.
    Been trying to get up to speed on op amp theory.
    I am trying to simplify a circuit diagram,
    The circuit is a single supply 8 volts.
    There are two inputs, 0.5 to 5V, going through buffers then potentiometers to allow for adjustment, then they need to be summed and inverted so a rising voltage becomes a falling voltage, so I'm doing this through a difference amp which has an adjustable reference on the positive input to allow level shifting up & down, this is also with a potentiometer coming directly off the power supply, rather than the "ideal" setups I've seen using a voltage divider to virtual ground.

    Questions are:
    1. Is there a fundamental problem in using a difference amp to sum on the negative input?
    2. As I am not looking for a gain at this stage, can I eliminate the resistor in the feedback loop, on the diff amp and the two resistors on the inputs and supply the difference amp negative input directly from the two input pot's?
    I have put a few formulas through an excel spreadsheet to size the resistors and using the same value resistors in the feedback loop and inputs gives the right outcome, so I assume taking out the three resistors will also give the same outcome.
    Is this assumption correct?
     
    Last edited: Oct 28, 2011
  2. crutschow

    Expert

    Mar 14, 2008
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    You need to post a diagram of your circuit.

    2. If I understand, you want to remove the feedback and input resistors from the op amp and feed the input directly to the op amp inputs. Since the open loop gain of an op amp is typically well over 100,000 you will have a lot of gain instead of a gain of one, so the circuit will act as a comparator with the output going to either rail. It's those resistors that determine the closed-loop gain so you can't eliminate them.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You should post a schematic of what you are trying to do. If you are referring to op-amps, it is possible to use the inverting input as a summing point but resistors will be required to do this. Why would you try to avoid them?

    If the input is taken directly from a pot then the impedance from the inverting input to ground will vary radically as the pot is adjusted. The negative feedback would be effectively shorted out if the signal comes from a pot slider which has been wound down to common. That cannot be satisfactory.
     
  4. 02Troll

    Thread Starter New Member

    Oct 22, 2011
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    Thanks for replies, below is the circuit diagram,
    Just trying to keep it as simple as possible, if this is not possible have thought about inverting amp instead of buffers on the input, then just doing a straight input into the difference amp, just need to do the figures, but wanted confirmation if current set up is usable.
    I have included min & max inputs with pots adjusted to 35% and reference voltage to diff amp at 1.65V, which then gives me an appropriate output range.
    So input voltage ascending, comes out as descending voltage.
    As it is a single supply all voltages quoted are referenced to power supply earth, there is no virtual earth points.
     
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Still working on the vehicle turbo modification I see.
     
  6. Wendy

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    Mar 24, 2008
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    Hello,

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