Summing amplifier

Discussion in 'General Electronics Chat' started by ceidas, Jan 21, 2012.

  1. ceidas

    Thread Starter Member

    Dec 26, 2011
    50
    1
    [​IMG]

    Hello everyone,
    I am trying to implement an op-amp that will subtract two voltages using this topology.
    The thing is when the correct output voltage tends to be less than one volt, I cant get the correct result unless I use another power supply so I can give to the op-amp (741) negative supply voltage.

    So, can I take correct result with only one positive DC supply voltage? Thanx in advance.
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    The 741 is not able to deliver output voltages near to its supply rails. The functional range of input voltages for a 741 is also restricted to well within the supply voltages.

    http://www.ti.com/lit/ds/symlink/lm741.pdf

    You will get better results using an op-amp designed for single-supply operation. These often have inputs and outputs whose voltages can closely approach the negative (or ground) rail.

    Even more flexibility is available from devices with "rail to rail" inputs and outputs, which cover the full supply voltage range, or very nearly so.
     
  3. ceidas

    Thread Starter Member

    Dec 26, 2011
    50
    1
    OK, I used the LM358 and my problem got solved. Anyway, I was more interested in implementing a solution with the 741, since I dont have any LM358 right now, but thanx anyway.
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    The inputs could be shifted positively by nominally equal amounts, using resistors tied to the positive supply, so that input voltages down to zero could lie within the common-mode range of the 741. This would have the disadvantage that any error in the resistance values would turn up as a voltage offset at the input. Since a shift of a few volts would be needed, resistor errors of a few percent could cause many tens of milllivolts error.

    Shifting the output voltage range down to ground would be less easy: a diode level shifter feeding an emitter follower might be an approach, but this seems a clumsy thing to do, and could cause issues with stability.

    What is the motivation for this - are you unable to obtain any other ICs?
     
  5. ceidas

    Thread Starter Member

    Dec 26, 2011
    50
    1
    Well, I am trying to create my own stable dc supply for my workshop, but the lm317 that I use cant get lower than 1.25V. So I subtract it from it.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    For anything but a tiny power supply, you will require a much heavier output current than the old 741 op-amp could supply.

    A Darlington or MOSFET follower output stage could be arranged to lose a couple of volts at the output, and as explained earlier the reference and input (feedback?) voltages could be raised positively.

    Using a decent op-amp with inputs able to go to negative rail would be so much easier anyway. The set-point could then come in to the non-inverting input from the slider of a pot tied across the LM317 (or whatever): no need to subtract anything.
     
  7. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    You can probably use a 741 ( but why, oh you only have them ) for the supply with a darlington transistor emitter follower booster. The 741 will not go all the way to 0 with a single ended supply, but it takes about 1.4 volts to drive the darlington, so I would guess you could get pretty close. Why not give the 741 some negative voltage, it need little current, modify your supply to the 741 for the negative voltage or try a 555 charge pump.

    You do not need a summing amplifier. A follower ( non inverting ) with a pot input from a reference is all you need. I have several old 741 based supplies and they work quite nicely ( I would not use the 741 now ) One extra small signal transistor and a resistor will add short circuit protection.
     
  8. ceidas

    Thread Starter Member

    Dec 26, 2011
    50
    1
    I am about to hit my head to the wall for not thinking these simple but awesome solutions. Thank you very much.
     
Loading...