An alternative to a rail-to-rail Op Amp?

Discussion in 'The Projects Forum' started by GiovaGiov, Sep 16, 2015.

  1. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
    30
    0
    Hi, for a didactic project (this one: http://forum.allaboutcircuits.com/threads/triangle-wave-generator.109269/#post-840873 ) I need a rail-to-rail op amp, eg an LT1498 or a TLV271: the problem is that in the electronic local stores I am unable to find these op amp; I've found them on Digikey, Mouser and on Ebay, but in the first two online shop I have to achieve a minumum order (and the shipping cost is expensive) and on Ebay (with free shipping) these ICs costs about $7 for a single item, which, honestly, is too much by my point of view.

    So: considering the circuit: http://forum.allaboutcircuits.com/threads/triangle-wave-generator.109269/#post-840873

    There is a way to replace the rail-to-rail op amp with another common op-amp? Or can I achieve the same result using some transistors? Or, again, there would be an alternative circuit using common components to achieve the same triangle wave generator?

    Consider that this project is just for study purposes and for fun, since I also want to take confidence with an oscilloscope software which I've bought.

    Kind regards.
     
  2. dannyf

    Well-Known Member

    Sep 13, 2015
    1,768
    357
    Depending on your needs. R2R only matters if you want the output to reach as close to the rails as possible -> that matters in an environment needing high amplitude DC output.

    If you only care about AC output, or small output amplitude, or can increase rail voltage, non-R2R opamps work just fine.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Even a lowly LM358 will pull down very close to Vss (Gnd?), and pull up to within ~1.2V of Vdd; a sort of half-way rail-to-rail...

    If you can capacitively-couple the output, then the 555 circuit with an op-amp buffer to isolate the RC timing network from the load is not a bad way to go...
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,981
    3,221
    A non-R2R op amp inputs and outputs can generally can only to to within about 2V of the plus and minus voltage rails powering the amp, so if you can keep the signal withing those limits, you don't need a R2R type, as dannyf noted.

    For example, for the triangular wave generator you referenced, if you reduce the gain of the output amp by reducing the value of R4 to about 6kΩ, the output should be within a normal op amps output range.
    If you still get clipping of the signal, you can reduce the value of R4 further.
     
    Last edited: Sep 16, 2015
  5. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
    30
    0
    Thank you all for the nice and precise replies.
    Well: looking at the mentioned circuit, I just need to increase the output of the NE555 close to the VCC (would be OK equal or at least -1 VCC).

    Currently, without the op amp, I get a sawtooth wave which goes from 1.9V to 10V:
    [​IMG]

    If i go with a LM358 I will increase the max output from 10V near to 15V, keeping (or lowering) the min value?

    Can someone suggest me a schematic using the LM358, if this op amp will be ok for my goal? I know, since is a dual op amp, that I should connect the unused inputs to the ground, right?

    I am very sorry for my stupid questions, but I am a novice, so not very expert.

    Kind regards.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Here is my quick and dirty favorite using a LM358:

    You could try adding a 10K pull-up resistor between the output pin of the LM358 and the Positive rail to see how close it pulls toward the rail.


    osc.gif
     
    GiovaGiov likes this.
  7. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
    30
    0
    Mike, thank you a lot for the schematic, you were very gentle! ;-)

    However: so I have to abandon the NE555 version and go with this new schematic? Initially I understood that I had to replace the LT1498 with the LM358 in my schematic. There would be a way to keep my circuit using the LM358 to increase the sawtooth wave?

    Kind regards.
     
    Last edited: Sep 16, 2015
  8. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
    30
    0
    To be clear: I'd wish to keep the sawtooth generator with the NE555 because I'm studying/doing experiments with this nice IC.
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    The 555 RC node naturally swings from 1/3 Vcc to 2/3 Vcc. The node is high impedance, and cannot drive a load, so you must use the opamp as a buffer. If you want to use the opamp to act as an amplifier, you can do that too, but at some point you will run into the ability of the opamp to pull high and low. Make the gain of the opamp adjustable, and you can see the onset of clipping...

    The opamp-only circuit I posted is a way of making a similar multivibrator (oscillator) without a 555. The oscillator and buffer function are done by just the opamp alone...
     
  10. #12

    Expert

    Nov 30, 2010
    16,250
    6,746
    In my opinion, the ultimate cure for not having a rail-to-rail op-amp is to give it both supply voltages, and plenty of it, like, +/- 12 volts.
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    So the output of a multivibrator like in post #6 would swing from -12V to +10V (using the LM358). The output would still be asymmetrical. Some opamps run out of push-pull more-or-less symmetrically.
     
  12. #12

    Expert

    Nov 30, 2010
    16,250
    6,746
    You are allowed to adjust your rail voltages for the correct output swing.
     
  13. crutschow

    Expert

    Mar 14, 2008
    12,981
    3,221
    Which works, of course, only if you don't want/need a rail-to-rail output voltage. :rolleyes:
     
    #12 likes this.
Loading...