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Voltage follower output — Parallax Forums

Voltage follower output

eiboydeiboyd Posts: 14
edited 2007-08-28 16:01 in BASIC Stamp
Hello, I have one of the BS2 pins going to 5V and this is being fed into a voltage follower (opamp = LM358N) that came with the digital input/output kit.· The output of the follower is 3.7V with the 5V input?·This appears to be an upper limit as I have a A to D and D to A converter working that will output·0 to 5V depending on the setting of a pot.· The D to A functions well (0 to 5V is output with full scale input on the pot) but when this is fed into the voltage follower the output tracks the input upto 3.7V and then rails at this level?· Does anyone know if this is a limitation of the op-amp or does it sound more like something else.

thanks much

Boyd

Comments

  • TechnoRobboTechnoRobbo Posts: 323
    edited 2007-08-28 01:02
    http://www.allaboutcircuits.com/vol_3/chpt_8/4.html
    ...all the way at the bottom of the page.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Have Fun


    TR
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-08-28 01:53
    If you want a full 5V output from an LM358, you will need to connect its positive suppy terminal (pin 8) to at least 6.5V. Assuming the load is no more than 10K ohms, the low end of the output range could still be as much as 20mV.

    If you want full 0-Vcc input/output from an op-amp, one of the newer CMOS "rail-to-rail" chips (e.g. Microchip MCP6022) could be your answer. But be aware that the current drive capability at either extreme of its output range will be limited due to a lack of voltage "headroom".

    -Phil

    Post Edited (Phil Pilgrim (PhiPi)) : 8/28/2007 2:34:00 AM GMT
  • eiboydeiboyd Posts: 14
    edited 2007-08-28 15:54
    Thanks for the input.· Looks like I should try a different chip or try the higher supply voltage suggestion depending on the limits of the LM358.· I will be using an npn to boost current so I will give both a shot.· Thanks again!



    boyd
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-08-28 16:01
    If you use the NPN as an emitter follower, your supply voltage for the LM358 will have to be at least 7.1V (even 9V is okay) to cover the additional 0.6V VBE drop. Your output won't be able to sink any current, except via whatever load resistor you add between the emitter and ground; but that may be okay, depending on your app.

    -Phil
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