In a confusion with IC 741

Discussion in 'General Electronics Chat' started by Tusher Chakraborty, Dec 15, 2013.

  1. Tusher Chakraborty

    Thread Starter New Member

    Dec 15, 2013
    I'm going to implement a circuit with photodiode and IC741(op-amp). The task of the circuit is responding quickly with the change of light.

    But I think I'm facing problems with IC741. Is it a poor choice? In that case, what will be the right choice?
  2. Potato Pudding

    Well-Known Member

    Jun 11, 2010

    Avoid the 741.

    There are many much better Op Amps out there, thought I suspect what you really want is a Comparator.
  3. Carlos Martínez 1

    New Member

    Nov 23, 2016
    The 741 is bad specifically for that. It's the slower OpAmp you could use. Nowadays it's only used for educational purposes.
  4. wayneh


    Sep 9, 2010
    Ditto. A comparator such as LM393 is better if you want a switch function and something like LM358 is likely easier if you want a proportional (not just on or off) response.

    That said, the old 741 will work just fine as long as you design around its properties.
  5. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    Usually with photodiode amps you have to decide if you want a photovoltaic or photocurrent response. What kind of bias you need.

    The bigger parameters of interest is Vos and Ib. Vos has a strong temperature dependence.
  6. WBahn


    Mar 31, 2012
    It's probably not the best choice, but it might be adequate. You don't give any indication of what "responding quickly" means for your application. Do you need a response in nanoseconds? In microseconds? In milliseconds?

    I remember a thread a few years back where the person wanted something that responded "as fast as possible" and, after many pages of posts with people trying to give suggestions on how to get the fastest possible response they could imagine, it turned out that the TS was thinking on the time-scale of minutes and anything that was faster than a handful of seconds was more than good enough for his application.

    In general, phrases like "responding quickly" have no place in engineering discussions (and, yes, that's pushing things too far). You need to start really trying to quantify your requirements. Ask yourself questions like: If the light goes instantly from full intensity to no intensity, what is the longest amount of time that I can tolerate my circuit output taking in going from a full-on indication to a full-off indication? Try to narrow this down with specifics -- if it took one millisecond, would that be good enough? If not, why not and would one hundred microseconds be good enough. If yes, would ten milliseconds be good enough. And remember, good enough is... good enough! If you insist on 1 microsecond when 1 millisecond will do, don't be surprised to end up paying $50 when 50 cents would have been sufficient.

    Also, ask how quickly the light intensity can change. There is little point having the detection circuit be able to change much more quickly than the signal being detected.
  7. hp1729

    Well-Known Member

    Nov 23, 2015
    The LM741 is used in education because it is a good example of all the flaws in op amps. Show your schematic you have so far.
    DickCappels likes this.
  8. dannyf

    Well-Known Member

    Sep 13, 2015
    it is unlikely 741 is your problem. but you provided so little information for anyone to say definitively.

    any opamp can be a problem for some applications. the 741 for its time was a high performance opamp.