Opamp related questions

Discussion in 'General Electronics Chat' started by msr, Jun 19, 2009.

  1. msr

    Thread Starter Active Member

    Jul 8, 2008

    I have some basic questions about opamps.
    Can I use them without feedback? If no, why?

    Im trying the following experiment (simulating in pspice):

    Im using a uA741 opamp. (datasheet)
    In the V+ I put a 60.5uV amplitude signal. V- is connected do ground. There's no feedback configurtion and opamp powersupply is +-15V.
    Taking in acount that Avd=200 V/mV=200 000 V/V with this configurantion shouldn't be a outpub signal with about 13V amplitude?

    Vo/Vi=200 000; Vo=13V => Vi=60.5uV

    I made the simulation in pspice but a DC component appear and the signal doesnt have the amplitude I espected (Vpk-pk=13-(-5.5)). I dont understand why it happens. I simply cannot/shouldn't use an ampop with feedback? Or theres something Im making wrong?

    Thanks in advance!
    Last edited: Jun 19, 2009
  2. KL7AJ

    Senior Member

    Nov 4, 2008
    You CAN use op amps without feedback, but only certain special types are really suitable for that. A comparator is one such application.

    Use of an unsuitable op amp without feedback will generally result in instability, and a pesky (though generally harmless) failure mode called LATCHUP...a state of INTERNAL POSITIVE feedback, where the output voltage goes "to the rails." In this condition there's nothing you can do except to kill the power to reset the op-amp to a normal state.

    The 741 is notorious for going into latch-up, and is not the want of device you want for a precision comparator.

  3. Audioguru

    New Member

    Dec 20, 2007
    A lousy old 741 opamp is too noisy to be used without any negative feedback.
    Also it has a fairly high input offset voltage and input offset current.
  4. AdrianN

    Active Member

    Apr 27, 2009
    Look in the datasheet you posted. On page 5 they say the offset is 1mV typ and 5mV maximum. This offset is amplified by the opamp open loop gain, and that is why you get a DC voltage at output. In fact, I am sure the opamp rails, at about 2-3V from the power supply.
  5. SgtWookie


    Jul 17, 2007
    If you really need open-loop gain, then you should be using a comparator.

    Operating an op amp without feedback causes the output stage to be in constant saturation; this dissipates a good bit of power in the form of heat.

    Comparators generally have open-collector outputs, and are designed to be operated in saturation. Because they have open-collector outputs, you must use a suitably sized pull-up resistor to Vcc in order to see an output. For an LM339, a 1.2k to 1.5k pull-up resistor will work well over most of the allowed Vcc range.
  6. hgmjr


    Jan 28, 2005
    If you need a comparator function then I too would recommend that you resort to a comparator rather try to enlist an opamp to do the job. Opamps are not that well suited to open loop operation since they are designed to operate on and reproduce analog signals.

  7. axeman22

    Active Member

    Jun 8, 2009
    seems OP AMP's are just not the best for voltage comparators.. I am using one right now on the bench and it's working quite ok. But.. what would be some suggested comparison chips ? (url ?)
  8. axeman22

    Active Member

    Jun 8, 2009
    also.. I am using (this chip) and my little comparator draws 10mA on the Op-Amp output (pin6) - I've got no idea which spec of the Datasheet says the max current (continuous) one can pull through pin6 without toasting the chip - how do I work that out?
  9. SgtWookie


    Jul 17, 2007
    Here are a couple:
    LM393/LM2903; dual comparator: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=296-6601-5-ND
    LM339; quad comparator: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=296-1393-5-ND

    Datasheets and pricing available on the pages linked to.
    The LM2903 is very similar to the LM393, but has a wider temperature range than the LM393; it's suitable for automotive use. Since the LM2903 is about the same price as the LM393, you'll get more for your money.

    If you have a Radio Shack store nearby, they may carry the LM339 quad comparator, but it'll be around $1.70 each.

    There are literally thousands of different comparators available, in myriad pin configurations and specifications. However, these two will be adequate for most hobbyist-type applications, are inexpensive and widely available. Those I've posted links to are also 8-pin DIP and 14-pin DIP configuration, which make them easy to breadboard and connect in pre-drilled prototype boards.
  10. SgtWookie


    Jul 17, 2007
    Absolute maximum specification for power dissipation in the package is 500mW, or 1/2 Watt, derated at higher temperatures. It would be pretty difficult to get to that point at room temperature, if you kept within recommended rail voltages.

    This opamp can have it's output shorted to ground indefinitely without damage.