• State
  • Locked Locked
  • Replies 6 replies
  • Subscribers 48 subscribers
  • Views 924 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TL084M: Looking for part behavior in comparator application

Dan Herbison
Prodigy 65 points
Part Number: TL084M
Other Parts Discussed in Thread: TL084M, LM139, LM311

I am evaluating a design submitted for evaluation by a vendor.  I am responsible to review the design and give feedback about possible problems.  The circuit uses a pair of TL084M op amps, one in a low pass filter application, which drives the 2nd op amp configured as a comparator (positive feedback).  A snip of the circuit is shown, below: (Sorry for the low res.  I can describe details needed.

The LPF is driven by the output of an LM139 comparator, with a pullup to +15 VDC via a 5.76 kOhm  resistor.  The LPF has a passband of about 9500 Hz.  TL084M comparator has 100k feedback resistance and 20k source resistance.

I don't know this op amp, but would like to give some better feedback than "it is generally not a good idea to use a saturating op amp as a comparator, especially for slowly varying inputs".  LM139 comparator is driven by a variable frequency, variable amplitude quasi-sine wave (fundamental with some unknown distortion) in the 100s Hz to 1000s Hz range.  Comparator output drives a discrete BJT common-emitter inverter.

Any criticisms of this circuit or suggestions on specific problems it might see would be appreciated.

  • 0
    TI__Genius 17125 points

    Hello Dan,

    When create a new post on e2e, you can either use the paper clip to attach a document, or click on the Insert media box.

    When you reply to an existing thread, you shoul see "Insert files, attach code and more...." below the entry window.

    Can you provide the exact part number you will be using and provide a little more detail on what you are requesting?

  • 0 in reply to Kirby Kruckmeyer96848
    Prodigy 65 points
    I edited the initial post to attach a file showing the relevant snippet of the schematic.

    I do not have a more specific part number than TL084M. Doesn't the rest of the PN just specify packaging? Your datasheet lists behaviors for TL084M, doesn't it?

    I am asking for comments about whether this is an advisable application for the TL084M, or not. (I think I read in some of your material you welcomed such questions about using op amps as comparators?) And, if not advisable, for some reason, to talk about the reason. I'd like to give some meaningful feedback to my supplier, other than, "Well, you have to be careful using op amps as comparators." They can hardly get dressed in the morning, by themselves. Yet, I am told I have to use them.
  • 0 in reply to Dan Herbison
    TI__Genius 17125 points
    Hi Dan
    I am moving this thread to the group that covers the general purpose amplifiers.
  • 0 in reply to Kirby Kruckmeyer96848
    TI__Mastermind 30600 points

    Hi Dan,

    I've tried to put your circuit into TINA and attached a screenshot here. Can you verify that I have the circuit drawn correctly with the right component values and understand the inputs?

    Could you describe the application and what this circuit is trying to accomplish from your perspective? That would help us provide some feedback.

    Thanks,

    Paul

    TL084M.TSC

  • 0 in reply to Paul Goedeke
    Prodigy 65 points

     Paul,

    Yes, that sim circuit would capture the essentials of the application circuit.  Your V2 may need to be +15 to give VEE = -15? There is an RC LPF between output of LM139 and input to active LPF.

    The circuit is basically to function as a differential crossing detector, operating on a floating differential signal.  Vout should toggle between extremes shortly after crossing of Vin across GND, in your sim circuit.  This is the front end of a tachometer circuit.

  • 0 in reply to Dan Herbison
    TI__Guru 63976 points
    Hello Dan,

    The function you are trying to accomplish is exactly what the LM311 comparator was designed for. It can accept a bipolar input on split supplies and output to single supply (ground referenced) output. It duplicates the 'LM139" stage and the output transistor.

    For a ground referenced output, tie the LM311 emitter pin to ground, as shown in Figure 11 of the LM311 datasheet. For a 15V supply, I would increase the pull-up resistor to 10k to keep the pull-up current at a reasonable level. 15V/2k is 7.5mA when the output is low...

    Are they trying to add a delay to the output? That's all the "filter" stage is going to do...