• State TI Thinks Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 1 answer
  • Subscribers 51 subscribers
  • Views 453 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

LM124A: Power supply sequencing and use as a voltage follower

Philip Beucler
Prodigy 40 points
Part Number: LM124A
Other Parts Discussed in Thread: LM124, LM124-N

I’m using the LM124A in a static condition as a voltage follower and have a couple application questions.  First, if I am biasing at +5/-5V, what is the max positive output voltage that it will go if it attempts to rail to the positive supply?  I cannot go over 3.3V output under any condition.

Second, I am biasing it with +5/-5V. Due to power supply sequencing that I cannot control, I will be applying up to -1.5V to the non-inverting input prior both to the +5 and -5 volts coming up.  The approximate timing is -1V applied at Time=0.  The +5V comes up ~40 mS later and the -5V comes up 40 mS after the +5V.  What will the output do during the power supply bring-up when -1V is applied to the input with no bias and then only +5V bias?  Will the output go to 0V with only +5V turned on?

 

lastly, I’m using this as a voltage follower to provide a DC low impedance steady state.   It won’t be switched or changing amplitude so frequency is not an issue.  I am questioning the follower in terms of any potential oscillations, do you have any cause for concern using the LM124A?

  • 0
    TI__Genius 17035 points

    Hi Philip,

    Can you clarify which version of the LM124 you will be using?   What is the exact part number?   TI offers many different versions of the LM124 and although they all mostly operate the same, there could be some differences minor differences in regards to your question.

    Also, I am moving your question to the group that supports the amplifiers.

  • 0
    Guru 140200 points

    Hi Philip,

    exactly! See the differencies in the "absolute maximum ratings" referring to negative input voltages in the datasheets of LM124A and LM124-N, e.g., ...

    Kai

  • 0
    TI__Guru*** 139926 points

    Philip,

    Welcome to the e2e forums.

    The output high will vary with load and supply VCC+ voltage. With higher VCC+ and lighter load, and higher temperature, the output will be greater than 3.3V ; I see no reason that more than 3.3V would be terrible. Perhaps the next stage could be protected with a diode or series resistance. 

    The input being -1V first will require an input resistor between input and non inverting input pin. 1k or more will be fine in this case. During the time that VCC+ is 5V and input is below VCC- pin (pin 11) the output might be VOL or it might be VOH. The input common mode is being violated so the correct output cannot be certain. After the VCC- also becomes powered, the output will be correct. 

    Even in a static DC voltage application, stability is still a factor because the op amp still amplifies over its full bandwidth. LM124 also has a time delay that may affect the output voltage if the load current changes polarity such that the LM124 has to switch been the PNP emitter follower and NPN Darlington drivers. See application note section "4.5 Second Crossover Example"

  • 0 in reply to Kirby Kruckmeyer96848
    Prodigy 40 points

    I'm using a bare die device purchased through a die distributor, not a packaged part so the only ordering information is LM124A, there are no other letter or number variants when ordering this part.

  • 0 in reply to Ron Michallick
    Prodigy 40 points

    The load is light, only a few microamps of negative current.  There is no stage following the LM124A, the output feeds my customer's input and they have dictated the n3.3V max.  I have a 1K series resistor at the output that feeds their input.

  • 0 in reply to Ron Michallick
    Prodigy 40 points

    Forgot to add, the -1V is set up with a -2.5V reference and a voltage divider to make the -1V.

  • 0 in reply to Philip Beucler
    TI__Guru*** 139926 points

    Philip,

    Try a Schottky diode from non-inverting input to -5V supply to limit the negative voltage seen by the LM124A input. This should prevent the op amp from going VOH, high out during power up. Better yet is to put the diode at -2.5V ref (made by a shunt regulator?) to -5V supply so the negative input voltage at LM124 will be even less.