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LM324 voltage follower configuration

Jonathan Newman
Prodigy 160 points

Other Parts Discussed in Thread: LM324

Hi,

I would like to use the LM324 in voltage follow configuration. This is depicted on pg. 9, figure 1 of the data sheet (http://www.ti.com/lit/ds/symlink/lm324.pdf). I am confused because I cannot decide whether this schematic is just showing the test circuit that was used to to produce the bandwidth and slew-rate results reported in table above the figure, or if the resistor and capacitor are a recommended stabilization network that should be included in the user's design. Can somebody help me with this? Thanks.

-Jon

  • 0
    TI__Guru 66726 points

    Hello Jonathan,

    The BW and slew specs are stated with a particular resistive and capacitive load on the output, RL and CL, as specified in the test conditions column above (1Meg and 20/30pF).

    RL and CL are not needed for stability. These are the loads applied to the device for this particular test.

    You are probably thinking of a "snubber" network, which is common on audio and power amplifiers, where there is a low value R in series with a fairly large C to prevent high frequency oscillations under some loading conditions...this is not normally required for the 324. (Never say never...)

    Regards,

  • 0 in reply to Paul Grohe
    TI__Guru**** 171046 points

    Paul,

    To answer the questions not asked, I have the this information (room temperature).

    The output follows the input until the input common mode is exceeded, about VCC-1.5V. Above this voltage the out may hold constant or jump to "the positive rail" which is also about VCC -1.5V.  The output will follow the input to ground unless the output needs to sink over 12uA. If its over 12uA the output may not go below 0.6V.

  • 0 in reply to Ron Michallick
    Prodigy 160 points

    OK. Thank you both for your help!

    So my plan of action is to include the snubber network as a part of my design that I can jumper in if I see ringing in the output, but otherwise I can ignore. 

    I'm operating on a 0 to 12 volt supply, and the follower is listening to 0 to 5 volts, so the supply should not be a problem unless I encounter this current sinking issue. Thanks for highlighting the word sink, Paul, because I would have missed it otherwise.  Can you please explain  a situation in which the device would need to sink more than 12 uA? I understand that if I am driving a low impedance than it might be required to source more than 12 uA, but I cannot think of why the output would ever need to sink 12 uA. Thanks.

  • 0 in reply to Jonathan Newman
    Prodigy 160 points

    Oops, that last question was actually direct at Ronald.

  • 0 in reply to Jonathan Newman
    TI__Guru**** 171046 points

    Jonathan,

    Any resistor that is connected to the output with the other side of the resistor being connected to a more positive voltage (than the output) can cause the output sink current.