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LM324： LM324 output current

jacky wang1
Intellectual 340 points
Part Number: LM324

Hi，

I see 6.5 Electrical Characteristics for LMx24 and LM324K  in datasheet，i do not understand output current，As shown above

for example ：1.when VCC = 15V,VO=0V,i think that  if VO>0, output current is sink and is not source，why ?

                2. ,I do not understand VID=1V,because V+ = V-, vid = 0V,why?

                3. when VCC = 15V,VO=15V,i think that  output current  is source ,is not sink. why?

                4. when VCC = 15V,VO=15V, I think vo range(VICR) :0---VDD-1.5V,and do not up to VDD,why?

Thanks a lot!

  • 0
    TI__Intellectual 775 points

    Jacky,

    In 1, the supply of an opamp (VCC) is connected to the 15V supply, the output VO is forced to 0V (with external supply), in this condition the op-amp has to source the current.  The condition VID = 1V is to provide the maximum sourcing drive to the output stage of an opamp. This is the condition forced at the input of the opamp. The condition of V+=V- is true when the amplifier is in closed-loop and in a linear mode of operation.  In 3, it is opposite, the output is forced at 15V and the opamp has the sink the current because VID = -1. 

    In 4, the input common-mode voltage range is different than the output swing range. The condition you are saying is true only if you are doing non-inverting unity gain configuration. Here is the link to TI Precision Lab Video Lecture, https://training.ti.com/ti-precision-labs-op-amps  lecture 3 (Input and output limitation) will explain this concept clearly.

    Regards,

    Sanjeev

  • 0 in reply to Sanjeev Manandhar
    Intellectual 340 points

    Hi

    1.when LM324 work in linear amplification area ，it output voltage range is 0--13V(vdd=15V),if output volatge is 0.2V,very low，I think that it do have sink source，only source current ，because it be equal to Power Supply，Power Supply only output current to the external load.I do not understand ，why lm324 have sink current?

    2.VID= 1,VID =-1,I do not understand，do you provide schematic ？

    Thanks a lot!

  • 0 in reply to jacky wang1
    TI__Guru**** 151256 points

    Jacky,

    Does this schematic help.

    Also try reading new LM324 design app note, http://www.ti.com/lit/pdf/sloa277

  • 0 in reply to jacky wang1
    TI__Intellectual 775 points

    Jacky,

    An Op-amp can both sink or source current. I do not understand what is the confusion here. The specification table is to show the maximum current it can sink and source. This is how these currents are specified.

    When VID =1, the opamp will try to swing Output close to the positive rail. If Output of the opamp is forced (externally) to 0V the opamp has to source maximum current it can to pull up the output. At 25C the typical value of this sourcing current is 30mA for LM324.

    Similarly, when VID = -1, the opamp will try to swing output close to the negative rail (which is GND in this case). If Output of the opamp is forced (externally) to 15V, the opamp has to sink maximum current it can to pull down the Output.  At 25C the typical value of this sinking current is 20mA.

    Ron's message has the schematic. 

    Sanjeev

  • 0 in reply to Sanjeev Manandhar
    Intellectual 340 points

    Hi，

    Thank you for the reply from the above engineers.

    If LM324 work in the enlarge area，in other words，it operate non-inverting unity gain configuration  and analog voltage output 

    ,for example 0.2V，however，low voltage is very low，LM324 still supply source current to the external load.

    If external load just is pull-up resistance 5Kohm to 5V.external current can flow into LM324, if allow LM324 happen sink current or not？What effect and damage to the opamp？

    Thanks a lot！

  • 0 in reply to jacky wang1
    TI__Intellectual 775 points

    Hi Jacky,

    I do not understand your questions that well but I will attempt to answer them.

    If you have 0.2 V in a non-inverting unity gain configuration then you would expect to see 0.2 V at the output. If the load (external) is tied to GND, then LM324 will be able to source the current to maintain 0.2V.

    In this configuration, if you tie 5Kohm resistor to 5V to out pin you would expect almost 1mA of current to be sunk (current sink) by LM324. At 0.2V output voltage, LM324 can sink 30uA of current, so your output voltage will rise up to 0.7V or so.  This will not damage the part.

    On the side, I would suggest you go through TIPL video lectures. This will help you understand the spec and applications of an op-amp. Both experts and beginners have found these video lectures very helpful.

    https://training.ti.com/ti-precision-labs-op-amps

    Sanjeev