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LM6152: feedback resistor optimization

RAMAKRISHNA NAGARAJU GUDLA
Prodigy 120 points
Part Number: LM6152
Other Parts Discussed in Thread: TINA-TI

Hello Team,

With respect to attached differential amplifier circuit

1) what is the minimum differential voltage that can be applied at the input so that the signal gets amplified by the gain (100/10 = 10) of the circuit.  

2) looks like datasheet recommends max RF is 10Kohm while the circuit is implemented with RF 100Kohm. Is there any reason for not to have 100KOHM for RF ?

Thanks and Regards

G R K Nagaraju

  • 0
    Guru 140200 points

    Hi Ramakrishna,

    that is not a true differential amplifier. A 100k resistor is missing from the +input of OPAmp to 0V. And, eventually, a 10nF cap in parallel to it is also missing.

    Kai

  • 0
    Guru 48395 points

    Morning, Kai is correct, you are missing some elements - however

    1. Your feedback pole is at 159Hz, not sure why you using a 75MHz device for that

    2.higher R values add to the total output noise

    3. They also can impair phase margin - so whatever device or RC you end up with check that (add the load for that as well)

  • 0
    Prodigy 120 points

    Thanks for response. Given that the elements are not missing,  what is the minimum differential voltage that can be applied at the input so that the signal gets amplified by the gain (100/10 = 10) of the circuit ?

    Best Regards

    G R K Nagaraju

  • 0 in reply to RAMAKRISHNA NAGARAJU GUDLA
    Guru 140200 points

    Hi Ramakrishnan,

    why not simulating the circuit with TINA-TI? It's free and can be downloaded here:

    Kai

  • 0 in reply to kai klaas69
    Prodigy 120 points

    Hi Kai,

    The intent is to amplify the error (difference in the inputs). so is it still recommended to have the 100k resistor from the +input of OPAmp to 0V & a 10nF cap in parallel to it? Also how it will be different if 100K resistor from the +input of OPAmp is connected to reference voltage instead of  0V? 

    Hope my question make sense

    Best Regards

    G R  K Nagaraju

  • 0 in reply to RAMAKRISHNA NAGARAJU GUDLA
    Guru 140200 points

    Hi Ramakrishnan,

    yes, for working as a differential amplifier the additional 100k is needed.

    Connecting the additional 100k to a reference voltage instead of 0V will shift the output signal by the value of this reference voltage. But assure that the reference voltage is driven by a low impedant source. Otherwise due to resistance missmatch the balance of differential amplifier will suffer and the differential amplifier is no longer working properly. (Wrong gain of differential input signal and improper suppressing of common mode input voltage.)

    But all these questions can be much better answered by running a TINA-TI simulation. I again encourage you to become familiar with TINA-TI...

    Kai