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.

  • TI Thinks Resolved

LM7321: Output oscillation with capacitive load

Expert 8830 points

Replies: 3

Views: 43

Part Number: LM7321

Customer is using LM7321 to bias some RF amplifiers and is observing large output oscillations.  They selected the LM7321 because of its ability to handle capacitive loads without a RISO resistor. Adding a resistor is not desired in this application.  The bias circuit consists of two LM7321, with the output of the first feeding into the second with a resistor divider.  See attached for basic circuit and captured oscillations.

Amplifiers are powered from 0V and -7.9V. Spice simulation does not show this response, but maybe the model is not accurate for this type of load.  

I have more data that I can provide if you contact me offline. 

LM7321 oscillation.pptx

  • Mark,

    LM7321 is good at output capacitance. However, like all op amps, it needs some feedback capacitance to prevent the feedback network from creating any lag.

    I will take a closer look at the data tomorrow and make a recommendation.

    Regards,
    Ronald Michallick
    Linear Applications

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

     

  • In reply to Ron Michallick:

    Hi Ron,

    Thanks for looking at this.  I will send you some other information via email.

  • In reply to Mark Guastaferro:

    It is always important to understand the phase lost in the feedback path. Assuming IN- node is 2pF for op amp and 2pF for board traces. 

    With moderate (330pF) to heavy capacitance (1uF) output load, the expected bandwidth of LM7321 is between 100k and 2.5MHz

    Here is the feedback phase lost for this frequency band.

    Adding a 4.7pF feedback capacitor changes the strong phase loss into a small phase gain.

    Regards,
    Ronald Michallick
    Linear Applications

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

     

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.