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