Hi ,
I am trying to design OPA657 transimpedance amplifier for input signal ranging from 5-40MHz.
In my earlier post mentioned in the below link, we were able to design a stable circuit for 17MHz signal using OPA 657.For 17Mhz, Feedback resistor Rf is 9.36k and feedback capacitor Cf is 1pF.
1) Is it possible to design OPA657 to work stable for a frequency ranging from 5MHz to 40MHz ?
2)How to check the stability of the op-amp at different frequency ?
Thanks in advance .
Livingstone.
Hello Livingstone,
To answer the questions you had raised:
Question:
1) Is it possible to design OPA657 to work stable for a frequency ranging from 5MHz to 40MHz ?
Answer:
I'm not 100% sure what you are asking.
If your input diode capacitance is still around 70pF and you like to increase the I-V bandwidth to around 40MHz (from previously 17MHz), you could:
a) Use a device with higher GBW product. e.g. The LMH6629 has 4GHz GBW and is a candidate.
b) You could reduce your gain (RF).
You can use LMH6629 Equation 8 and 9 to compute the compensation capacitor (CF) and to estimate the resulting -3dB bandwidth for any CIN (input capacitance) and RF (transimpedance gain):
Done this way, your circuit will be stable.
Question:
2)How to check the stability of the op-amp at different frequency?
Answer:
The expressions above were derived with the premise of "stability" built-in (minus the op amp higher frequency pole / shift near 0dB gain crossing and its minimum stable gain considerations). So, if you use these, you will normally be stable. The TINA-TI stability technique presented in the E2E link provided, can be utilized to ensure there is enough phase margin for the other op amp details I've mentioned.
Regards,
Hooman
