OPA547: How to know its Ro?

Xueliang Wang

Part Number: OPA547

Hello everyone,

I was trying to find out the open loop output resistance Ro of opa547 with the Ro-drive method proposed by Tim Green in "Operational Amplifier Stability Part 3: Ro and Rout".

1. When doing the simulation I found there is always DC offset dominating the rms value, so the Ro-drive method did not apply. What made this happen?

2. When changing the feedback network from 1k&100k to 100k&10MEG, the AC nodal valtage change greatly. How to understand this?

3. Any suggestion to improve the simulation and help to clarify the Ro of opa547?

Regards,

Xueliang

over 4 years ago

0 Michael Steffes over 4 years ago

Guru 47485 points

Morning Xueliang

Here is an AC sim of the open loop Zol in the model - this is dBohms, and the file below

OPA547 Zol.TSC

0 Xueliang Wang over 4 years ago in reply to Michael Steffes

Prodigy 230 points

Hi Michael

Thanks for your reply and it really helps to clarify Zo. Do you know something about the reason why the Ro-drive method does not work as previously mentioned?

Regards,

Xueliang

0 Michael Steffes over 4 years ago in reply to Xueliang Wang

Guru 47485 points

Well, the model probably has a 0.1uA input bias current - with a 10Mohm feedback R that is in fact 1V output DC offset.

Also, with those very high Rf||Rg values, it is possible the simulation is oscillating due to input capacitance - total input C is 10pF, for your 10Mohm feedback R, that is another loop pole at 160kHz - very likely unstable.

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OPA547: How to know its Ro?