[SOLVED] High noise levels at frequencies above unity-gain

Hello

I have been getting some strange results when trying to do noise analysis on my opamp design.

The frequency characteristics looks ok when doing stb analysis with unity gain at 1 MHz with 65 degree phase-margin, and behaves nicely in transient.

When doing the noise analysis i see a strong increase of noise from around 200 KHz and all the way to 1 GHz. Anybody have any ideas what can cause this when the stb analysis shows a sharp rollof of gain at frequencies above 1 MHz?



If i turn off the flicker noise in the noise contributions menu i get the following.



The design uses Multi-Path hybrid nested miller compensation and I suspect this can have something to do with it, but I dont understand how this can happen when the stb shows very little gain for frequencies beyond 1 MHz.

That's easy to achieve if you managed to put a strong noise source in your amplifier. It's suppressed by feedback, but suppression is fading toward unity gain.

You want to check the contribution of individual transistors to see what the noise source is. You can get listing for specific frequencies.

Ahh I see, thanks man.

By looking trough the noise summary I found that the strong noise sources were caused by the big resistors used as part of the ideal common-mode feedback.