Here's a piece of straight wire through a ferrite toroid.

Toroid ------ | | wire======================== | | ------

I recall seeing an experiment where a wire is poked through a sheet of paper, some current applied and iron fillings revealing a concentric ring pattern. That's in air..

Replacing air with ferrite, the flux density at a radius from the wire is:

B = (uo***ur***I)/2***pi***r

say Idc = 1A say ferrite ur = 4000 B is in tesla r in meters uo= 4***pi***10E-7

Therefore the flux density at 1mm away from the wire is ...Yikes!...

8000 gauss. Well... that'll saturate a typical ferrite material.At that radius, that part of the core is saturated and acts like air. The magnetic field continues to spread outward.

At r=2mm ..Still kinda saturated..Core material acts like air.

At r=3mm...Not saturated I supposed the field gets more contained here as opposed to fanning out beyond the core.

So..if I haven't botched the math and theory:

Can I conclude I need approximately >3mm toroid radius to get the most small signal inductance when there's a 1Adc bias point?

D from BC myrealaddress(at)comic(dot)com British Columbia Canada