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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Interesting inductor
Date: Wed, 13 Mar 2024 23:55:24 +1100
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On 13/03/2024 2:17 pm, Phil Hobbs wrote:
> So I'm doing a new lab amp product.
> Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).
> 
> The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz).  The 
> spherical cows love it, so we'll see when the test boards arrive later 
> this week.
> 
> As part of the design, I wanted to make an emitter follower with a 
> decent amount of inductance in series with its tail resistor, to avoid 
> the transistor turning off on fast negative edges and causing linearity 
> problems.
> 
> Searching on Digikey, I found this very interesting part:
> <https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.
> 
> 4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_, 
> which is several times higher than many other parts of that description. 
>    That corresponds to an effective parallel capacitance of 0.12 pF, 
> about that of a resistor of the same size, despite all the copper windings.
> 
> Pretty nifty, if true.  (Parts on order.)

Anything over 1uH has a ferrite core - probably a nickel-zinc ferrite at 
those sorts of frequencies.

Minimising parallel capacitance is supposed to demand spacing the 
winding wires by their own diameter, but that doesn't show up on the 
drawing (and probably wouldn't even if they were doing it).

Definitely interesting.

-- 
Bill Sloman, Sydney