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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Interesting inductor
Date: Wed, 13 Mar 2024 12:49:04 -0000 (UTC)
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John Larkin <jl@997PotHill.com> wrote:
> On Tue, 12 Mar 2024 23:17:57 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> 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.)
>> 
>> Cheers
>> 
>> Phil Hobbs
> 
> Couldn't you have a high tail voltage and a big resistor, or maybe a
> string of smaller inductors? Or something. We've made super wideband
> inductors from a string of various values.

The first stage (paralleled pHEMTs with a BFU520A cascode and BFU520A
follower) has a gain of about 40 and flatband 1-Hz noise of 0.2 nV. That
means that the noise of the follower and the second stage is not
insignificant. 

The second stage is a VCVS active lowpass using an OPA818 at a gain of 10,
and the output stage is an OPA695 CFA inverter, to make the overall circuit
noninverting and provide a gain adjustment. (TE now makes a low-inductance
pot that’s nearly as good as the old Murata PVA2 ones that you use. )

Keeping the supplies simple is important, and so is avoiding ground loops.
The box actually makes +7 and -5 by railsplitting a 24V wall wart, and then
using regulating cap multipliers. (The second and third stages’ supplies
are followers running off the quiet ones, to prevent unwanted feedback.)

Sooo, I want to run the follower on +7/0 if possible, which is where the
inductor comes in.  It doesn’t save any power, on account of the
railsplitter, so I can probably use the -5 rail instead. 

There’s no overall feedback in this version, because it’s hard to do
without trashing the noise performance and/or stability. 

> 
> I'm hassling with inductors now too, but at the other end of the speed
> spectrum.
> 
> We want a programmable inductor, from maybe 1 mH to 500 mH or so,
> maybe 100 mA. Sounds like an inductive DAC, a series string of
> inductors with shorting relays. If the step inductance ratio were,
> say, 1.8:1 we could have some hidden bits, more than the customer
> sees, so we could get pretty close to his requested value.
> 
> We could test all 2^n steps, make a list, and select the closest to
> his request.

We did something similar for choosing resistor taps in a low noise PGA.
Works okay, but is a bit of a pain. 
> 
> We're simulating loads to an engine control computer, torque motors
> and solenoids and steppers.

Fun.  Analog computers forever!

Cheers 

Phil Hobbs 



-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics