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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Interesting inductor
Date: Wed, 13 Mar 2024 12:27:08 -0400
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On 2024-03-13 12:04, John Larkin wrote:
> On Wed, 13 Mar 2024 11:49:31 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
>> On 2024-03-13 10:59, John Larkin wrote:
>>> On Wed, 13 Mar 2024 12:49:04 -0000 (UTC), Phil Hobbs
>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>
>>>> 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!
>>>>

>>>
>>> We are about to publicly announce the P940, our modular power system.
>>> It would be tragic if I make my fortune selling power supplies and
>>> dummy loads that work in the single digits of KHz.
>>
>> If that happens, I'll commiserate appropriately. ;)
> 
> Buy me a beer that I can cry in.
> 
>>>
>>> Making DACs with relays is humiliating.
>>
>> Nah, relays are amazing.  There are low-power muxes that come close,
>> e.g. the TMUX1511 (5 ohms R_on, 2 pF C_off), but nothing that will take
>> any sort of power.
>>
>> Of course you can do similar things with tubes. ;)

> 
> If Ron * Coff is the figure of merit, in femtoseconds, no semi can
> come within miles of a relay. We use a $1 DPDT telecom relay that is a
> damned good 3 GHz 50 ohm switch.
> 
> Tubes don't score well by that standard, except krytrons maybe.
> 
Not identical things, just similar.  Dragging a grid up to +200V quickly 
and then leaving it there, with no turn-off charge injection and nearly 
no capacitive loading, is a job for a tube.  (I used an 811A for that 
BITD--it even had a B battery for the plate and a C battery for the grid 
bias.) :)

Cheers

Phil Hobbs



-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com