Path: ...!feeds.phibee-telecom.net!2.eu.feeder.erje.net!feeder.erje.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Phil Hobbs Newsgroups: sci.electronics.design Subject: Re: Interesting inductor Date: Wed, 13 Mar 2024 12:49:04 -0000 (UTC) Organization: A noiseless patient Spider Lines: 86 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Wed, 13 Mar 2024 12:49:04 -0000 (UTC) Injection-Info: dont-email.me; posting-host="57581a49e97a1082fe4776357f5c28f6"; logging-data="1019208"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1/3xQYE89pB4I+IQnee+V1k" User-Agent: NewsTap/5.5 (iPhone/iPod Touch) Cancel-Lock: sha1:CJRvuubdG1gupmvy834l54MGnB8= sha1:3js9kC6Hx68jQjb6k5NUkTRxrHE= Bytes: 4647 John Larkin wrote: > On Tue, 12 Mar 2024 23:17:57 -0400, 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: >> . >> >> 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