Some "Free" BW in a Cascode Amp - Page 1
Forum section: electronics / projects

author: mawyatt
author_rank: Super Contributor
timestamp: September 29, 2021, 05:32:04 pm
content: Long ago when we were designing chips with SiGe bipolars we utilized a simple technique to extend the Cascode BW without increasing the power consumption. The usual technique employed collector peaking on the top transistor with a series inductor to increase the impedance as "seen" from the collector thus increasing the higher frequency gain to partially compensate for the roll-off.Another technique we used was to split the effective shunt capacitance from the lower transistor collector and upper transistor emitter which are normally connected together in a conventional Cascode. By placing a small inductor to split the collector and emitter capacitances the BW can be extended without increasing the device currents. It works by means of creating a 3 lumped element "pi" equivalent to a transmission line, the 2 shunt capacitances and the series inductor.Here's a simple Cascode simulation, quickly done with 2N3904 transistors. The bias is set at 1ma by the emitter current sources, and the voltage gain is 20dBv which is set by the collector resistors of ~264 ohms since the emitter resistance is kt/(qIc) + re, or ~26 ohms. The emitter capacitance controls the lower frequency corner.The Green plot shows the standard Cascode AC gain, the Blue the Modified Cascode with the Capacitive Splitting inductor and the Red with both Collector Peaking and the Capacitive Splitting.As you can see the -3dB BW increases from ~68 to 121 and 177MHz utilizing these techniques.Anyway, this is just a very simple circuit to show the techniques mentioned and maybe helpful to someone interested in discrete Cascode amplifiers.Best,

author: David Hess
author_rank: Super Contributor
timestamp: September 29, 2021, 07:05:18 pm
content: T-coils are also used to increase bandwidth.https://www.electronicdesign.com/technologies/analog/article/21807815/whats-all-this-tcoil-stuff-anyhowhttps://www.eevblog.com/forum/testgear/tektronix-common-design-parts-volumes-1-to-4/

author: mawyatt
author_rank: Super Contributor
timestamp: September 29, 2021, 07:40:45 pm
content: Knew about the Ross inspired "T" coils, but hadn't seen the Electronics Design article with Dr. Tom Lee, thanks for the linkRumor has it that with the "T" coils and the Ft doubler circuit, Tek kept a hefty BW advantage over HP for well over a decade.Tom is a great electronics history buff, and really gets into obscure circuits. He referenced one of my circuits about 8 years ago at the IEEE ISSCC conference evening forum, it was from an EDN article and later published in the "Current Sources & Voltage References" text by Harrison. It's also sort of a cascode.Best

author: 2N3055
author_rank: Super Contributor
timestamp: September 29, 2021, 07:41:44 pm
content: Nice mini design note, Mike.David is also right, T-coil networks (made famous by guys in Tek) is interesting technique too..Good book on topic from my neighbours in Slovenia:"Wideband amplifiers" Starič, Marganhttps://www.springer.com/gp/book/9780387283401Best

author: mawyatt
author_rank: Super Contributor
timestamp: September 29, 2021, 08:02:55 pm
content: I had that book but someone borrowed it and conveniently forgot to return itWhat a brilliant collection of highly developed networks and the math behind them.It's the best collection of techniques and such for squeezing out the most BW from an amp. Used it to help develop the high linearity ultra wide-band amps and mixers in InP and SiGe utilizing a modified Cherry-Hooper and Caprio Cell Differential In/Out structure. We were pushing towards 100GHzDidn't know anyone knew about this treasure of a book, so thanks for the reminderNow I need to try and get it returnedBest,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 29, 2021, 09:19:32 pm
content: Is that actually adding three poles, or just two? Seems to me it would be two, as the emitter isn't exactly a free node (you've also not indicated anything else on the schematic, it would be pretty lucky for both capacitances to work out just right). Also has to be done carefully, in case the emitter impedance goes squirrelly up there.Looks like even just the one (or two) does quite a lot though, good to know!Question, do you know of any peaking networks better than the, uh what is it 20 or 30%, you get with plain old series peaking,for a resistive load?The classic networks are all about capacitive loads.Tim

author: RoGeorge
author_rank: Super Contributor
timestamp: September 29, 2021, 11:45:11 pm
content: Quote from: mawyatt on September 29, 2021, 08:02:55 pmFirst, thanks for posting about the BW trick.This rare magic spell I didn't know before!Just in case someone lost your book, it can be downloaded from one of the author's webpages, using the link posted byT3sl4co1lhere:https://www.eevblog.com/forum/testgear/tektronix-common-design-parts-volumes-1-to-4/msg1913948/#msg1913948Out of curiosity parsed the first chapter (Laplace) and it's one of the most clear approach I've read so far.Didn't know the book before, thank you all for mentioning it.There are some supplemental materials (e.g. helping animations, programs), not essential but linked by the text in the pdf book. Searched online and couldn't find the supplemental materials, anybody knows where to find those supplemental files?

author: mawyatt
author_rank: Super Contributor
timestamp: September 30, 2021, 01:12:37 am
content: Quote from: T3sl4co1l on September 29, 2021, 09:19:32 pmWe had worked this out over 20 years ago in a highly simplified manner, but can't remember the exact details. It wasn't by accident or luck tho. The approach we took was to consider the lumped element transmission line interface created by splitting the capacitances with the series inductance. The full up theoretical analysis would make a nice PhD dissertation, or maybe it's already been done.There's nothing else to indicate on the schematic, it's just that simple. Of course for a practical implementation, a different biasing scheme and decoupling might be used. Also works with Fets. Honestly don't know if we were the first to utilize this cascode structure, it's so simple I'd be surprised if someone else wasn't also doing this back then.Think this might have been shown to the grad students when I was teaching grad school as an adjunct back in 2000, although very little of our work "leaked" out since it was all "proprietary".Anyway, glad to find some folks find this interesting.Edit, forgot to answer your question about peaking networks. We did have a series peaking network that was differential (almost everything we did was differential), but can't remember the exact details but recall it employed some cross coupling between the integrated inductors which were intertwined and used shunt resistors. Probably something along the "T" network equivalent.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: September 30, 2021, 01:31:30 am
content: Quote from: RoGeorge on September 29, 2021, 11:45:11 pmThis is indeed quite a surprising result from such a simple change to the classic cascode, glad you like itThanks for the point to the online book, this is definitely a jewel for those interestingHopefully I can remember some other old ideas/circuits that folks will find interesting today.Best,

author: 2N3055
author_rank: Super Contributor
timestamp: September 30, 2021, 06:07:23 am
content: Quote from: RoGeorge on September 29, 2021, 11:45:11 pmOnly thing I have is latest errata for PDF version..http://www-f9.ijs.si/~margan/WBAmps/Corrections4.pdfPeter Starič died last year. He actually worked for Tek from 1967-70. Brilliant fellow.Margan too. He is originally from my home town Rijeka..His very funny CV:http://www-f9.ijs.si/~margan/WBAmps/RXon_cv.pdfAnd article..http://www.croatia.org/crown/articles/9605/1/Erik-Margan-an-artistic-scientist-in-search-for-invisible-particles.htmlTwo very renaissance guys... No wonder book is brilliant..

author: mawyatt
author_rank: Super Contributor
timestamp: September 30, 2021, 01:30:42 pm
content: An interesting extension of this Cascode device is an implementation where the top device is replaced with a depletion mode GaN FET (or JFET). The GaN gate is tied to the bipolar emitter, and the GaN source to the bipolar collector, yielding a 3 terminal device with emitter/gate, base, and drain leads. We coined this as a GaNsistor (see patent 7939857). The device is self biasing in that the depletion mode top device provides the collector emitter bias voltage for the lower bipolar device. Of course you can replace the bipolar with a NMOS device for CMOS compatibility.This led to the creation of a high power, frequency and voltage DAC (see patent 7903016) which was coined as Direct Digital to Antenna, or DD2A. It was used to create high power RF/MW signals where the final RF analog signal was only created at the antenna, and also the basis of a DARPA program called Power DAC. A very clever augmentation of this concept by a former colleague used the DD2A to replace the amplifiers in a Traveling Wave Amplifier to achieve higher power MW/MMW levels directly under digital control.Anyway, seems Cascode amplifiers can be very useful in many diverse applications and a great resource for the designerBest,

author: magic
author_rank: Super Contributor
timestamp: September 30, 2021, 05:36:41 pm
content: JFET cascode tied to the source/emitter is an old and effective trick, albeit one which requires a somewhat oversized JFET (not running it at close to 100% IDSS).

author: RoGeorge
author_rank: Super Contributor
timestamp: September 30, 2021, 07:39:09 pm
content: Quote from: mawyatt on September 30, 2021, 01:30:42 pmPDF links for the 'Fast & Curious' like me to click:https://patentimages.storage.googleapis.com/3a/85/80/9fd69da1303572/US7939857.pdfhttps://patentimages.storage.googleapis.com/c3/fa/32/f59bba16a4c921/US7903016.pdfMeanwhile found the supplemental materials (companion CD) for the "Wideband Amplifiers" book, too:https://extras.springer.com/2006/978-0-387-28340-1.zip

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 01, 2021, 12:30:16 am
content: Such devices are currently available in power GaN -- do you know if they license the same patent? Seems like they ought, but I didn't read in detail. Or maybe they even say it on the tin, I haven't read their datasheets in that much detail either...Tim

author: David Hess
author_rank: Super Contributor
timestamp: October 01, 2021, 04:29:24 am
content: Quote from: magic on September 30, 2021, 05:36:41 pmThe various manufacturers of depletion mode MOSFETs also show that cascode configuration in their application notes.I do not think I have ever seen it used in test instrumentation.

author: magic
author_rank: Super Contributor
timestamp: October 01, 2021, 06:16:31 am
content: There are examples in small signal, small frequency amplifiers.Alex Nikitin used a JFET/JFET configuration in audio applications to counteract gm variation with Id in a common source stage by modulating its Vds with the Vgs of the cascode device.http://www.ant-audio.co.uk/Theory/Parametric_Linearisation.pdfBurr-Brown may or may not have used it for bootstrapped cascode over the input stage in OPA111, OPA2107 and similar. They did mention the possibility in their patent, 1985.QuoteWouldn't be surprised if there is an audio amp somewhere using such input stage arrangement, but with high IDSS devices instead of the "means for reducing the current level in the cascode FET".The problem with this "lazy" approach is that, all else being equal, the JFET/DMOSFET could be made smaller (cheaper, less capacitance) if you can afford to reduce Vgs and offset the gate potential as needed. For example, OPA445 uses several JFET cascodes and it always level-shifts the gate.

author: mawyatt
author_rank: Super Contributor
timestamp: October 01, 2021, 02:24:53 pm
content: The GaNsistor and DD2A mentioned in patents 7939857 & 7903016 were intend to cover RF/MW/MMW frequency ranges at high power levels. They were and are employed in Advanced Electronic Warfare (Jammer) applications where complex waveforms are created very quickly "on the fly" and at very high power levels. The mentioned augmentation of the DD2A was an application where the individual power amplifiers in a Traveling Wave Amplifier were replaced with individual DD2As. There was also some work done with a Silicon MESFET version, although GaN provided better performance.Here's a couple low res images of early test chips in IBM 9HP SiGe BiCMOS process.@T3sl4co1lWe were aware of potential use by others and alerted the patent legal office of my former employer (now Harris/L3), the patents were assigned to ITT Defense which became Excelis, which was acquired by Harris, and then merged with L3. We haven't been contacted regarding this, so assume they are not moving forward.Best,