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emitter follower gain

I'm not really a solid state guy--I've learned everything I know about HBTs from my trusty tome by Pulfrey, "Understanding modern transistors and diodes". (Great book, btw.) It says that a lot of the unusual properties of SiGe HBTs come from two features: a heterojunction between base and emitter, and a Ge concentration gradient across the base. The heterojunction produces a much higher barrier for holes than electrons, and so allows much higher doping density in the base without the whole thing going metallic.

The very high doping density produces an impurity band below the conduction band, so that to some degree the majority carriers are delocalized even below the ionization temperature. That may be why the beta doesn't tank below 100K the way it usually does. On the other hand, I may be making all that up. ;)

The concentration gradient more or less eliminates the effect of base narrowing, which gives these devices their amazingly high Early voltage and low capacitance. The datasheet of my fave, the BFP640, has plots where the Early slope goes the other way, i.e. collector current _drops_ slightly towards higher V_CE, but the slopes are all pretty nearly zero. A 40-GHz transistor with super low capacitance, high beta, and a practically infinite V_A lets you do magical things, e.g. a cascoded pHEMT front end with 0.3 nV 1-Hz noise, < 1 pF of capacitance, and an RC-coupled voltage gain of 50 from DC to some ridiculous frequency. (The pHEMT has a 1/f corner of ~10 MHz, unfortunately.)

I posted a circuit like that a year or so back, in the "electrometer front end" thread.

Cheers

Phil Hobbs

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

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs
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Lots of mesfets and phemts do that. Would that make a source follower have gain?

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

It might. The ones I've tried (mostly the Avago ATF3x143 series and the Skyworks SKY65050) have pretty low drain impedances. The ATF38143's drain impedance is only about 160 ohms, which makes it useless as a follower, as I discovered to my cost. The Skyworks part is a bit better, enough to be usable.

You're a fan of the NEC parts, as I recall. Do they really have negative drain impedances? That would be pretty useful--I'm always looking for ways to get a decent amount of gain in the first stage, because it makes everything else easier.

Cheers

Phil Hobbs

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

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

John,John,

I did not see how you put the voltage bias the BJT base respect to the emitter. Generally we have to put statically the base at VDD/2 (5V in your suggestion). Please no offense John, this schematic is not realistic IMHO.

Habib.

Reply to
Habib Bouaziz-Viallet

But it works!

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin 

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Hmm, now I'm tempted to fetch a #24A and see what some consequences of the tetrode region are. (Namely: electrons at excess velocity strike the plate, producing secondary emission, which is picked up by the screen; the result is a slanted negative resistance band where plate current effectively diverts to the screen.

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Tim

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Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

No idea. Mikko is a smart guy, though, and a careful worker, so I wouldn't have any _a_priori_ reason to doubt it. It would be very interesting to see the data!

Cheers

Phil Hobbs

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

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Thanks, Phil.

The book is an interesting read, particularly when I never owned the 2nd ed. It was amusing to encounter the LT1028 'undocumented' noise peaks, which seem to be standard knowledge nowadays. I'm mildly curious whether the first hints might have originated from doi:10.1109/77.403007 , it would be great to have made an impact, but of course everybody and his brother owning a spectrum analyzer probably had observed those.

Anyway, I have learned a lot from what you and JL and many others have shared their wisdom here. Thanks.

Regards, Mikko

Reply to
mikkivir

Yes and no, How could it be possible that Emmiter voltage become negative with a current generator referenced to 0V ... Spice magic !

H.

Reply to
Habib Bouaziz-Viallet

It is real. We have a plenty of data from various transistors, but most of it is still in numerical form and I haven't had an opportunity to plot it into neat curves and maybe write a paper. We published one set of chars in doi:10.1063/1.4729665 .

The top hFE seems to be batch dependent, the batch giving

27000 was purchased from Mouser in 2013, we have some two dozen of them left. But the other batch we purchased for stockpiling the transistors only peaked at ~6000. We have the third batch, which we haven't measured yet. Still, the NESG3031 seems to be the most consistent hFE-performer, all the batches I have obtained since the -96 batch doi:10.1088/0953-2048/19/12/014 have had at least ~20 times the room-temperature hFE when cooled. The NESG4030 works well, too, but NESG7030 does not. And there are many SiGe's which do freeze out when cooled, I don't know whether we've had a bad batch or what. Weinreb et al reported hFE = 5000 for the BFU725A in doi: 10.1063/1.3103939 but we measured only the rather unimpressive hFE = 360 max from our samples. The attractive thing is that the super-high hFE kicks in already at LN2 temperatures, which implies much more applications than the LHe operation which is our main interest.

The reason why SiGe works at those temperatures is AFAIK exactly what Phil says, that you can dope the base region much more heavily before the junctions start to leak. That was btw a good reason to expect a low Rbb', too, at room temperature. It's amusing to see than my very first post (Why extreme S-parameter values) in this forum was due to being puzzled about their high Early voltage, and their consquently huge voltage gain. SiGe's would have been nice gadgets to have included in the AoE.

We utilized the high hFE in making a cryogenic amplifier with i_N in the ballpark of 50 fA/rtHz and e_N roughly

0.3 nV/rtHz, doi: doi:10.1016/j.cryogenics.2013.06.004 . This would not be possible if the hFE was not there. We also made a couple years back a 0.1 - 1.5 GHz 50-ohm matched LNA which we haven't published however.

Regards, Mikko

Reply to
mikkivir

Does that R1 milli-ohm resistor do anything, is it a hang-over from an earlier circuit or needed as a Spice-ism to tame simulations?

piglet

Reply to
piglet

I was afraid they would never manage it. There must be a point at which, if you try to cover too much of the field in too much detail, it changes faster than you can write!

--

John Devereux
Reply to
John Devereux

A Spice current source will pull to infinity unless limited by other circuit constraints.

That's why I developed current sources that behave like the real deal... can't pass thru zero.

See CompliantParts.zip on the Device Models & Subcircuits Page of my website.

Limiting is by TANH, so these devices have no convergence problems. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

It's a Spice source, not a real-life current sink. It will output -5 gigavolts if you let it.

You could return the low end of the current source to -10 volts if you prefer; it doesn't matter.

In my gadget, I do want to run an emitter follower with a constant-current load, namely the drain of a jfet that's itself running constant-current. The NPN bootstraps the drain slope and capacitance. It's just something I'm playing with, with no serious intent at present.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

Don Knuth has had that problem for most of his adult life, it seems like. He still managed to invent all the algorithms everybody else uses. ;)

Cheers

Phil Hobbs

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

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

It's just an easy place to probe the base current.

In real life, but not in Spice, emitter followers tend to oscillate at mucho-MHz, so a resistor or a ferrite bead in the base is prudent. 33 to 100 ohms usually works, if you don't mind the Johnson noise.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

Yes i know John,

Please see what i meant in my previous post : Considering the E node, the current direction with respect to the E voltage, the I generator should not be considered as a generator when E voltage becomes negative. May be I missed something but It's impossible to implement that schematic in the real life.

Oh i see (i think) why not publish the idea ?

Habib.

Reply to
Habib Bouaziz-Viallet

Ok thanks, certainly i will see.

Habib.

Reply to
Habib Bouaziz-Viallet

Lots of Spice things can't be implemented in real life. But the Spice models can still be educational. I can build a simple current sink that's so close to the Spice "I" component, that it doesn't matter.

Well, for one reason, it's not my idea.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

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