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It's sure a lot easier if you can flip the circuit into a sink. Then you've got more choices than just a BFT92.
James
follower,
I only eyeballed the beta cancellation. It may well be that VAF drops out as well... interesting! ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| 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.
One way of looking at the Early effect is a parasitic C-B conductance, so I think Fred's circuit gets rid of that too, but only at low frequency, since it requires the op amp and FET to do their thing. Using a BFP650 ideally makes the output stiff at all frequencies from DC to several gigahertz. I have _got_ to try that in a diode laser driver.
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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Wonder if there's duality? I could certainly do with stiffer CMOS mirrors. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| 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.
follower,
That is a very weird transistor.
But if I wanted a negative current source, I'd just cascode into a PHEMT. I need positive current, and the people who make gaasfets stubbornly refuse to make p-channel ones.
I could put a compound inductor directly in series with my mosfet current source, I suppose.
-- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Sadly, I need a positive current source. My customer wants really precise laser bias current, which I frankly think is unjustified, but unfortunately he's the one with the money.
-- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
follower,
BFP640.
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It's hotter than a two-dollar pistol, too. Pity there aren't any PNPs.
With the beta cancellation trick, it might be interesting to try using a BFT92 in inverted mode as the cascode device. Inverted RF transistors have lower capacitance, and their f_T can even be higher than the same device right-way-up.
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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
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Oh, this is it here:
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It says 2006, but it sure seems longer.
-------------- keywords (for future searches): EDN current source current sink base current compensation error cancellation
"Error compensation improves bipolar-current sinks.(design ideas) October 1, 2006 | Thompson, Brad; Granville, Fran
You can improve a current sink's accuracy by at least two orders of magnitude by adding two standard 1%-tolerance resistors. As a bonus, you also compensate for errors that a low-current-gain pass transistor's base current introduces. To do so, you measure the transistor's base current and add a proportionally scaled error term to the source's reference voltage."
--------------
-- Cheers, James Arthur
But for ordinary rail voltages it'll break down base-to-emitter, that's the rub.
I was going to suggest John could just use a high rail voltage and a bootstrapped resistor--then lots of current-source problems go away.
E.g., to be absurd, 1kV and a resistor gives 10ppm stability into a
10mV compliance range. Resistors are excellent, r.f.-wise. To be less absurd, bootstrap the resistor.-- Cheers, James Arthur
Phil Hobbs wrote: : I started by taking your advice, which was pretty valuable--thanks
Don't mention it. I have already gained more from your, Larkin's, Win Hill's and many other's posts here in SED.
: again. I noticed the change in specs as well, when I ordered another : batch a couple of months ago. They changed the type number to BFP650H : instead of BFP650E when they changed the process. (The dogs.)
They did? The datasheet still list the part as BFP650 without suffixes even though it is the SiGe:C version.
: I'm using it as a cascode stage for a SKY65050 pHEMT, and it works
Interesting hint, I should give it a try.
: great. One of the best things about it is that it has effectively : infinite Early voltage, so you can get a lot of voltage gain out of a : single stage. Even for situations where you don't care so much about : ultralow noise, the combination of a gigantic f_T with a very high V_A : is unique in my collection. It does want to oscillate at 14 GHz if you : look at it crosswise, but a nice 5-ohm bead in series with the base : cleans that right up.
The high Early seems to be characteristic to the SiGe's, it puzzled me already in the thread
Regards, Mikko
John Larkin wrote: : But if I wanted a negative current source, I'd just cascode into a : PHEMT. I need positive current, and the people who make gaasfets : stubbornly refuse to make p-channel ones.
The same with SiGe PNP's - I would have wanted to make a push-pull for LHe but there are no discretes available. An NPN-NPN push-pull is a possibility, but awkward. Maybe the discretes will become available soon, at least there are complementary IC processes producing such devices as the OPA835/6 (which actually work in LHe but only marginally).
Regards, Mikko
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If you sense and compensate base current perfectly, the current source will be perfect. Maybe a flying cap sampling the base resistor would do better than Fred's and the EDN ckts?
For speed beyond the BFT92, easiest are the hybrids you've already done--outboard additions (L and R) that improve GHz response, the bjt handles mid-range, and the op-amp handles d.c. accuracy.
Cascoding two BFT92s helps, eliminating Cfb feedback of the exposed device.
To do better than that, bootstrap the exposed BFT92. I can think of a few ways--pretty hairy.
No, it's actually good he's the one with the money. If you had it, you wouldn't need him!
-- Cheers, James Arthur
Unfortunately, most of the good fast parts are designed for RF use, which means we only get n-types, and very badly characterized ones at that. S-params and load pulls don't tell you much about DC or switching behavior. You're lucky if they tell you Idss, very lucky if they give you DC curves, and heaven is getting a Spice model.
I asked Mini-Circuits if one of their MMICs inverts the signal or not. They didn't know.
Grrrrr.
-- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
he
nWhat if you separate the a.c. and d.c. paths at the pass element too? (Maybe this is what you meant by "lowpass filter at the base"?)
| |-
One could just use a p-channel mosfet as the cascode. That could get the source capacitance down to 20 pF, maybe better if one found the right fet (I'm talking 50-100 mA maybe.) Then put a good wideband inductor in its drain. "Good wideband inductor" is non-trivial. The Piconics conical things are good, but expensive and hard to handle. Making your own takes a bunch of parts... several different FBs and Ls in series, with damping resistors.
-- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
The distributors' stock of the E version all ran out a few months ago, and all they have now is the H version. I haven't managed to get the straight story as to just what they did.
I've ordered some of those too, thanks. It's a bit worrying that there are seem to be only about 100 of them in the world, according to FindChips.
Cheers
Phil
-- 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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
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Would someone be kind enough to help me see how this compensates for the base current? I've done the Vref-opamp-fet-resistor current source.
Color me slightly confused. (I'd be happy to pay the, one beer licensing fee,... to understand it.)
George H.
The summing junction is held still by feedback, and we assume the 5V reference is also stable. Any base current thus divides 50:50 between the two 1k resistors. Thus I_B /2 goes to the SJ, pulling it down by (2k)(I_B/2) = 1000 ohms * I_B. To restore balance, the source of the FET has to go up by the same amount, which requires an increase in drain current of 1000*I_B/1000, i.e. exactly I_B.
That'll work great at DC, but any fast voltage swing at the collector will make a mess, due to the slow loop dynamics and high impedance at the base. The simple way to fix it is to use a transistor with infinite Early voltage and bypass the daylights out of the base, which is how the BFP650 came up.
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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Just run the math. Assume Ic is output, base current is Ic/beta, emitter current is Ic*(beta+1)/beta
You'll find a need for 2*VR-VH > 0 (VR=3, VH=5 in this example)
And Ic can't be 3mA as shown in the example ;-) ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| 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.
But that part has negative (positive?) Early voltage, namely negative output impedance. I wonder if that's thermal or wideband.
So, it would oscillate with an LC in its collector and no base drive!
-- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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