On Feb 12, 2:15=A0pm, John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:> On Sun, 12 Feb 2012 13:39:37 -0500, Phil Hobbs > > > > > > > > > > <pcdhSpamMeSensel...@electrooptical.net> wrote: > >Okkim Atnarivik wrote: > > >> Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote: > >> : Looks good. =A0Another approach would be something like a BFP650 fol=lower,> > >> =A0 Phil, I was about to suggest that if you've ended up using the > >> BFP650 by following my example, you might be better off with the BFP64=0.> >> Or more modern ones with a higher hFE (I'm fond of the NESG3031 and > >> NESG4030). > > >> =A0 Namely, my choice of BFP650 (and not the then more widely availabl=e> >> BFP640) was driven by the search of the lowest possible R_BB, because > >> I was deemed to encounter lower source resistances than the 50ohms for > >> which SiGe devices are generally optimized. I reasoned that the base > >> geometry of the BFP650, as a medium power device, is more likely yield > >> a low R_BB. In your applications BFP640 may work better with the "more > >> ordinary" impedance levels and ambient temperatures. > > >> =A0 The "I was about.." part means that I'm somewhat puzzled now that =I> >> looked up the recent Infineon data sheets. The BFP650 is now listed > >> as a SiGe:C part (the datasheet 2010-10-22) and its Gummel-Poon > >> model reads RB=3D6.376 . The preliminary datasheet I have, dated > >> Aug-16-2004, lists it as an ordinary SiGe part with RB=3D1.036 . So, I='m> >> wondering how much the device has changed from the versions I've been > >> using. I seem to recall that also in some other more recent data sheet > >> versions it was listed as a SiGe, not SiGe:C . > > >> =A0 Given the fact that the most recent BFP640 datasheet (dated 2007-0=5-29)> >> reads RB=3D3.129, the BFP650 does not seem to have any noise advantage > >> any more, either. > > >> =A0 In fact, there was a recent paper in the RSI where > >> the Jena group claim to have achieved 15 pV/rtHz with BFP640 in LHe, > >> as compared with my 75 pV/rtHz with BFP650. This is a puzzling result, > >> actually, because it is better than one would expect in the picture wh=ere> >> u_N originates from the collector shot noise acting on the r_E - provi=ded> >> that the thermal voltage saturates at ~5..7 meV even when the ambient > >> temperature keeps going down. According to my data it does saturate in > >> BFP640 just like it does in all other SiGe's I've tried, when measured=by> >> the transconductance. I'm wondering whether my transistors have always > >> been oscillating so high that I cannot see it (ie. >27 GHz) and the > >> V_T saturation is an artefact due to it... > > >> =A0 Regards, > >> =A0 =A0 =A0 =A0 =A0 =A0Mikko > > >I started by taking your advice, which was pretty valuable--thanks > >again. =A0I noticed the change in specs as well, when I ordered another > >batch a couple of months ago. =A0They changed the type number to BFP650H > >instead of BFP650E when they changed the process. =A0(The dogs.) > > >I'm using it as a cascode stage for a SKY65050 pHEMT, and it works > >great. =A0One 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. =A0Even 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. =A0It does want to oscillate at 14 GHz if yo=u> >look at it crosswise, but a nice 5-ohm bead in series with the base > >cleans that right up. > > >I'm going to be using it in a new front end design, as a > >bootstrapped-bootstrap wrapped round a couple of parallelled BF862s. =A0=DC> >coupling the bootstrap and using a current source load gets rid of the > >thermal tails on the BF862s by keeping their dissipation constant. > >(Running them just slightly above I_DSS gives them a zero tempco anyway, > >but this one needs really good dc stability.) > > > Funny you should say "cascode." I was just about to post the > following: > > If I want a fast, low-capacitance, accurate, positive current source, > I could build a fairly slow active current source with a voltage > reference, a resistor, an opamp, and a p-fet. But it would have a lot > of capacitance and would be fairly slow. So I could cascode that with > a microwave-type PNP transistor, with a bit of base resistance or a > ferrite bead to keep it stable. But then I'd have the base current > error, and the good fast PNPs, what few there are, have mediocre > betas. > > A PNP darlington is too slow. > > So, how to correct for the base current error? > > Two ideas so far: make a Darlington, but add a lowpass filter from the > higher-current transistor base, into the emitter of the second > transistor. The main transistor needed a base resistor anyhow. > > Or sense the base current of the PNP, with opamps and such, and > increase the current of the precision/slow source to make up for it.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
picosecond test points
Started by ●February 11, 2012
Reply by ●February 12, 20122012-02-12
Reply by ●February 12, 20122012-02-12
On Sun, 12 Feb 2012 18:39:45 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>Jim Thompson wrote: >> >> On Sun, 12 Feb 2012 22:49:26 +0100, Fred Bartoli <" "> wrote: >> >> >John Larkin a �crit : >> >> On Sun, 12 Feb 2012 13:39:37 -0500, Phil Hobbs >> >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >> >>> Okkim Atnarivik wrote: >> >>>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>>> : Looks good. Another approach would be something like a BFP650 follower, >> >>>> >> >>>> Phil, I was about to suggest that if you've ended up using the >> >>>> BFP650 by following my example, you might be better off with the BFP640. >> >>>> Or more modern ones with a higher hFE (I'm fond of the NESG3031 and >> >>>> NESG4030). >> >>>> >> >>>> Namely, my choice of BFP650 (and not the then more widely available >> >>>> BFP640) was driven by the search of the lowest possible R_BB, because >> >>>> I was deemed to encounter lower source resistances than the 50ohms for >> >>>> which SiGe devices are generally optimized. I reasoned that the base >> >>>> geometry of the BFP650, as a medium power device, is more likely yield >> >>>> a low R_BB. In your applications BFP640 may work better with the "more >> >>>> ordinary" impedance levels and ambient temperatures. >> >>>> >> >>>> The "I was about.." part means that I'm somewhat puzzled now that I >> >>>> looked up the recent Infineon data sheets. The BFP650 is now listed >> >>>> as a SiGe:C part (the datasheet 2010-10-22) and its Gummel-Poon >> >>>> model reads RB=6.376 . The preliminary datasheet I have, dated >> >>>> Aug-16-2004, lists it as an ordinary SiGe part with RB=1.036 . So, I'm >> >>>> wondering how much the device has changed from the versions I've been >> >>>> using. I seem to recall that also in some other more recent data sheet >> >>>> versions it was listed as a SiGe, not SiGe:C . >> >>>> >> >>>> Given the fact that the most recent BFP640 datasheet (dated 2007-05-29) >> >>>> reads RB=3.129, the BFP650 does not seem to have any noise advantage >> >>>> any more, either. >> >>>> >> >>>> In fact, there was a recent paper in the RSI where >> >>>> the Jena group claim to have achieved 15 pV/rtHz with BFP640 in LHe, >> >>>> as compared with my 75 pV/rtHz with BFP650. This is a puzzling result, >> >>>> actually, because it is better than one would expect in the picture where >> >>>> u_N originates from the collector shot noise acting on the r_E - provided >> >>>> that the thermal voltage saturates at ~5..7 meV even when the ambient >> >>>> temperature keeps going down. According to my data it does saturate in >> >>>> BFP640 just like it does in all other SiGe's I've tried, when measured by >> >>>> the transconductance. I'm wondering whether my transistors have always >> >>>> been oscillating so high that I cannot see it (ie. >27 GHz) and the >> >>>> V_T saturation is an artefact due to it... >> >>>> >> >>>> Regards, >> >>>> Mikko >> >>> I started by taking your advice, which was pretty valuable--thanks >> >>> 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.) >> >>> >> >>> I'm using it as a cascode stage for a SKY65050 pHEMT, and it works >> >>> 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. >> >>> >> >>> I'm going to be using it in a new front end design, as a >> >>> bootstrapped-bootstrap wrapped round a couple of parallelled BF862s. DC >> >>> coupling the bootstrap and using a current source load gets rid of the >> >>> thermal tails on the BF862s by keeping their dissipation constant. >> >>> (Running them just slightly above I_DSS gives them a zero tempco anyway, >> >>> but this one needs really good dc stability.) >> >>> >> >>> Cheers >> >>> >> >>> Phil Hobbs >> >> >> >> >> >> Funny you should say "cascode." I was just about to post the >> >> following: >> >> >> >> If I want a fast, low-capacitance, accurate, positive current source, >> >> I could build a fairly slow active current source with a voltage >> >> reference, a resistor, an opamp, and a p-fet. But it would have a lot >> >> of capacitance and would be fairly slow. So I could cascode that with >> >> a microwave-type PNP transistor, with a bit of base resistance or a >> >> ferrite bead to keep it stable. But then I'd have the base current >> >> error, and the good fast PNPs, what few there are, have mediocre >> >> betas. >> >> >> >> A PNP darlington is too slow. >> >> >> >> So, how to correct for the base current error? >> >> >> > >> >Try this : >> > >> > >> > >> > >> > >> > Ic = 3mA >> > >> > 1K | >> > ___ |/ >> > 5V>-----|___|----+------| >> > | |> >> > .-. | >> > | | | >> > 1K | | | >> > '-' | >> > | | >> > |\ | ||-+ >> > 3V>--|+\ | ||<- >> > | >---------|-----||-+ >> > .-|-/ | | >> > | |/ | ___ | >> > '--------------+--|___|-+ >> > | >> > 1K .-. >> > | | >> > | |1K >> > '-' >> > | >> > === >> > GND >> > >> > >> >Delta Ic < 3nA for 45<Beta<65 for ex. >> >> Interesting cancellation of beta terms. But doesn't VAF of the NPN >> bung thing up? > >If you use a BFP650, VAF is huge (or even negative), even with a 40 GHz >f_T. See Figure 3 on the datasheet, http://tinyurl.com/6rkavak > >Cheers > >Phil HobbsI 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.
Reply by ●February 12, 20122012-02-12
Jim Thompson wrote:> > On Sun, 12 Feb 2012 18:39:45 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >Jim Thompson wrote: > >><snipped Fred's cute beta cancellation circuit>> >> > >> Interesting cancellation of beta terms. But doesn't VAF of the NPN > >> bung thing up? > > > >If you use a BFP650, VAF is huge (or even negative), even with a 40 GHz > >f_T. See Figure 3 on the datasheet, http://tinyurl.com/6rkavak > > > >Cheers > > > >Phil Hobbs > > I only eyeballed the beta cancellation. It may well be that VAF drops > out as well... interesting!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
Reply by ●February 12, 20122012-02-12
On Sun, 12 Feb 2012 19:27:03 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>Jim Thompson wrote: >> >> On Sun, 12 Feb 2012 18:39:45 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >Jim Thompson wrote: >> >> > ><snipped Fred's cute beta cancellation circuit> > >> >> >> >> Interesting cancellation of beta terms. But doesn't VAF of the NPN >> >> bung thing up? >> > >> >If you use a BFP650, VAF is huge (or even negative), even with a 40 GHz >> >f_T. See Figure 3 on the datasheet, http://tinyurl.com/6rkavak >> > >> >Cheers >> > >> >Phil Hobbs >> >> I only eyeballed the beta cancellation. It may well be that VAF drops >> out as well... interesting! > >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 HobbsWonder 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.
Reply by ●February 12, 20122012-02-12
On Sun, 12 Feb 2012 18:39:45 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>Jim Thompson wrote: >> >> On Sun, 12 Feb 2012 22:49:26 +0100, Fred Bartoli <" "> wrote: >> >> >John Larkin a �crit : >> >> On Sun, 12 Feb 2012 13:39:37 -0500, Phil Hobbs >> >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >> >>> Okkim Atnarivik wrote: >> >>>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>>> : Looks good. Another approach would be something like a BFP650 follower, >> >>>> >> >>>> Phil, I was about to suggest that if you've ended up using the >> >>>> BFP650 by following my example, you might be better off with the BFP640. >> >>>> Or more modern ones with a higher hFE (I'm fond of the NESG3031 and >> >>>> NESG4030). >> >>>> >> >>>> Namely, my choice of BFP650 (and not the then more widely available >> >>>> BFP640) was driven by the search of the lowest possible R_BB, because >> >>>> I was deemed to encounter lower source resistances than the 50ohms for >> >>>> which SiGe devices are generally optimized. I reasoned that the base >> >>>> geometry of the BFP650, as a medium power device, is more likely yield >> >>>> a low R_BB. In your applications BFP640 may work better with the "more >> >>>> ordinary" impedance levels and ambient temperatures. >> >>>> >> >>>> The "I was about.." part means that I'm somewhat puzzled now that I >> >>>> looked up the recent Infineon data sheets. The BFP650 is now listed >> >>>> as a SiGe:C part (the datasheet 2010-10-22) and its Gummel-Poon >> >>>> model reads RB=6.376 . The preliminary datasheet I have, dated >> >>>> Aug-16-2004, lists it as an ordinary SiGe part with RB=1.036 . So, I'm >> >>>> wondering how much the device has changed from the versions I've been >> >>>> using. I seem to recall that also in some other more recent data sheet >> >>>> versions it was listed as a SiGe, not SiGe:C . >> >>>> >> >>>> Given the fact that the most recent BFP640 datasheet (dated 2007-05-29) >> >>>> reads RB=3.129, the BFP650 does not seem to have any noise advantage >> >>>> any more, either. >> >>>> >> >>>> In fact, there was a recent paper in the RSI where >> >>>> the Jena group claim to have achieved 15 pV/rtHz with BFP640 in LHe, >> >>>> as compared with my 75 pV/rtHz with BFP650. This is a puzzling result, >> >>>> actually, because it is better than one would expect in the picture where >> >>>> u_N originates from the collector shot noise acting on the r_E - provided >> >>>> that the thermal voltage saturates at ~5..7 meV even when the ambient >> >>>> temperature keeps going down. According to my data it does saturate in >> >>>> BFP640 just like it does in all other SiGe's I've tried, when measured by >> >>>> the transconductance. I'm wondering whether my transistors have always >> >>>> been oscillating so high that I cannot see it (ie. >27 GHz) and the >> >>>> V_T saturation is an artefact due to it... >> >>>> >> >>>> Regards, >> >>>> Mikko >> >>> I started by taking your advice, which was pretty valuable--thanks >> >>> 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.) >> >>> >> >>> I'm using it as a cascode stage for a SKY65050 pHEMT, and it works >> >>> 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. >> >>> >> >>> I'm going to be using it in a new front end design, as a >> >>> bootstrapped-bootstrap wrapped round a couple of parallelled BF862s. DC >> >>> coupling the bootstrap and using a current source load gets rid of the >> >>> thermal tails on the BF862s by keeping their dissipation constant. >> >>> (Running them just slightly above I_DSS gives them a zero tempco anyway, >> >>> but this one needs really good dc stability.) >> >>> >> >>> Cheers >> >>> >> >>> Phil Hobbs >> >> >> >> >> >> Funny you should say "cascode." I was just about to post the >> >> following: >> >> >> >> If I want a fast, low-capacitance, accurate, positive current source, >> >> I could build a fairly slow active current source with a voltage >> >> reference, a resistor, an opamp, and a p-fet. But it would have a lot >> >> of capacitance and would be fairly slow. So I could cascode that with >> >> a microwave-type PNP transistor, with a bit of base resistance or a >> >> ferrite bead to keep it stable. But then I'd have the base current >> >> error, and the good fast PNPs, what few there are, have mediocre >> >> betas. >> >> >> >> A PNP darlington is too slow. >> >> >> >> So, how to correct for the base current error? >> >> >> > >> >Try this : >> > >> > >> > >> > >> > >> > Ic = 3mA >> > >> > 1K | >> > ___ |/ >> > 5V>-----|___|----+------| >> > | |> >> > .-. | >> > | | | >> > 1K | | | >> > '-' | >> > | | >> > |\ | ||-+ >> > 3V>--|+\ | ||<- >> > | >---------|-----||-+ >> > .-|-/ | | >> > | |/ | ___ | >> > '--------------+--|___|-+ >> > | >> > 1K .-. >> > | | >> > | |1K >> > '-' >> > | >> > === >> > GND >> > >> > >> >Delta Ic < 3nA for 45<Beta<65 for ex. >> >> Interesting cancellation of beta terms. But doesn't VAF of the NPN >> bung thing up? > >If you use a BFP650, VAF is huge (or even negative), even with a 40 GHz >f_T. See Figure 3 on the datasheet, http://tinyurl.com/6rkavak > >Cheers > >Phil HobbsThat 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
Reply by ●February 12, 20122012-02-12
On Sun, 12 Feb 2012 16:07:51 -0800 (PST), dagmargoodboat@yahoo.com wrote:>On Feb 12, 2:15�pm, John Larkin ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> On Sun, 12 Feb 2012 13:39:37 -0500, Phil Hobbs >> >> >> >> >> >> >> >> >> >> <pcdhSpamMeSensel...@electrooptical.net> wrote: >> >Okkim Atnarivik wrote: >> >> >> Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote: >> >> : Looks good. �Another approach would be something like a BFP650 follower, >> >> >> � Phil, I was about to suggest that if you've ended up using the >> >> BFP650 by following my example, you might be better off with the BFP640. >> >> Or more modern ones with a higher hFE (I'm fond of the NESG3031 and >> >> NESG4030). >> >> >> � Namely, my choice of BFP650 (and not the then more widely available >> >> BFP640) was driven by the search of the lowest possible R_BB, because >> >> I was deemed to encounter lower source resistances than the 50ohms for >> >> which SiGe devices are generally optimized. I reasoned that the base >> >> geometry of the BFP650, as a medium power device, is more likely yield >> >> a low R_BB. In your applications BFP640 may work better with the "more >> >> ordinary" impedance levels and ambient temperatures. >> >> >> � The "I was about.." part means that I'm somewhat puzzled now that I >> >> looked up the recent Infineon data sheets. The BFP650 is now listed >> >> as a SiGe:C part (the datasheet 2010-10-22) and its Gummel-Poon >> >> model reads RB=6.376 . The preliminary datasheet I have, dated >> >> Aug-16-2004, lists it as an ordinary SiGe part with RB=1.036 . So, I'm >> >> wondering how much the device has changed from the versions I've been >> >> using. I seem to recall that also in some other more recent data sheet >> >> versions it was listed as a SiGe, not SiGe:C . >> >> >> � Given the fact that the most recent BFP640 datasheet (dated 2007-05-29) >> >> reads RB=3.129, the BFP650 does not seem to have any noise advantage >> >> any more, either. >> >> >> � In fact, there was a recent paper in the RSI where >> >> the Jena group claim to have achieved 15 pV/rtHz with BFP640 in LHe, >> >> as compared with my 75 pV/rtHz with BFP650. This is a puzzling result, >> >> actually, because it is better than one would expect in the picture where >> >> u_N originates from the collector shot noise acting on the r_E - provided >> >> that the thermal voltage saturates at ~5..7 meV even when the ambient >> >> temperature keeps going down. According to my data it does saturate in >> >> BFP640 just like it does in all other SiGe's I've tried, when measured by >> >> the transconductance. I'm wondering whether my transistors have always >> >> been oscillating so high that I cannot see it (ie. >27 GHz) and the >> >> V_T saturation is an artefact due to it... >> >> >> � Regards, >> >> � � � � � �Mikko >> >> >I started by taking your advice, which was pretty valuable--thanks >> >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.) >> >> >I'm using it as a cascode stage for a SKY65050 pHEMT, and it works >> >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. >> >> >I'm going to be using it in a new front end design, as a >> >bootstrapped-bootstrap wrapped round a couple of parallelled BF862s. �DC >> >coupling the bootstrap and using a current source load gets rid of the >> >thermal tails on the BF862s by keeping their dissipation constant. >> >(Running them just slightly above I_DSS gives them a zero tempco anyway, >> >but this one needs really good dc stability.) >> >> >> Funny you should say "cascode." I was just about to post the >> following: >> >> If I want a fast, low-capacitance, accurate, positive current source, >> I could build a fairly slow active current source with a voltage >> reference, a resistor, an opamp, and a p-fet. But it would have a lot >> of capacitance and would be fairly slow. So I could cascode that with >> a microwave-type PNP transistor, with a bit of base resistance or a >> ferrite bead to keep it stable. But then I'd have the base current >> error, and the good fast PNPs, what few there are, have mediocre >> betas. >> >> A PNP darlington is too slow. >> >> So, how to correct for the base current error? >> >> Two ideas so far: make a Darlington, but add a lowpass filter from the >> higher-current transistor base, into the emitter of the second >> transistor. The main transistor needed a base resistor anyhow. >> >> Or sense the base current of the PNP, with opamps and such, and >> increase the current of the precision/slow source to make up for it. > >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. > >JamesSadly, 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
Reply by ●February 12, 20122012-02-12
John Larkin wrote:> > On Sun, 12 Feb 2012 18:39:45 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >Jim Thompson wrote: > >> > >> On Sun, 12 Feb 2012 22:49:26 +0100, Fred Bartoli <" "> wrote: > >> > >> >John Larkin a �crit : > >> >> On Sun, 12 Feb 2012 13:39:37 -0500, Phil Hobbs > >> >> <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> >> > >> >>> Okkim Atnarivik wrote: > >> >>>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> >>>> : Looks good. Another approach would be something like a BFP650 follower, > >> >>>> > >> >>>> Phil, I was about to suggest that if you've ended up using the > >> >>>> BFP650 by following my example, you might be better off with the BFP640. > >> >>>> Or more modern ones with a higher hFE (I'm fond of the NESG3031 and > >> >>>> NESG4030). > >> >>>> > >> >>>> Namely, my choice of BFP650 (and not the then more widely available > >> >>>> BFP640) was driven by the search of the lowest possible R_BB, because > >> >>>> I was deemed to encounter lower source resistances than the 50ohms for > >> >>>> which SiGe devices are generally optimized. I reasoned that the base > >> >>>> geometry of the BFP650, as a medium power device, is more likely yield > >> >>>> a low R_BB. In your applications BFP640 may work better with the "more > >> >>>> ordinary" impedance levels and ambient temperatures. > >> >>>> > >> >>>> The "I was about.." part means that I'm somewhat puzzled now that I > >> >>>> looked up the recent Infineon data sheets. The BFP650 is now listed > >> >>>> as a SiGe:C part (the datasheet 2010-10-22) and its Gummel-Poon > >> >>>> model reads RB=6.376 . The preliminary datasheet I have, dated > >> >>>> Aug-16-2004, lists it as an ordinary SiGe part with RB=1.036 . So, I'm > >> >>>> wondering how much the device has changed from the versions I've been > >> >>>> using. I seem to recall that also in some other more recent data sheet > >> >>>> versions it was listed as a SiGe, not SiGe:C . > >> >>>> > >> >>>> Given the fact that the most recent BFP640 datasheet (dated 2007-05-29) > >> >>>> reads RB=3.129, the BFP650 does not seem to have any noise advantage > >> >>>> any more, either. > >> >>>> > >> >>>> In fact, there was a recent paper in the RSI where > >> >>>> the Jena group claim to have achieved 15 pV/rtHz with BFP640 in LHe, > >> >>>> as compared with my 75 pV/rtHz with BFP650. This is a puzzling result, > >> >>>> actually, because it is better than one would expect in the picture where > >> >>>> u_N originates from the collector shot noise acting on the r_E - provided > >> >>>> that the thermal voltage saturates at ~5..7 meV even when the ambient > >> >>>> temperature keeps going down. According to my data it does saturate in > >> >>>> BFP640 just like it does in all other SiGe's I've tried, when measured by > >> >>>> the transconductance. I'm wondering whether my transistors have always > >> >>>> been oscillating so high that I cannot see it (ie. >27 GHz) and the > >> >>>> V_T saturation is an artefact due to it... > >> >>>> > >> >>>> Regards, > >> >>>> Mikko > >> >>> I started by taking your advice, which was pretty valuable--thanks > >> >>> 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.) > >> >>> > >> >>> I'm using it as a cascode stage for a SKY65050 pHEMT, and it works > >> >>> 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. > >> >>> > >> >>> I'm going to be using it in a new front end design, as a > >> >>> bootstrapped-bootstrap wrapped round a couple of parallelled BF862s. DC > >> >>> coupling the bootstrap and using a current source load gets rid of the > >> >>> thermal tails on the BF862s by keeping their dissipation constant. > >> >>> (Running them just slightly above I_DSS gives them a zero tempco anyway, > >> >>> but this one needs really good dc stability.) > >> >>> > >> >>> Cheers > >> >>> > >> >>> Phil Hobbs > >> >> > >> >> > >> >> Funny you should say "cascode." I was just about to post the > >> >> following: > >> >> > >> >> If I want a fast, low-capacitance, accurate, positive current source, > >> >> I could build a fairly slow active current source with a voltage > >> >> reference, a resistor, an opamp, and a p-fet. But it would have a lot > >> >> of capacitance and would be fairly slow. So I could cascode that with > >> >> a microwave-type PNP transistor, with a bit of base resistance or a > >> >> ferrite bead to keep it stable. But then I'd have the base current > >> >> error, and the good fast PNPs, what few there are, have mediocre > >> >> betas. > >> >> > >> >> A PNP darlington is too slow. > >> >> > >> >> So, how to correct for the base current error? > >> >> > >> > > >> >Try this : > >> > > >> > > >> > > >> > > >> > > >> > Ic = 3mA > >> > > >> > 1K | > >> > ___ |/ > >> > 5V>-----|___|----+------| > >> > | |> > >> > .-. | > >> > | | | > >> > 1K | | | > >> > '-' | > >> > | | > >> > |\ | ||-+ > >> > 3V>--|+\ | ||<- > >> > | >---------|-----||-+ > >> > .-|-/ | | > >> > | |/ | ___ | > >> > '--------------+--|___|-+ > >> > | > >> > 1K .-. > >> > | | > >> > | |1K > >> > '-' > >> > | > >> > === > >> > GND > >> > > >> > > >> >Delta Ic < 3nA for 45<Beta<65 for ex. > >> > >> Interesting cancellation of beta terms. But doesn't VAF of the NPN > >> bung thing up? > > > >If you use a BFP650, VAF is huge (or even negative), even with a 40 GHz > >f_T. See Figure 3 on the datasheet, http://tinyurl.com/6rkavak > > > >Cheers > > > >Phil Hobbs > > 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.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
Reply by ●February 12, 20122012-02-12
On Feb 12, 8:37=A0pm, dagmargoodb...@yahoo.com wrote:> On Feb 12, 4:49=A0pm, Fred Bartoli <" "> wrote: > > > > > John Larkin a =E9crit : > > > Funny you should say "cascode." I was just about to post the > > > following: > > > > If I want a fast, low-capacitance, accurate, positive current source, > > > I could build a fairly slow active current source with a voltage > > > reference, a resistor, an opamp, and a p-fet. But it would have a lot > > > of capacitance and would be fairly slow. So I could cascode that with > > > a microwave-type PNP transistor, with a bit of base resistance or a > > > ferrite bead to keep it stable. But then I'd have the base current > > > error, and the good fast PNPs, what few there are, have mediocre > > > betas. > > > > A PNP darlington is too slow. > > > > So, how to correct for the base current error? > > > Try this : > > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 =A0 =A0 =A0 =A0 =A0Ic =3D 3mA> > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A01K =A0 ==A0 =A0 =A0 =A0 =A0 =A0 |> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0___ =A0 ==A0 =A0 =A0 =A0 =A0|/> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 5V>-----|___|----+------| > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0| =A0 =A0 =A0|>> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 .-. =A0 =A0 =A0 |> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 | | =A0 =A0 =A0 |> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0=1K | | =A0 =A0 =A0 |> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 '-' =A0 =A0 =A0 |> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0| =A0 =A0 =A0 =A0|> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 |\ =A0 =A0 =A0 =A0 ==A0 | =A0 =A0 ||-+> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A03V>--|+\ =A0 =A0 =A0 =A0 =A0=| =A0 =A0 ||<-> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0>---------|---=--||-+> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 .-|-/ =A0 =A0 =A0 =A0 ==A0| =A0 =A0 =A0 =A0|> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | |/ =A0 =A0 =A0 =A0 ==A0 | =A0 ___ =A0|> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 '--------------+--|___|=-+> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 =A0 =A0 =A0 =A0 |> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 =A0 =A01K =A0.-.> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 =A0 =A0 =A0 =A0| |> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 =A0 =A0 =A0 =A0| |1K> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 =A0 =A0 =A0 =A0'-'> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 =A0 =A0 =A0 =A0 |> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 =A0 =A0 =A0 =A0=3D=3D=3D> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 =A0 =A0 =A0 =A0GND> > > Delta Ic < 3nA for 45<Beta<65 for ex. > > I saw a similar (but I think more complicated) current sink scheme > decades(?) ago in one of the magazines. It used an op amp diff amp, > bjt pass element, sensed base resistor drop to sense the base current, > and fed that back. > > I don't think it was a cascode--just one pass element, IIRC. > > I might have saved it somewhere, but I've got no idea where. =A0Maybe > that's enough to jog someone's memory?Oh, this is it here: http://www.edn.com/article/463825-Error_compensation_improves_bipolar_curre= nt_sinks.php R5 .--47K---. .---<--< Iout | | | R4 | |\ | R2 47 | Vref >---1K---+--|+\ | ___ |/ | >--+---|___|---+-| Q1 .-|-/ | | |>. 2n3020 | |/ --- Ccomp | | | --- R6 | | | | ___ 47K| | | +---|___|---' | | | ___ | '-------+---|___|-------+ R3 1k | .-. | | R1 | | 1 ohm '-' | =3D=3D=3D GND 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
Reply by ●February 12, 20122012-02-12
On Feb 12, 9:27=A0pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote:> 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. > > > I could put a compound inductor directly in series with my mosfet > > current source, I suppose. > > It's hotter than a two-dollar pistol, too. =A0Pity 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. =A0Inverted RF transistors > have lower capacitance, and their f_T can even be higher than the same > device right-way-up.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
Reply by ●February 13, 20122012-02-13
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> 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 http://tinyurl.com/8a44pzn . If you check the page 4 of http://tinyurl.com/6qmnd4u you see it there too (and additionally you get the large hFE = 400 ). Regards, Mikko
