John Larkin wrote:> > On Fri, 17 Aug 2012 15:07:40 +0000 (UTC), Frank Miles > <fpm@u.washington.edu> wrote: > > >On Thu, 16 Aug 2012 16:39:55 -0700, John Larkin wrote: > > > >> On Thu, 16 Aug 2012 15:08:21 -0700 (PDT), dagmargoodboat@yahoo.com > >> wrote: > >> > >>>On Aug 16, 5:15 pm, John Larkin <jlar...@highlandtechnology.com> wrote: > >>>> On Thu, 16 Aug 2012 13:12:50 -0700 (PDT), dagmargoodb...@yahoo.com > >>>> wrote: > >>>> > >>>> > >>>> > >>>> > >>>> > >>>> > >>>> > >>>> > >>>> > >>>> >On Aug 16, 3:59 pm, Jim Thompson > >>>> ><To-Email-Use-The-Envelope-I...@On-My- Web-Site.com> wrote: > >>>> >> On Thu, 16 Aug 2012 12:24:51 -0700 (PDT), dagmargoodb...@yahoo.com > >>>> >> wrote: > >>>> > >>>> >> >On Aug 16, 11:41 am, John Larkin > >>>> >> ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >>>> >> >> On Thu, 16 Aug 2012 08:29:46 -0700, Jim Thompson > >>>> > >>>> >> >> <To-Email-Use-The-Envelope-I...@On-My-Web-Site.com> wrote: > >>>> >> >> >On Thu, 16 Aug 2012 07:39:04 -0700 (PDT), > >>>> >> >> >dagmargoodb...@yahoo.com wrote: > >>>> > >>>> >> >> >>On Aug 15, 10:43 pm, John Larkin > >>>> >> >> >><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >>>> > >>>> >> >> >>> Hey, how about this? > >>>> > >>>> >> >> >>>https://dl.dropbox.com/u/53724080/Circuits/Isrc_simpler.JPG > >>>> > >>>> >> >> >>It's kind of a variation on Phil's active inductor. That > >>>> >> >> >>defeats all the oscillation modes I can think of so far. (c-b, > >>>> >> >> >>e-b, c-e)(Which means little, since circuits are so ingenious > >>>> >> >> >>inventing new ones.) > >>>> > >>>> >> >> >>> Ludicrously simple. No Miller nonsense. Might not oscillate. > >>>> >> >> >>> MMBTH81 would have about 2 pF out, or BFT92 would have about > >>>> >> >> >>> 0.75 but lower beta. > >>>> > >>>> >> >> >>You could use the beta-cancellation trick. But, stray and > >>>> >> >> >>parasitic capacitance will be big factors compared to a 10pF > >>>> >> >> >>integration cap. > >>>> > >>>> >> >> >>> If this works, I should have done it years ago. > >>>> > >>>> >> >> >Yup. But I'd still add a base resistor, for the very reasons > >>>> >> >> >Gerhard Hoffmann brought up... the output Z of the OpAmp is a > >>>> >> >> >big unknown (*). > >>>> > >>>> >> >> The base and emitter are shorted at AC. The opamp Zout doesn't > >>>> >> >> matter any more. > >>>> > >>>> >> >Yep, that's a good point. If the op amp can drive that load all > >>>> >> >by itself, it seems okay. That load being 1nF in series with > >>>> >> >120ohms, it should be a snap. > >>>> > >>>> >> >> >(*) Which brings to mind... maybe add some DC load to the OpAmp > >>>> >> >> >to keep it out of the class-B region. > >>>> > >>>> >> >> LM7301 is a r-r output design. It isn't an ancient class-B like > >>>> >> >> an LM324. > >>>> > >>>> >> >I love LM324As. Yeah, I know you hate 'em. > >>>> > >>>> >> CMOS R-R OpAmps inherently have load-dependent gain and phase, so I > >>>> >> avoid them when I can. > >>>> > >>>> >But John rightly points out that in the ramp timescale, Vbe is held > >>>> >constant by the 1nF cap. So, the transistor's fine. > >>>> > >>>> >In the longer term a load-step on the op amp's output may make it > >>>> >overshoot or ring. That would happen opening and closing the > >>>> >shorting switch. Probably not a big deal, AFAICT. > >>>> > >>>> >I was initially concerned the op amp is effectively absent on the 1nS > >>>> >response level, so there's no base drive, but John's right, that > >>>> >behavior is set by the 1nF b-e cap. IOW, it doesn't matter. > >>>> > >>>> The opamp is slowly servoing Vbe to get the average current right. The > >>>> ramp is so fast that the transistor is still running in constant Vbe > >>>> mode during the ramp. The cap could actually be much bigger... doesn't > >>>> matter much. > >>>> > >>>> It looks very weird. I like that. > >>> > >>>You can cancel the d.c. base current error (and use the faster > >>>transistor without fear) if you tack on a bunch of resistors--I worked > >>>up a PNP version of the technique whit3rd's citing--but the Phil- > >>>lington is cuter and mostly eliminates the need. > >> > >> > >> Here's a "shorted cascode" with Ib correction. > >> > >> https://dl.dropbox.com/u/53724080/Circuits/Isrc_cascode_Ib.JPG > >> > >> It would need some work on the slow loop dynamics. > > > >Very cute! This configuration may not be the best if you've got ripple > >on the positive supply, but other than that (and the whole Ccb > >nonlinearity problem, which none of the configurations address) it looks > >promising! In fact, if you reconnect the ground lead of the lowest > >capacitor to the positive supply, the whole thing is referenced to the > >positive supply which should reduce any ripple problems. Oscillation - > >that's another problem, you'll want to have a low HF impedance on the V+ > >supply anyway. > > > > -F > > The killer is probably the Early slope of the PNP, which will make the > current source look resistive. Since RF parts are not specified well > for DC behavior, one would have to measure some transistors to see how > bad that may be. I think the LPTM said that SiGe transistors have huge > Early voltages, but they probably don't come in PNP. > > Still, the Ib correction is cute, and its loop compensation turns out > to be interesting. Perhaps some Master Circuit Designer will address > that issue. > > We could dump the b-e cap to fix the Early problem and add a base > resistor to kill RF oscillation in the PNP. Something like this: > > https://dl.dropbox.com/u/53724080/Circuits/Isrc_Ib_2.JPG > > As you note, C1 might better return to V+. V+ would be a bypassed > copper pour, pretty stiff from DC to daylight. > > Pity, I really liked the Shorted Cascode.A BFP640 in that floating simulated-inductor circuit would fix that--its Early voltage is like a kilovolt. 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
fast ramp follies
Started by ●August 14, 2012
Reply by ●August 17, 20122012-08-17
Reply by ●August 17, 20122012-08-17
On Fri, 17 Aug 2012 12:45:35 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Larkin wrote: >> >> On Fri, 17 Aug 2012 15:07:40 +0000 (UTC), Frank Miles >> <fpm@u.washington.edu> wrote: >> >> >On Thu, 16 Aug 2012 16:39:55 -0700, John Larkin wrote: >> > >> >> On Thu, 16 Aug 2012 15:08:21 -0700 (PDT), dagmargoodboat@yahoo.com >> >> wrote: >> >> >> >>>On Aug 16, 5:15 pm, John Larkin <jlar...@highlandtechnology.com> wrote: >> >>>> On Thu, 16 Aug 2012 13:12:50 -0700 (PDT), dagmargoodb...@yahoo.com >> >>>> wrote: >> >>>> >> >>>> >> >>>> >> >>>> >> >>>> >> >>>> >> >>>> >> >>>> >> >>>> >> >>>> >On Aug 16, 3:59 pm, Jim Thompson >> >>>> ><To-Email-Use-The-Envelope-I...@On-My- Web-Site.com> wrote: >> >>>> >> On Thu, 16 Aug 2012 12:24:51 -0700 (PDT), dagmargoodb...@yahoo.com >> >>>> >> wrote: >> >>>> >> >>>> >> >On Aug 16, 11:41 am, John Larkin >> >>>> >> ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> >>>> >> >> On Thu, 16 Aug 2012 08:29:46 -0700, Jim Thompson >> >>>> >> >>>> >> >> <To-Email-Use-The-Envelope-I...@On-My-Web-Site.com> wrote: >> >>>> >> >> >On Thu, 16 Aug 2012 07:39:04 -0700 (PDT), >> >>>> >> >> >dagmargoodb...@yahoo.com wrote: >> >>>> >> >>>> >> >> >>On Aug 15, 10:43 pm, John Larkin >> >>>> >> >> >><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> >>>> >> >>>> >> >> >>> Hey, how about this? >> >>>> >> >>>> >> >> >>>https://dl.dropbox.com/u/53724080/Circuits/Isrc_simpler.JPG >> >>>> >> >>>> >> >> >>It's kind of a variation on Phil's active inductor. That >> >>>> >> >> >>defeats all the oscillation modes I can think of so far. (c-b, >> >>>> >> >> >>e-b, c-e)(Which means little, since circuits are so ingenious >> >>>> >> >> >>inventing new ones.) >> >>>> >> >>>> >> >> >>> Ludicrously simple. No Miller nonsense. Might not oscillate. >> >>>> >> >> >>> MMBTH81 would have about 2 pF out, or BFT92 would have about >> >>>> >> >> >>> 0.75 but lower beta. >> >>>> >> >>>> >> >> >>You could use the beta-cancellation trick. But, stray and >> >>>> >> >> >>parasitic capacitance will be big factors compared to a 10pF >> >>>> >> >> >>integration cap. >> >>>> >> >>>> >> >> >>> If this works, I should have done it years ago. >> >>>> >> >>>> >> >> >Yup. But I'd still add a base resistor, for the very reasons >> >>>> >> >> >Gerhard Hoffmann brought up... the output Z of the OpAmp is a >> >>>> >> >> >big unknown (*). >> >>>> >> >>>> >> >> The base and emitter are shorted at AC. The opamp Zout doesn't >> >>>> >> >> matter any more. >> >>>> >> >>>> >> >Yep, that's a good point. If the op amp can drive that load all >> >>>> >> >by itself, it seems okay. That load being 1nF in series with >> >>>> >> >120ohms, it should be a snap. >> >>>> >> >>>> >> >> >(*) Which brings to mind... maybe add some DC load to the OpAmp >> >>>> >> >> >to keep it out of the class-B region. >> >>>> >> >>>> >> >> LM7301 is a r-r output design. It isn't an ancient class-B like >> >>>> >> >> an LM324. >> >>>> >> >>>> >> >I love LM324As. Yeah, I know you hate 'em. >> >>>> >> >>>> >> CMOS R-R OpAmps inherently have load-dependent gain and phase, so I >> >>>> >> avoid them when I can. >> >>>> >> >>>> >But John rightly points out that in the ramp timescale, Vbe is held >> >>>> >constant by the 1nF cap. So, the transistor's fine. >> >>>> >> >>>> >In the longer term a load-step on the op amp's output may make it >> >>>> >overshoot or ring. That would happen opening and closing the >> >>>> >shorting switch. Probably not a big deal, AFAICT. >> >>>> >> >>>> >I was initially concerned the op amp is effectively absent on the 1nS >> >>>> >response level, so there's no base drive, but John's right, that >> >>>> >behavior is set by the 1nF b-e cap. IOW, it doesn't matter. >> >>>> >> >>>> The opamp is slowly servoing Vbe to get the average current right. The >> >>>> ramp is so fast that the transistor is still running in constant Vbe >> >>>> mode during the ramp. The cap could actually be much bigger... doesn't >> >>>> matter much. >> >>>> >> >>>> It looks very weird. I like that. >> >>> >> >>>You can cancel the d.c. base current error (and use the faster >> >>>transistor without fear) if you tack on a bunch of resistors--I worked >> >>>up a PNP version of the technique whit3rd's citing--but the Phil- >> >>>lington is cuter and mostly eliminates the need. >> >> >> >> >> >> Here's a "shorted cascode" with Ib correction. >> >> >> >> https://dl.dropbox.com/u/53724080/Circuits/Isrc_cascode_Ib.JPG >> >> >> >> It would need some work on the slow loop dynamics. >> > >> >Very cute! This configuration may not be the best if you've got ripple >> >on the positive supply, but other than that (and the whole Ccb >> >nonlinearity problem, which none of the configurations address) it looks >> >promising! In fact, if you reconnect the ground lead of the lowest >> >capacitor to the positive supply, the whole thing is referenced to the >> >positive supply which should reduce any ripple problems. Oscillation - >> >that's another problem, you'll want to have a low HF impedance on the V+ >> >supply anyway. >> > >> > -F >> >> The killer is probably the Early slope of the PNP, which will make the >> current source look resistive. Since RF parts are not specified well >> for DC behavior, one would have to measure some transistors to see how >> bad that may be. I think the LPTM said that SiGe transistors have huge >> Early voltages, but they probably don't come in PNP. >> >> Still, the Ib correction is cute, and its loop compensation turns out >> to be interesting. Perhaps some Master Circuit Designer will address >> that issue. >> >> We could dump the b-e cap to fix the Early problem and add a base >> resistor to kill RF oscillation in the PNP. Something like this: >> >> https://dl.dropbox.com/u/53724080/Circuits/Isrc_Ib_2.JPG >> >> As you note, C1 might better return to V+. V+ would be a bypassed >> copper pour, pretty stiff from DC to daylight. >> >> Pity, I really liked the Shorted Cascode. > >A BFP640 in that floating simulated-inductor circuit would fix that--its >Early voltage is like a kilovolt. > >Cheers > >Phil HobbsOK, I'll think about that soon. In some cases, I could use a negative ramp, so the Shorted Cascode with the SiGe might be interesting. I invented a pseudo-inductor once, for one of the C5A projects of my youth. To get the impedance up, I bootstrapped the base of a darlington from its own emitter with a double RC. It oscillated at some low frequency, 10 Hz or something. I didn't realize it at the time, but I'd stumbled onto that RCRC circuit that has voltage gain. I fixed it without really understanding it. This was back in pre-Spice days, when simulation was a chore. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Reply by ●August 17, 20122012-08-17
On Thu, 16 Aug 2012 22:01:35 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Larkin wrote: >> >> On Thu, 16 Aug 2012 17:28:16 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >On 08/16/2012 05:19 PM, John Larkin wrote: >> >> On Thu, 16 Aug 2012 12:20:49 -0700 (PDT), dagmargoodboat@yahoo.com >> >> wrote: >> >> >> >>> On Aug 16, 11:29 am, Jim Thompson<To-Email-Use-The-Envelope-I...@On- >> >>> My-Web-Site.com> wrote: >> >>>> On Thu, 16 Aug 2012 07:39:04 -0700 (PDT), dagmargoodb...@yahoo.com >> >>>> wrote: >> >>>> >> >>>> >> >>>> >> >>>> >> >>>> >> >>>> >> >>>> >> >>>> >> >>>> >> >>>>> On Aug 15, 10:43 pm, John Larkin >> >>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> >>>> >> >>>>>> Hey, how about this? >> >>>> >> >>>>>> https://dl.dropbox.com/u/53724080/Circuits/Isrc_simpler.JPG >> >>>> >> >>>>> It's kind of a variation on Phil's active inductor. That defeats all >> >>>>> the oscillation modes I can think of so far. (c-b, e-b, c-e)(Which >> >>>>> means little, since circuits are so ingenious inventing new ones.) >> >>>> >> >>>>>> Ludicrously simple. No Miller nonsense. Might not oscillate. MMBTH81 >> >>>>>> would have about 2 pF out, or BFT92 would have about 0.75 but lower >> >>>>>> beta. >> >>>> >> >>>>> You could use the beta-cancellation trick. But, stray and parasitic >> >>>>> capacitance will be big factors compared to a 10pF integration cap. >> >>>> >> >>>>>> If this works, I should have done it years ago. >> >>>> >> >>>> Yup. But I'd still add a base resistor, for the very reasons Gerhard >> >>>> Hoffmann brought up... the output Z of the OpAmp is a big unknown (*). >> >>>> >> >>>> (*) Which brings to mind... maybe add some DC load to the OpAmp to >> >>>> keep it out of the class-B region. >> >>> >> >>> I agree with both those points, and meant to mention them. I don't >> >>> trust the op-amp to behave nicely to impulses, or its output to be >> >>> stiff. I'd expect the output to be hi-z on this timescale. >> >>> >> >>> If anything I'd be tempted to bypass the bjt's base to the supply >> >>> rail. The BJT then handles the fast stuff. Isolate that capacitive >> >>> load from the op amp's output with a series resistor, and stabilize >> >>> the op amp loop itself with a feed-forward cap. Standard stuff. >> >>> >> >>> Does it all matter? Unclamping the integration cap will feed-thru a >> >>> small impulse through to the bjt / CCS. After that, it's pretty >> >>> clean. So, the op-amp's output impedance might not matter, but I >> >>> suspect it will. >> >>> >> >>> I invented loading LM324 outputs all by myself waayyyy back as a punk >> >>> kid, to stiffen them up and kill the crossover THD. Of course >> >>> everyone else on the planet thought of it too. :-) >> >> >> >> >> >> Me too! And I also invented loading the charge pump phase detector in >> >> the 4046. >> >> >> >> I also invented the successive-detection log video detector, and the >> >> dual-slope ADC. I did those at times when I could have patented them. >> >> I'd be smoking seegars on the Riviera. >> >> >> >> >> >DLVAs have been around since at least the war: e.g. US Patent 2577506 to >> >Belleville, filed 1945. >> > >> >Cheers >> > >> >Phil Hobbs >> >> Oh well. But I did invent the dual-slope some years before Fairchild >> (?) patented it. >> > >I used to be a big fan of the Intersil ICL7109 back in the day. Dual >slope was magic. > >Cheers > >Phil Hobbs... and educated a lot of people about dielectric absorption. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Reply by ●August 17, 20122012-08-17
On Fri, 17 Aug 2012 10:24:23 -0700, John Larkin <jlarkin@highlandtechnology.com> wrote:>On Thu, 16 Aug 2012 22:01:35 -0400, Phil Hobbs ><pcdhSpamMeSenseless@electrooptical.net> wrote: > >>John Larkin wrote: >>> >>> On Thu, 16 Aug 2012 17:28:16 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>> >On 08/16/2012 05:19 PM, John Larkin wrote: >>> >> On Thu, 16 Aug 2012 12:20:49 -0700 (PDT), dagmargoodboat@yahoo.com >>> >> wrote: >>> >> >>> >>> On Aug 16, 11:29 am, Jim Thompson<To-Email-Use-The-Envelope-I...@On- >>> >>> My-Web-Site.com> wrote: >>> >>>> On Thu, 16 Aug 2012 07:39:04 -0700 (PDT), dagmargoodb...@yahoo.com >>> >>>> wrote: >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>>> On Aug 15, 10:43 pm, John Larkin >>> >>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>> >>>> >>> >>>>>> Hey, how about this? >>> >>>> >>> >>>>>> https://dl.dropbox.com/u/53724080/Circuits/Isrc_simpler.JPG >>> >>>> >>> >>>>> It's kind of a variation on Phil's active inductor. That defeats all >>> >>>>> the oscillation modes I can think of so far. (c-b, e-b, c-e)(Which >>> >>>>> means little, since circuits are so ingenious inventing new ones.) >>> >>>> >>> >>>>>> Ludicrously simple. No Miller nonsense. Might not oscillate. MMBTH81 >>> >>>>>> would have about 2 pF out, or BFT92 would have about 0.75 but lower >>> >>>>>> beta. >>> >>>> >>> >>>>> You could use the beta-cancellation trick. But, stray and parasitic >>> >>>>> capacitance will be big factors compared to a 10pF integration cap. >>> >>>> >>> >>>>>> If this works, I should have done it years ago. >>> >>>> >>> >>>> Yup. But I'd still add a base resistor, for the very reasons Gerhard >>> >>>> Hoffmann brought up... the output Z of the OpAmp is a big unknown (*). >>> >>>> >>> >>>> (*) Which brings to mind... maybe add some DC load to the OpAmp to >>> >>>> keep it out of the class-B region. >>> >>> >>> >>> I agree with both those points, and meant to mention them. I don't >>> >>> trust the op-amp to behave nicely to impulses, or its output to be >>> >>> stiff. I'd expect the output to be hi-z on this timescale. >>> >>> >>> >>> If anything I'd be tempted to bypass the bjt's base to the supply >>> >>> rail. The BJT then handles the fast stuff. Isolate that capacitive >>> >>> load from the op amp's output with a series resistor, and stabilize >>> >>> the op amp loop itself with a feed-forward cap. Standard stuff. >>> >>> >>> >>> Does it all matter? Unclamping the integration cap will feed-thru a >>> >>> small impulse through to the bjt / CCS. After that, it's pretty >>> >>> clean. So, the op-amp's output impedance might not matter, but I >>> >>> suspect it will. >>> >>> >>> >>> I invented loading LM324 outputs all by myself waayyyy back as a punk >>> >>> kid, to stiffen them up and kill the crossover THD. Of course >>> >>> everyone else on the planet thought of it too. :-) >>> >> >>> >> >>> >> Me too! And I also invented loading the charge pump phase detector in >>> >> the 4046. >>> >> >>> >> I also invented the successive-detection log video detector, and the >>> >> dual-slope ADC. I did those at times when I could have patented them. >>> >> I'd be smoking seegars on the Riviera. >>> >> >>> >> >>> >DLVAs have been around since at least the war: e.g. US Patent 2577506 to >>> >Belleville, filed 1945. >>> > >>> >Cheers >>> > >>> >Phil Hobbs >>> >>> Oh well. But I did invent the dual-slope some years before Fairchild >>> (?) patented it. >>> >> >>I used to be a big fan of the Intersil ICL7109 back in the day. Dual >>slope was magic. >> >>Cheers >> >>Phil Hobbs > >... and educated a lot of people about dielectric absorption.Yup. The 7109 is pretty dead, but the related 7106/7 are still kicking vigorously after all the years- pretty amazing for a part introduced in the mid-seventies. Do we know who designed those?
Reply by ●August 17, 20122012-08-17
On Fri, 17 Aug 2012 10:24:23 -0700, John Larkin <jlarkin@highlandtechnology.com> wrote:>On Thu, 16 Aug 2012 22:01:35 -0400, Phil Hobbs ><pcdhSpamMeSenseless@electrooptical.net> wrote: > >>John Larkin wrote: >>> >>> On Thu, 16 Aug 2012 17:28:16 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>> >On 08/16/2012 05:19 PM, John Larkin wrote: >>> >> On Thu, 16 Aug 2012 12:20:49 -0700 (PDT), dagmargoodboat@yahoo.com >>> >> wrote: >>> >> >>> >>> On Aug 16, 11:29 am, Jim Thompson<To-Email-Use-The-Envelope-I...@On- >>> >>> My-Web-Site.com> wrote: >>> >>>> On Thu, 16 Aug 2012 07:39:04 -0700 (PDT), dagmargoodb...@yahoo.com >>> >>>> wrote: >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>>> On Aug 15, 10:43 pm, John Larkin >>> >>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>> >>>> >>> >>>>>> Hey, how about this? >>> >>>> >>> >>>>>> https://dl.dropbox.com/u/53724080/Circuits/Isrc_simpler.JPG >>> >>>> >>> >>>>> It's kind of a variation on Phil's active inductor. That defeats all >>> >>>>> the oscillation modes I can think of so far. (c-b, e-b, c-e)(Which >>> >>>>> means little, since circuits are so ingenious inventing new ones.) >>> >>>> >>> >>>>>> Ludicrously simple. No Miller nonsense. Might not oscillate. MMBTH81 >>> >>>>>> would have about 2 pF out, or BFT92 would have about 0.75 but lower >>> >>>>>> beta. >>> >>>> >>> >>>>> You could use the beta-cancellation trick. But, stray and parasitic >>> >>>>> capacitance will be big factors compared to a 10pF integration cap. >>> >>>> >>> >>>>>> If this works, I should have done it years ago. >>> >>>> >>> >>>> Yup. But I'd still add a base resistor, for the very reasons Gerhard >>> >>>> Hoffmann brought up... the output Z of the OpAmp is a big unknown (*). >>> >>>> >>> >>>> (*) Which brings to mind... maybe add some DC load to the OpAmp to >>> >>>> keep it out of the class-B region. >>> >>> >>> >>> I agree with both those points, and meant to mention them. I don't >>> >>> trust the op-amp to behave nicely to impulses, or its output to be >>> >>> stiff. I'd expect the output to be hi-z on this timescale. >>> >>> >>> >>> If anything I'd be tempted to bypass the bjt's base to the supply >>> >>> rail. The BJT then handles the fast stuff. Isolate that capacitive >>> >>> load from the op amp's output with a series resistor, and stabilize >>> >>> the op amp loop itself with a feed-forward cap. Standard stuff. >>> >>> >>> >>> Does it all matter? Unclamping the integration cap will feed-thru a >>> >>> small impulse through to the bjt / CCS. After that, it's pretty >>> >>> clean. So, the op-amp's output impedance might not matter, but I >>> >>> suspect it will. >>> >>> >>> >>> I invented loading LM324 outputs all by myself waayyyy back as a punk >>> >>> kid, to stiffen them up and kill the crossover THD. Of course >>> >>> everyone else on the planet thought of it too. :-) >>> >> >>> >> >>> >> Me too! And I also invented loading the charge pump phase detector in >>> >> the 4046. >>> >> >>> >> I also invented the successive-detection log video detector, and the >>> >> dual-slope ADC. I did those at times when I could have patented them. >>> >> I'd be smoking seegars on the Riviera. >>> >> >>> >> >>> >DLVAs have been around since at least the war: e.g. US Patent 2577506 to >>> >Belleville, filed 1945. >>> > >>> >Cheers >>> > >>> >Phil Hobbs >>> >>> Oh well. But I did invent the dual-slope some years before Fairchild >>> (?) patented it. >>> >> >>I used to be a big fan of the Intersil ICL7109 back in the day. Dual >>slope was magic. >> >>Cheers >> >>Phil Hobbs > >... and educated a lot of people about dielectric absorption.S&Hs were my instructor. ;-)
Reply by ●August 17, 20122012-08-17
On Friday, August 17, 2012 8:55:05 AM UTC-7, John Larkin wrote:> We could dump the b-e cap to fix the Early problem and add a base > > resistor to kill RF oscillation in the PNP. Something like this: > > > > https://dl.dropbox.com/u/53724080/Circuits/Isrc_Ib_2.JPGIt looks like the FET is expected to function without any Vds bias? And, the base-emitter capacitor was intended to combat Miller effect; so I'm thinking Rbb is the only resistance you want in its base connection. And you don't even WANT that.
Reply by ●August 17, 20122012-08-17
John Larkin wrote:> > On Thu, 16 Aug 2012 22:01:35 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >John Larkin wrote: > >> > >> On Thu, 16 Aug 2012 17:28:16 -0400, Phil Hobbs > >> <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> > >> >On 08/16/2012 05:19 PM, John Larkin wrote: > >> >> On Thu, 16 Aug 2012 12:20:49 -0700 (PDT), dagmargoodboat@yahoo.com > >> >> wrote: > >> >> > >> >>> On Aug 16, 11:29 am, Jim Thompson<To-Email-Use-The-Envelope-I...@On- > >> >>> My-Web-Site.com> wrote: > >> >>>> On Thu, 16 Aug 2012 07:39:04 -0700 (PDT), dagmargoodb...@yahoo.com > >> >>>> wrote: > >> >>>> > >> >>>> > >> >>>> > >> >>>> > >> >>>> > >> >>>> > >> >>>> > >> >>>> > >> >>>> > >> >>>>> On Aug 15, 10:43 pm, John Larkin > >> >>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >> >>>> > >> >>>>>> Hey, how about this? > >> >>>> > >> >>>>>> https://dl.dropbox.com/u/53724080/Circuits/Isrc_simpler.JPG > >> >>>> > >> >>>>> It's kind of a variation on Phil's active inductor. That defeats all > >> >>>>> the oscillation modes I can think of so far. (c-b, e-b, c-e)(Which > >> >>>>> means little, since circuits are so ingenious inventing new ones.) > >> >>>> > >> >>>>>> Ludicrously simple. No Miller nonsense. Might not oscillate. MMBTH81 > >> >>>>>> would have about 2 pF out, or BFT92 would have about 0.75 but lower > >> >>>>>> beta. > >> >>>> > >> >>>>> You could use the beta-cancellation trick. But, stray and parasitic > >> >>>>> capacitance will be big factors compared to a 10pF integration cap. > >> >>>> > >> >>>>>> If this works, I should have done it years ago. > >> >>>> > >> >>>> Yup. But I'd still add a base resistor, for the very reasons Gerhard > >> >>>> Hoffmann brought up... the output Z of the OpAmp is a big unknown (*). > >> >>>> > >> >>>> (*) Which brings to mind... maybe add some DC load to the OpAmp to > >> >>>> keep it out of the class-B region. > >> >>> > >> >>> I agree with both those points, and meant to mention them. I don't > >> >>> trust the op-amp to behave nicely to impulses, or its output to be > >> >>> stiff. I'd expect the output to be hi-z on this timescale. > >> >>> > >> >>> If anything I'd be tempted to bypass the bjt's base to the supply > >> >>> rail. The BJT then handles the fast stuff. Isolate that capacitive > >> >>> load from the op amp's output with a series resistor, and stabilize > >> >>> the op amp loop itself with a feed-forward cap. Standard stuff. > >> >>> > >> >>> Does it all matter? Unclamping the integration cap will feed-thru a > >> >>> small impulse through to the bjt / CCS. After that, it's pretty > >> >>> clean. So, the op-amp's output impedance might not matter, but I > >> >>> suspect it will. > >> >>> > >> >>> I invented loading LM324 outputs all by myself waayyyy back as a punk > >> >>> kid, to stiffen them up and kill the crossover THD. Of course > >> >>> everyone else on the planet thought of it too. :-) > >> >> > >> >> > >> >> Me too! And I also invented loading the charge pump phase detector in > >> >> the 4046. > >> >> > >> >> I also invented the successive-detection log video detector, and the > >> >> dual-slope ADC. I did those at times when I could have patented them. > >> >> I'd be smoking seegars on the Riviera. > >> >> > >> >> > >> >DLVAs have been around since at least the war: e.g. US Patent 2577506 to > >> >Belleville, filed 1945. > >> > > >> >Cheers > >> > > >> >Phil Hobbs > >> > >> Oh well. But I did invent the dual-slope some years before Fairchild > >> (?) patented it. > >> > > > >I used to be a big fan of the Intersil ICL7109 back in the day. Dual > >slope was magic. > > > >Cheers > > > >Phil Hobbs > > ... and educated a lot of people about dielectric absorption. >Yup. Using some gnarly disc ceramic made for some pretty amusing INL. 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 ●August 17, 20122012-08-17
On Fri, 17 Aug 2012 15:06:02 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> >Yup. Using some gnarly disc ceramic made for some pretty amusing INL. > >Cheers > >Phil HobbsEven polyester was none too good in that application.
Reply by ●August 17, 20122012-08-17
Spehro Pefhany wrote:> > On Fri, 17 Aug 2012 15:06:02 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > > > >Yup. Using some gnarly disc ceramic made for some pretty amusing INL. > > > >Cheers > > > >Phil Hobbs > > Even polyester was none too good in that application.IIRC BITD I usually used polystyrene, and it seemed to work fine. 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 ●August 17, 20122012-08-17
On Fri, 17 Aug 2012 16:11:01 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>Spehro Pefhany wrote: >> >> On Fri, 17 Aug 2012 15:06:02 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> > >> >Yup. Using some gnarly disc ceramic made for some pretty amusing INL. >> > >> >Cheers >> > >> >Phil Hobbs >> >> Even polyester was none too good in that application. > >IIRC BITD I usually used polystyrene, and it seemed to work fine. > >Cheers > >Phil HobbsI would have thought PS caps would be too bulky in that size range. We used PC (polycarbonate) where it mattered. I did a design that used a synthetic high-DA cap made from resistors and relatively good caps in order to linearize a sensor. Nowadays we tend to use micros for that, of course.
