On Aug 16, 11:41=A0am, 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=A0pm, 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. =A0That 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. =A0But, 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. =A0But 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. -- Cheers, James Arthur
fast ramp follies
Started by ●August 14, 2012
Reply by ●August 16, 20122012-08-16
Reply by ●August 16, 20122012-08-16
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 don't trust a BJT current mirror to be that well-behaved under transient conditions. My inclination would be to have the BJT "charge" an inductor that takes all the abuse. The OpAmp + PNP is likely to fart.> >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. :-)YOU invented that? I was doing that in the mid '70's so I could use LM324's in active filters. I even banned Motorola as a supplier to OmniComp/GenRad of LM324's because of excessive dead-band. ...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 ●August 16, 20122012-08-16
On Thu, 16 Aug 2012 12:24:51 -0700 (PDT), dagmargoodboat@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. ...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 ●August 16, 20122012-08-16
On Aug 16, 3:59=A0pm, 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. MMBTH=81> >> >>> would have about 2 pF out, or BFT92 would have about 0.75 but lowe=r> >> >>> 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. =A0If the op amp can drive that load all by > >itself, it seems okay. =A0That 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. =A0Yeah, 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. -- Cheers, James Arthur
Reply by ●August 16, 20122012-08-16
On Thu, 16 Aug 2012 13:12:50 -0700 (PDT), dagmargoodboat@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. -- 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 16, 20122012-08-16
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. -- 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 16, 20122012-08-16
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 -- 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 16, 20122012-08-16
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 HobbsOh well. But I did invent the dual-slope some years before Fairchild (?) patented it. -- 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 16, 20122012-08-16
On Aug 16, 2:57=A0pm, whit3rd <whit...@gmail.com> wrote:> On Wednesday, August 15, 2012 1:17:18 PM UTC-7, (unknown) wrote: > > On Aug 15, 2:58=A0pm, whit3rd <whit...@gmail.com> wrote: > > > > On Monday, August 13, 2012 8:49:33 PM UTC-7, John Larkin wrote: > > > > > On Mon, 13 Aug 2012 20:38:56 -0700, miso <m...@sushi.com> wrote: > > > > > >> OK, I need to charge a capacitor with a stable constant current.=The> > > > > >> desired slope is about a volt per nanosecond. > > > > > >I'd be more inclined to cascode the current source output rather t=han> > > > > >add an inductor. > > > > > Would that help? The beta error would increase, and cascodes can > > > > > oscillate > > > > If beta 'error' is a problem,... > > > you can add a transistor base resistor and feed back to the op amp > > > > to completely remove base current > > Is this the one you're thinking of? > > >http://groups.google.com/group/sci.electronics.design/browse_thread/t... > > Looks like the same thing; BAD ASCII ART FOLLOWS > > =A0 =A0 =A0 =A0 =A0 =A0 =A0+---- 47k-----+ =A0 =A0 =A0 =A0 =A0 =A0 (Iout) > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 |> =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 |\ =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 |> Vref>--1k----+---| \ =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 |/ > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0>---+--+---47ohm----+--| > =A0 =A0 =A0 =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 =A0 =A0 =A0 =A0 =A0 =A0 ==A0 | =A0|\> =A0 =A0 =A0 =A0 =A0 =A0 =A0+----------+---+--47k------+ =A0 =A0V > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =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(1ohm)> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 =A0|> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 GND> > The use of a cascode can be folded into this scheme, by (1) using matched > transistors for the cascode (2) doubling the feedback (to compensate two > base currents, while only sensing one). =A0 A 1.2V reference as well as 2=.5V cascode> base bias can be had from a LV431 style programmable zener; set it for 2.=5V> and the sense node gives you a stable 1.25V..Yes. As cute as i(b) cancellation is, Phil's pseudo-darlington has two advantages-- 1. by reducing i(b), the i(b) error becomes less important, and 2. it should be more linear, since it removes Early's effect. -- Cheers, James Arthur
Reply by ●August 16, 20122012-08-16
On Aug 16, 5:15=A0pm, 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=A0pm, 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)(Whic=h> >> >> >>means little, since circuits are so ingenious inventing new ones.=)> > >> >> >>> Ludicrously simple. No Miller nonsense. Might not oscillate. MM=BTH81> >> >> >>> would have about 2 pF out, or BFT92 would have about 0.75 but l=ower> >> >> >>> beta. > > >> >> >>You could use the beta-cancellation trick. But, stray and parasit=ic> >> >> >>capacitance will be big factors compared to a 10pF integration ca=p.> > >> >> >>> If this works, I should have done it years ago. > > >> >> >Yup. But I'd still add a base resistor, for the very reasons Gerha=rd> >> >> >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 matt=er> >> >> any more. > > >> >Yep, that's a good point. =A0If the op amp can drive that load all by > >> >itself, it seems okay. =A0That 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. =A0Yeah, 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. =A0So, the transistor's fine. > > >In the longer term a load-step on the op amp's output may make it > >overshoot or ring. =A0That would happen opening and closing the shorting > >switch. =A0Probably 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. =A0IOW, 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. -- Cheers, James Arthur
