On Tue, 14 Aug 2012 19:00:49 -0700 (PDT), dagmargoodboat@yahoo.com wrote:>On Aug 14, 8:10�pm, George Herold <gher...@teachspin.com> wrote: >> On Aug 14, 5:57�pm, dagmargoodb...@yahoo.com wrote: >> > On Aug 14, 4:56�pm, John Larkin wrote: >> >> > > >A bootstrap "current source" may be better in this instance. >> >> > > Probably. A follower opamp, a level shifting bandgap, and a resistor >> > > back down. >> >> > > Something like this: >> >> > >https://dl.dropbox.com/u/53724080/Circuits/Ramp.JPG >> >> > > >Alternatively, could your topology allow inverting the ramp, so you could >> > > >use NPNs? �Yes, you'd have to move your switch but that may be easier >> > > >than a positive current source. �{I asked about the switch due to ramp- >> > > >start issues, not ramp-end.} >> >> > > The switch is a PHEMT, fast and low capacitance, but definitely >> > > n-channel. >> >> > This ckt from Mr. Bloggs has pretty high cuteness: >> >> > � � � � � � � � �Vcc >> > � � � � � � � � � | >> > � � � � � � �.----+----. >> > � � � � � � �| � � � � | >> > � � � � � � �| � � � [120] >> > � � � � � � �|LM385adj | >> > � � � � � .-----. � � �| >> > � � � � � | �+ �|FB � �| >> > � � � � � | � � |------+ >> > � � � � � | �- �| � � �| >> > � � � � � '-----' � � �| >> > � � � � � � �| � � � |< >> > � � � � � � �+-------| �3906 >> > � � � � � � �| � � � |\ >> > � � � � � � �| � � � � | >> > � � � � � �[15K] � � � | >> > � � � � � � �| � � � � V >> > � � � � � � �| >> > � � � � � � === >> >> > That gets you an accurate CCS that's slow, all it needs is speed. >> >> >> Maybe hang the fast bits down below the 3906? >> >> Or isolate the lm385 from the higher frequencies? >> >> George H. > >Yes. I like miso's cascode idea, a fast pnp. > >John's original isolates the PNP with a collector bead/inductor. That >offers constant-current compliance that's potentially very fast, but >it's an LC tank. Parasitic feedback collector-to-emitter makes it >sing. I've done that many times at UHF on purpose--they oscillate >pretty nicely. > >So, I'd consider spoiling that LC's Q, in combo with Fred's ckt. If >that's too slow, cascode it. > >Total brute force: a resistor to +100v. Fast, and 1% linear over this >range.Most any transistor with a resistor in its emitter is a candidate for oscillation. The collector circuit could be soft or stiff. The usual fix is a base resistor to kill the Q of the base circuit. This is a great interview quiz: +10 | | | | c +5-----------b e | | 1K | | | gnd If they can tell you the b/c/e voltages, hire them. Currents, offer more. If they mention oscillation, offer them stock. 100 volts * 10 mA is, sadly, 1 watt. 10 volts through a resistor wouldn't be bad. A simple software hack could take out the curvature. I probably should have done that. Resistors don't often oscillate. I don't see how a cascode helps much. Beta error doubles, and now you have two transistors to oscillate. -- John Larkin 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
fast ramp follies
Started by ●August 14, 2012
Reply by ●August 15, 20122012-08-15
Reply by ●August 15, 20122012-08-15
On Tue, 14 Aug 2012 09:56:38 -0700 (PDT), "langwadt@fonz.dk" <langwadt@fonz.dk> wrote:>On 14 Aug., 05:11, John Larkin ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> OK, I need to charge a capacitor with a stable constant current. The >> desired slope is about a volt per nanosecond. >> >> So I did this: >> >> https://dl.dropbox.com/u/53724080/Circuits/Fast_Ramp.JPG >> >> Things like this tend to oscillate, so I used a fairly slow, high-beta >> transistor, BCX71K. The ferrite in the collector is supposed to >> isolate the ramp cap from the transistor capacitance and make the ramp >> linear. >> >> Well, the ramp looked not much better than an R-C curve, and the BCX71 >> oscillates at 80 MHz. Increasing R1 from 50 to 150 ohms kills the >> oscillation and makes the curvature worse. >> >> So we went to the opposite extreme, a BFT92, a 5 GHz PNP. Typ beta is >> 50! The ramp is now visually linear, and the oscillation frequency >> went up some. Tried a ferrite instead of R1, and it *really* >> oscillates. A 100 ohm base resistor seems to work. >> > >I wonder if you could (ab)use one side of a LVDS driver as a CCS? > > >-LasseI don't entirely understand LVDS drivers. Just today, Rob told me that they have pairs of dueling current sources plus a slow servo loop to center the output common-mode voltage. Sounds tricky. -- John Larkin 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 ●August 15, 20122012-08-15
On Tue, 14 Aug 2012 21:41:01 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Larkin wrote: >> >> On Tue, 14 Aug 2012 19:47:22 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >Jim Thompson wrote: >> >> >> >> On Mon, 13 Aug 2012 20:11:09 -0700, John Larkin >> >> <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >> >> >> > >> >> > >> >> >OK, I need to charge a capacitor with a stable constant current. The >> >> >desired slope is about a volt per nanosecond. >> >> > >> >> >So I did this: >> >> > >> >> >https://dl.dropbox.com/u/53724080/Circuits/Fast_Ramp.JPG >> >> > >> >> [snip] >> >> >> >> What FET is that which dumps the capacitor? >> >> >> >> At 1V/ns and 10mA, C=10pF, making the FET capacitance possibly an >> >> issue. >> >> >> >> Please provide the FET part number. >> >> >> > >> >I'm guessing an enhancement pHEMT, e.g. ATF55143. 100 mA, 5V, 0.1 pF >> >Cdg. Easily hot enough for the job. >> >> I used an NE3509, driven from an EP logic gate with an opamp based DC >> restore level shifter sort of thing, which was discussed here. I had >> to hack an 0402 resistor in series with the gate to slow it down. It >> turned off so fast that it made the early part of the ramp ring. >> >> I measured the off-state drain capacitance of the 3509 as 0.35 pF, >> mainly independent of Vd. >> >> -- >> > >Great part, but depletion mode. Do you have a negative supply?On this board, yes, for the ECL comparators. In another situation, I found that you can convert 3.3 volt CMOS to gate drive level with one 1206 size quad resistor pack, with -5 available. Even enhances a bit. Great parts. I hope NEC keeps making them. CMOS---+-----R-------+-------------gate | | | | | | +-----R-------+-----R----- -5 | | R | | gnd -- John Larkin 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 ●August 15, 20122012-08-15
John Larkin wrote:> > On Tue, 14 Aug 2012 21:41:01 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >John Larkin wrote: > >> > >> On Tue, 14 Aug 2012 19:47:22 -0400, Phil Hobbs > >> <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> > >> >Jim Thompson wrote: > >> >> > >> >> On Mon, 13 Aug 2012 20:11:09 -0700, John Larkin > >> >> <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >> >> > >> >> > > >> >> > > >> >> >OK, I need to charge a capacitor with a stable constant current. The > >> >> >desired slope is about a volt per nanosecond. > >> >> > > >> >> >So I did this: > >> >> > > >> >> >https://dl.dropbox.com/u/53724080/Circuits/Fast_Ramp.JPG > >> >> > > >> >> [snip] > >> >> > >> >> What FET is that which dumps the capacitor? > >> >> > >> >> At 1V/ns and 10mA, C=10pF, making the FET capacitance possibly an > >> >> issue. > >> >> > >> >> Please provide the FET part number. > >> >> > >> > > >> >I'm guessing an enhancement pHEMT, e.g. ATF55143. 100 mA, 5V, 0.1 pF > >> >Cdg. Easily hot enough for the job. > >> > >> I used an NE3509, driven from an EP logic gate with an opamp based DC > >> restore level shifter sort of thing, which was discussed here. I had > >> to hack an 0402 resistor in series with the gate to slow it down. It > >> turned off so fast that it made the early part of the ramp ring. > >> > >> I measured the off-state drain capacitance of the 3509 as 0.35 pF, > >> mainly independent of Vd. > >> > >> -- > >> > > > >Great part, but depletion mode. Do you have a negative supply? > > On this board, yes, for the ECL comparators. > > In another situation, I found that you can convert 3.3 volt CMOS to > gate drive level with one 1206 size quad resistor pack, with -5 > available. Even enhances a bit. > > Great parts. I hope NEC keeps making them. > > CMOS---+-----R-------+-------------gate > | | > | | > | | > +-----R-------+-----R----- -5 > | > | > R > | > | > gnd >Nice. R is sort of 1k-ish? Much more than that and you'd need a speedup cap, or so I'd think. 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 15, 20122012-08-15
On Tue, 14 Aug 2012 23:17:39 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Larkin wrote: >> >> On Tue, 14 Aug 2012 21:41:01 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >John Larkin wrote: >> >> >> >> On Tue, 14 Aug 2012 19:47:22 -0400, Phil Hobbs >> >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >> >> >Jim Thompson wrote: >> >> >> >> >> >> On Mon, 13 Aug 2012 20:11:09 -0700, John Larkin >> >> >> <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >> >> >> >> >> > >> >> >> > >> >> >> >OK, I need to charge a capacitor with a stable constant current. The >> >> >> >desired slope is about a volt per nanosecond. >> >> >> > >> >> >> >So I did this: >> >> >> > >> >> >> >https://dl.dropbox.com/u/53724080/Circuits/Fast_Ramp.JPG >> >> >> > >> >> >> [snip] >> >> >> >> >> >> What FET is that which dumps the capacitor? >> >> >> >> >> >> At 1V/ns and 10mA, C=10pF, making the FET capacitance possibly an >> >> >> issue. >> >> >> >> >> >> Please provide the FET part number. >> >> >> >> >> > >> >> >I'm guessing an enhancement pHEMT, e.g. ATF55143. 100 mA, 5V, 0.1 pF >> >> >Cdg. Easily hot enough for the job. >> >> >> >> I used an NE3509, driven from an EP logic gate with an opamp based DC >> >> restore level shifter sort of thing, which was discussed here. I had >> >> to hack an 0402 resistor in series with the gate to slow it down. It >> >> turned off so fast that it made the early part of the ramp ring. >> >> >> >> I measured the off-state drain capacitance of the 3509 as 0.35 pF, >> >> mainly independent of Vd. >> >> >> >> -- >> >> >> > >> >Great part, but depletion mode. Do you have a negative supply? >> >> On this board, yes, for the ECL comparators. >> >> In another situation, I found that you can convert 3.3 volt CMOS to >> gate drive level with one 1206 size quad resistor pack, with -5 >> available. Even enhances a bit. >> >> Great parts. I hope NEC keeps making them. >> >> CMOS---+-----R-------+-------------gate >> | | >> | | >> | | >> +-----R-------+-----R----- -5 >> | >> | >> R >> | >> | >> gnd >> > >Nice. R is sort of 1k-ish? Much more than that and you'd need a >speedup cap, or so I'd think.I used 10K recently, but I didn't need speed. A uP port pin was driving the gates to switch some range timing caps in and out. 1K is about right for fast switching. 250 ohms times, say, 0.6 pF gate capacitance is pretty speedy. -- John Larkin 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 ●August 15, 20122012-08-15
On Aug 14, 11:03=A0pm, John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:> On Tue, 14 Aug 2012 19:00:49 -0700 (PDT), dagmargoodb...@yahoo.com > wrote: > > > > > > > > > > >On Aug 14, 8:10 pm, George Herold <gher...@teachspin.com> wrote: > >> On Aug 14, 5:57 pm, dagmargoodb...@yahoo.com wrote: > > >> > This ckt from Mr. Bloggs has pretty high cuteness: > > >> >. Vcc > >> >. | > >> >. .----+----. > >> >. | | > >> >. | [120] > >> >. |LM385adj | > >> >. .-----. | > >> >. | + |FB | > >> >. | |------+ > >> >. | - | | > >> >. '-----' | > >> >. | |< > >> >. +-------| 3906 > >> >. | |\ > >> >. | | > >> >. [15K] | > >> >. | V > >> >. | > >> >. =3D=3D=3D > > >> > That gets you an accurate CCS that's slow, all it needs is speed. > > >> Maybe hang the fast bits down below the 3906? > > >> Or isolate the lm385 from the higher frequencies? > > >> George H. > > >Yes. =A0I like miso's cascode idea, a fast pnp. > > >John's original isolates the PNP with a collector bead/inductor. =A0That > >offers constant-current compliance that's potentially very fast, but > >it's an LC tank. =A0Parasitic feedback collector-to-emitter makes it > >sing. I've done that many times at UHF on purpose--they oscillate > >pretty nicely. > > >So, I'd consider spoiling that LC's Q, in combo with Fred's ckt. If > >that's too slow, cascode it. > > >Total brute force: a resistor to +100v. =A0Fast, and 1% linear over this > >range. > > Most any transistor with a resistor in its emitter is a candidate for > oscillation. The collector circuit could be soft or stiff. The usual > fix is a base resistor to kill the Q of the base circuit.The fix depends on the mode of oscillation, which I guess you could figure out probing the phase. The emitter-follower feeds back e-to-b. A resonant base ckt usually makes it oscillate easily at HF-to-VHF, so killing the base ckt Q cures that.> This is a great interview quiz: > > =A0 =A0 =A0 =A0 =A0 =A0 =A0+10 > =A0 =A0 =A0 =A0 =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0 c > +5-----------b > =A0 =A0 =A0 =A0 =A0 =A0 =A0 e > =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 =A0gnd > > If they can tell you the b/c/e voltages, hire them. Currents, offer > more. If they mention oscillation, offer them stock. > > 100 volts * 10 mA is, sadly, 1 watt.Yes, of course. Since you're kind of a devil-may-care guy, watt's a what or two, eh watt?> 10 volts through a resistor > wouldn't be bad. A simple software hack could take out the curvature. > I probably should have done that. Resistors don't often oscillate. > > I don't see how a cascode helps much. Beta error doubles, and now you > have two transistors to oscillate.You're right about the beta error of course. My first impulse would be to try RF-stabilizing the original stage by damping the collector (or VHF-bypassing the emitter), with Fred's LM385-adj for d.c. stability. Cascoded transistors (somewhat) protect each other from oscillating by breaking (or at least suppressing) the c-to-e feedback path. Hard- bypassing the cascode base is standard practice in UHF--prevents feedback nasties at the base. Cascodes don't oscillate, generally. If they did, we wouldn't love them as we do. From your frequencies, Q1's oscillation feedback is likely c-to-e (Cce). Collector-load L1 makes the stage high-gain, and L1 resonates with strays at *some* frequency. If the gain is high enough, it'll oscillate. Damping the resonance attenuates the feedback. Suppress the feedback sufficiently, kill the oscillation. -- Cheers, James Arthur
Reply by ●August 15, 20122012-08-15
On Wed, 15 Aug 2012 04:56:28 -0700 (PDT), dagmargoodboat@yahoo.com wrote:>On Aug 14, 11:03�pm, John Larkin ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> On Tue, 14 Aug 2012 19:00:49 -0700 (PDT), dagmargoodb...@yahoo.com >> wrote: >> >> >> >> >> >> >> >> >> >> >On Aug 14, 8:10 pm, George Herold <gher...@teachspin.com> wrote: >> >> On Aug 14, 5:57 pm, dagmargoodb...@yahoo.com wrote: >> >> >> > This ckt from Mr. Bloggs has pretty high cuteness: >> >> >> >. Vcc >> >> >. | >> >> >. .----+----. >> >> >. | | >> >> >. | [120] >> >> >. |LM385adj | >> >> >. .-----. | >> >> >. | + |FB | >> >> >. | |------+ >> >> >. | - | | >> >> >. '-----' | >> >> >. | |< >> >> >. +-------| 3906 >> >> >. | |\ >> >> >. | | >> >> >. [15K] | >> >> >. | V >> >> >. | >> >> >. === >> >> >> > That gets you an accurate CCS that's slow, all it needs is speed. >> >> >> Maybe hang the fast bits down below the 3906? >> >> >> Or isolate the lm385 from the higher frequencies? >> >> >> George H. >> >> >Yes. �I like miso's cascode idea, a fast pnp. >> >> >John's original isolates the PNP with a collector bead/inductor. �That >> >offers constant-current compliance that's potentially very fast, but >> >it's an LC tank. �Parasitic feedback collector-to-emitter makes it >> >sing. I've done that many times at UHF on purpose--they oscillate >> >pretty nicely. >> >> >So, I'd consider spoiling that LC's Q, in combo with Fred's ckt. If >> >that's too slow, cascode it. >> >> >Total brute force: a resistor to +100v. �Fast, and 1% linear over this >> >range. >> >> Most any transistor with a resistor in its emitter is a candidate for >> oscillation. The collector circuit could be soft or stiff. The usual >> fix is a base resistor to kill the Q of the base circuit. > >The fix depends on the mode of oscillation, which I guess you could >figure out probing the phase. The emitter-follower feeds back e-to-b. >A resonant base ckt usually makes it oscillate easily at HF-to-VHF, so >killing the base ckt Q cures that.I once theorized that the open-loop output impedance of an opamp would be about right to kill the q of a base. That was all wrong.> >> This is a great interview quiz: >> >> � � � � � � �+10 >> � � � � � � � | >> � � � � � � � | >> � � � � � � � | >> � � � � � � � | >> � � � � � � � c >> +5-----------b >> � � � � � � � e >> � � � � � � � | >> � � � � � � � | >> � � � � � � � 1K >> � � � � � � � | >> � � � � � � � | >> � � � � � � � | >> � � � � � � �gnd >> >> If they can tell you the b/c/e voltages, hire them. Currents, offer >> more. If they mention oscillation, offer them stock. >> >> 100 volts * 10 mA is, sadly, 1 watt. > >Yes, of course. Since you're kind of a devil-may-care guy, watt's a >what or two, eh watt?The specfic gadget here is powered by USB. What with crappy laptops and ultra-cheap cables (copperclad steel conductors? 28 gage? Copper is expensive!) you're lucky to get 4 volts at half an amp. Gotta start groveling for milliwatts again. I wish I had a fast current source that ran directly from V+, which might be as low as, say, 4.5 volts. That leaves 2 volts at the top of my ramp.> >> 10 volts through a resistor >> wouldn't be bad. A simple software hack could take out the curvature. >> I probably should have done that. Resistors don't often oscillate. >> >> I don't see how a cascode helps much. Beta error doubles, and now you >> have two transistors to oscillate. > >You're right about the beta error of course. My first impulse would >be to try RF-stabilizing the original stage by damping the collector >(or VHF-bypassing the emitter), with Fred's LM385-adj for d.c. >stability.Bypassing the emitter is interesting. That would be way outside the opamp loop territory. The LM7301 is 4 MHz, the oscillation is probably hundreds of MHz, so there's room to work. I can't simulate this, so I'd have to breadboard it. The base resistor works OK, but probably lowers the collector impedance and degrades the ramp a little.> >Cascoded transistors (somewhat) protect each other from oscillating by >breaking (or at least suppressing) the c-to-e feedback path. Hard- >bypassing the cascode base is standard practice in UHF--prevents >feedback nasties at the base. Cascodes don't oscillate, generally. >If they did, we wouldn't love them as we do. > >From your frequencies, Q1's oscillation feedback is likely c-to-e >(Cce). Collector-load L1 makes the stage high-gain, and L1 resonates >with strays at *some* frequency. If the gain is high enough, it'll >oscillate. Damping the resonance attenuates the feedback. Suppress >the feedback sufficiently, kill the oscillation.The ferrite isn't high-Q, and eliminating it doesn't kill oscillation. It does tend to improve ramp linearity by disconnecting the transistor capacitance from the ramp cap. Some. So the only difference between the upper transistor and the lower cascode transistor is that the cascode has a hard base bypass, and the upper is driven by the opamp? This is the thing that JT missed. He assumed we had a classic gain-phase loop dynamics oscillation (with an 800 KHz opamp and a 100 MHz oscillation! He never explained that.) What's really happening is that the transistor capacitances, the base wirebond and leads, the pcb trace, and the equivalent stuff inside the opamp (leads, wirebonds, esd diodes, finally transistors) are part of a resonant base circuit. 50 or 100 ohms of base resistor has no significant effect on closed-loop dynamics, but kills the Q of that resonant system. -- John Larkin 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 ●August 15, 20122012-08-15
On Wed, 15 Aug 2012 08:53:46 -0700, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:>On Wed, 15 Aug 2012 04:56:28 -0700 (PDT), dagmargoodboat@yahoo.com >wrote: > >>On Aug 14, 11:03�pm, John Larkin >><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>> On Tue, 14 Aug 2012 19:00:49 -0700 (PDT), dagmargoodb...@yahoo.com >>> wrote: >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >On Aug 14, 8:10 pm, George Herold <gher...@teachspin.com> wrote: >>> >> On Aug 14, 5:57 pm, dagmargoodb...@yahoo.com wrote: >>> >>> >> > This ckt from Mr. Bloggs has pretty high cuteness: >>> >>> >> >. Vcc >>> >> >. | >>> >> >. .----+----. >>> >> >. | | >>> >> >. | [120] >>> >> >. |LM385adj | >>> >> >. .-----. | >>> >> >. | + |FB | >>> >> >. | |------+ >>> >> >. | - | | >>> >> >. '-----' | >>> >> >. | |< >>> >> >. +-------| 3906 >>> >> >. | |\ >>> >> >. | | >>> >> >. [15K] | >>> >> >. | V >>> >> >. | >>> >> >. === >>> >>> >> > That gets you an accurate CCS that's slow, all it needs is speed. >>> >>> >> Maybe hang the fast bits down below the 3906? >>> >>> >> Or isolate the lm385 from the higher frequencies? >>> >>> >> George H. >>> >>> >Yes. �I like miso's cascode idea, a fast pnp. >>> >>> >John's original isolates the PNP with a collector bead/inductor. �That >>> >offers constant-current compliance that's potentially very fast, but >>> >it's an LC tank. �Parasitic feedback collector-to-emitter makes it >>> >sing. I've done that many times at UHF on purpose--they oscillate >>> >pretty nicely. >>> >>> >So, I'd consider spoiling that LC's Q, in combo with Fred's ckt. If >>> >that's too slow, cascode it. >>> >>> >Total brute force: a resistor to +100v. �Fast, and 1% linear over this >>> >range. >>> >>> Most any transistor with a resistor in its emitter is a candidate for >>> oscillation. The collector circuit could be soft or stiff. The usual >>> fix is a base resistor to kill the Q of the base circuit. >> >>The fix depends on the mode of oscillation, which I guess you could >>figure out probing the phase. The emitter-follower feeds back e-to-b. >>A resonant base ckt usually makes it oscillate easily at HF-to-VHF, so >>killing the base ckt Q cures that. > >I once theorized that the open-loop output impedance of an opamp would >be about right to kill the q of a base. That was all wrong. > >[snip] That's because an OpAmp's open-loop output impedance is not resistive. Unfortunately most manufacturers incorrectly model it that way. ...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 15, 20122012-08-15
Am 15.08.2012 18:47, schrieb Jim Thompson:>> I once theorized that the open-loop output impedance of an opamp would >> be about right to kill the q of a base. That was all wrong.> > That's because an OpAmp's open-loop output impedance is not resistive. > Unfortunately most manufacturers incorrectly model it that way.It is not so much the model but the use. A voltage source that drops 6 dB / octave looks inductive (unless one has done additional esoteric buffering etc) and adding a capacitive load yields a nice resonance. Also, someone above praised cascodes and that they would not oscillate. Nothing could be more wrong. At ReallyHighFrequency(tm) the circuit topology is not the same. The load at the collector is way above its resonance -> low, capacitive impedance. Base is grounded with the usual parasitic L, and the emitter looks into the high impedance of the driving stage. That is like the circuit of the usual UHF VCO, aka loaded follower. Base input impedance is capacitive with with say, -50 Ohm in series. Oscillates with parasitic base L. Hi, JL, does the usual negative CML gate provide enough level change to switch one of these SKY, NEC or Avago depletion FETs completely off? I think, I'll try it for my pulse stretchers :-) regards, Gerhard.
Reply by ●August 15, 20122012-08-15
On Wed, 15 Aug 2012 20:23:56 +0200, Gerhard Hoffmann <dk4xp@arcor.de> wrote:>Am 15.08.2012 18:47, schrieb Jim Thompson: > >>> I once theorized that the open-loop output impedance of an opamp would >>> be about right to kill the q of a base. That was all wrong. > >> >> That's because an OpAmp's open-loop output impedance is not resistive. >> Unfortunately most manufacturers incorrectly model it that way. > >It is not so much the model but the use. >A voltage source that drops 6 dB / octave looks inductive (unless one >has done additional esoteric buffering etc) and adding a capacitive load >yields a nice resonance.Neeeerp! OPEN-LOOP doesn't look that way. And that's where the models fail.> > > > >Also, someone above praised cascodes and that they would not oscillate. >Nothing could be more wrong.Yep. Tranny's can do gawd-awful things all by their lonesome.> >At ReallyHighFrequency(tm) the circuit topology is not the same. >The load at the collector is way above its resonance -> low, >capacitive impedance. Base is grounded with the usual parasitic L, >and the emitter looks into the high impedance of the driving stage. >That is like the circuit of the usual UHF VCO, aka loaded follower. >Base input impedance is capacitive with with say, -50 Ohm in series. >Oscillates with parasitic base L. > > > > >Hi, JL, does the usual negative CML gate provide enough level change >to switch one of these SKY, NEC or Avago depletion FETs completely off? >I think, I'll try it for my pulse stretchers :-) > >regards, Gerhard. > >...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.
