Forums

fast ramp follies

Started by John Larkin August 14, 2012
Am 15.08.2012 22:04, schrieb dagmargoodboat@yahoo.com:
> On Aug 15, 2:23 pm, Gerhard Hoffmann <dk...@arcor.de> wrote:
>> Also, someone above praised cascodes and that they would not oscillate. >> Nothing could be more wrong. > > That was me. They don't generally oscillate, meaning that's not a > general, usual result. They certainly can--I made a cascode > oscillator-frequency multiplier once. It worked amazingly well. > >> 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, > > Hopefully base inductance is minimal. If L(base bond wire) is a > typical SOT-23-ish 1.5nH, that resonates with 10pF at 1.3GHz. I > assumed that's not the feedback mechanism, since John's first > transistor oscillated at 70MHz.
1.5nH for the base lead is ok, but add 1nH for the via to gnd. And if you are not really lucky and have dual supplies, the base won't be at GND level but a few volts higher, so a cap is needed for the RF. That takes some mm, and also some nH. The base capacitance of a BFR93/BFT93 (my standard workhorses) also seems to be more than 10 pF, depending strongly on emitter impedance. So, while 70 MHz seems lowish, it's not impossible.
>> 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. > > If that's the mode, a base resistor would kill it, agreed?
Yes. But, even at 100 Ohms, the margin would not be really large. And it adds to Rbb, which is the primary noise generator of the transistor. (internal Rbb' = 5R typ.) Ok, in a cascode that would have to be divided by beta.
>> regards, Gerhard.
whit3rd wrote:
> > On Wednesday, August 15, 2012 1:34:08 PM UTC-7, Phil Hobbs wrote: > > whit3rd wrote: > > > > > If beta 'error' is a problem, either use a transistor that's sorted by beta > > > > > (rather than the less-expensive 'general purpose' part number), or > > > > > you can add a transistor base resistor and feed back to the op amp > > > > > to completely remove base current > > > The beta feedback trick is cute, but it relies on the op amp to > > > > compensate, and so only works at low frequency. It also doesn't fix > > > > Early effect at high frequency, whereas a properly designed cascode > > > > does. > > Just as voltage regulators rely on bypass capacitors at high frequency, > I'd think of a series inductor to keep the current source happy at high > frequencies. I wonder, though, how far one could get with B-E capacitance > (with a HF transistor that has relatively small B-C capacitance)? > Either way, oscillation is possible and layout will matter.
It's really tough to get the sort of SRF you'd need to do it with a real inductor. A good RF transistor has an output capacitance of well below 1 pF, and small NPNs can be below 0.25 pF. One interesting trick for this is to use a simulated inductor, i.e. a one-pole capacitance multiplier floating--like so: --> I_in 0--*-------- -------*--0 | \ A | | ----- | | | | *--RRRR----*----| |--* That has no base current error, and lets you use NPNs for the high-frequency task even in a current-sourcing application. Something like a BFG25AW is often a good choice. With another resistor across the capacitor, you could use a depletion pHEMT and get another order of magnitude in bandwidth and probably two orders in Q.
> Early effect should only add repeatable curvature, I'd be concerned > only because it isn't usually a well-tested parameter.
An RC would be a repeatable curvature too, and would have the additional advantage of being known *a priori*, as opposed to Early effect, which is actually not linear and at least as variable as beta. You're right, though, the Early curvature would be ~10x smaller. Fun stuff. 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
John Larkin wrote:
> > 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.
I'm doing a bunch of that at the moment too, but only at low frequency. It would be really great if analogue muxes were as good as a handful of 2N7002s. Of course DG408s are way under a buck now, which is some consolation, but still--15 pF and 100 ohms? 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
Am 15.08.2012 22:15, schrieb John Larkin:
> On Wed, 15 Aug 2012 20:23:56 +0200, Gerhard Hoffmann <dk4xp@arcor.de> > wrote:
>> 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. >
> The -6 rolloff is usually done somewhere deep inside the chip.
Usually in the VAS stage, but the output of the OpAmp is where the feedback is taken from, so loading that node will have a first-order effect.
> At 100 > MHz, the opamp gain as such is very low and the rolloff is a lot > steeper than -6.
A few octaves at -12 would guarantee > 180 deg at unity gain. Then we do not have to search for other culprits.
> Fast transistors used as classic emitter followers (stiff bypassed > base and collector) tend to oscillate. They don't need additional > inductance. A base resistor will kill the oscillation, but an opamp > output impedance usually won't.
Well, the output impedance of an OpAmp is meant as max. Ohms worst case, so a nice op amp will feature less... Somebody with a VNA could measure the
> impedance of some opamp outputs in the hundreds of MHz... that would > be interesting. I suspect the low frequency loop (follower, inverter, > whatever) wouldn't change things much at high frequencies.
Aaaaargh... exploding daytime schedule, and you drop sth. interesting b4 my feet... Preferred type, conditions? May take a week!
>> 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 :-) > > If you terminate them as you're supposed to, you only get 0.4 volts > single-ended swing, not enough. The NE3508/9 can be driven nicely from > a full-swing ECL (eclips lite, eclips plus) gate, with margin to > enhance a bit (+0.2 to -0.6 maybe), or full-swing ECL can drive the > neat Avago enhancement parts.
:-)
> MC10EL/EP89 swings about 2 volts, which can drive meatier depletion > fets that want more swing. Beyond that, it's hard to get fast edges > with volts of swing.
Yes, I got the promised double ECL swing from SY100EP89, but the ADCMP580 was so much faster :-) http://www.imagesup.de/bild-_Mgif-119362.htm I have played with the ADN2525 laser driver, I would prefer the 2531, but digikey does not stock them. Should yield 2 * 2V into 2* 50R with tr<30ns, just right for our time distribution system. It doesn't still work as I want it; maybe because I drove it single-ended, which is deprecated. I'll ask our local AD distributor for the 2531 (2*50R).
> I took a bunch of data on the NE3508/9 if you're interested.
YES YES YES!!!
> > Bummer that the RF boys usually furnish pitiful DC/time domain specs.
<sigh> Gerhard
On Aug 15, 4:34=A0pm, Phil Hobbs
<pcdhSpamMeSensel...@electrooptical.net> wrote:

[...]

> The NPN/PNP series-shunt pair (the one that folks with nothing better to > do are are slagging each other off about at the moment) runs the driver > transistor at constant I_C, and lets you use a much faster NPN to > stiffen the PNP at high frequency. =A0Try simulating it--it's really good > medicine. > > (I spent most of July up to my ears in high performance laser noise > cancellers, which rely on a lot of those sorts of tricks, so I'm pretty > well up on all this at the moment.)
I like the pseudo-Darlington, I'm just leery because I had those scream wickedly once using fast transistors. I don't think I ever fixed it. It has a bunch of advantages here, including reducing the base current i(b) error. Here's a skeleton idea for a Bloggs-Hobbs hybrid: Vcc Vcc -+- -+- | | | .-. R1 | U1 | | 120 | LM385 '-' | -adj | .---' | A<-----R2-----+---------. | | | | |<' Q1 | +-----+-----| BFT92 | | | |\ | R4 .-. --- C1 | |/ Q2 1K | | --- +--f.b.-| BFP640 '-' | | |>. | =3D=3D=3D .-. | =3D=3D=3D | | R3 | '-' 600 | | | '---------+----> R2's a pathetic provision for isolating U1 from the nS stuff. The whole thing needs stabilizing against UHF oscillation, these are just the bones. -- Cheers, James Arthur
dagmargoodboat@yahoo.com wrote:
> > On Aug 15, 4:34 pm, Phil Hobbs > <pcdhSpamMeSensel...@electrooptical.net> wrote: > > [...] > > > The NPN/PNP series-shunt pair (the one that folks with nothing better to > > do are are slagging each other off about at the moment) runs the driver > > transistor at constant I_C, and lets you use a much faster NPN to > > stiffen the PNP at high frequency. Try simulating it--it's really good > > medicine. > > > > (I spent most of July up to my ears in high performance laser noise > > cancellers, which rely on a lot of those sorts of tricks, so I'm pretty > > well up on all this at the moment.) > > I like the pseudo-Darlington, I'm just leery because I had those > scream wickedly once using fast transistors. I don't think I ever > fixed it. > > It has a bunch of advantages here, including reducing the base current > i(b) error. > > Here's a skeleton idea for a Bloggs-Hobbs hybrid: > > Vcc Vcc > -+- -+- > | | > | .-. R1 > | U1 | | 120 > | LM385 '-' > | -adj | > .---' | > A<-----R2-----+---------. > | | | > | |<' Q1 | > +-----+-----| BFT92 | > | | |\ | > R4 .-. --- C1 | |/ Q2 > 1K | | --- +--f.b.-| BFP640 > '-' | | |>. > | === .-. | > === | | R3 | > '-' 600 | > | | > '---------+----> > > R2's a pathetic provision for isolating U1 from the nS stuff. The > whole thing needs stabilizing against UHF oscillation, these are just > the bones. > > -- > Cheers, > James Arthur
It wouldn't need much more than that, I don't think. Series-shunt pairs have poor stability when the transistors are about the same speed, due to having two lags in the loop, but the BFP640 is ~10x faster than the BFT92, which helps a lot. (Reducing the BFT92's collector current makes the difference wider and helps with stability.) I used the BFP640 as a cascode for an ATF38143 pHEMT in a very quiet charge-sensitive preamp (noise floor ~10 electrons per inverse bandwidth), and all it needed was an 0402 bead in the base--no worries at all. 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
On Thu, 16 Aug 2012 00:48:30 +0200, Gerhard Hoffmann <dk4xp@arcor.de>
wrote:

>Am 15.08.2012 22:15, schrieb John Larkin: >> On Wed, 15 Aug 2012 20:23:56 +0200, Gerhard Hoffmann <dk4xp@arcor.de> >> wrote: > >>> 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. >> > > >> The -6 rolloff is usually done somewhere deep inside the chip. > >Usually in the VAS stage, but the output of the OpAmp is where >the feedback is taken from, so loading that node will have a >first-order effect. > >> At 100 >> MHz, the opamp gain as such is very low and the rolloff is a lot >> steeper than -6. > >A few octaves at -12 would guarantee > 180 deg at unity gain. >Then we do not have to search for other culprits. > > >> Fast transistors used as classic emitter followers (stiff bypassed >> base and collector) tend to oscillate. They don't need additional >> inductance. A base resistor will kill the oscillation, but an opamp >> output impedance usually won't. > >Well, the output impedance of an OpAmp is meant as max. Ohms worst case, >so a nice op amp will feature less... > > > >Somebody with a VNA could measure the >> impedance of some opamp outputs in the hundreds of MHz... that would >> be interesting. I suspect the low frequency loop (follower, inverter, >> whatever) wouldn't change things much at high frequencies. > >Aaaaargh... exploding daytime schedule, and you drop sth. >interesting b4 my feet... >Preferred type, conditions? May take a week! > > > >>> 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 :-) >> >> If you terminate them as you're supposed to, you only get 0.4 volts >> single-ended swing, not enough. The NE3508/9 can be driven nicely from >> a full-swing ECL (eclips lite, eclips plus) gate, with margin to >> enhance a bit (+0.2 to -0.6 maybe), or full-swing ECL can drive the >> neat Avago enhancement parts. > >:-) > >> MC10EL/EP89 swings about 2 volts, which can drive meatier depletion >> fets that want more swing. Beyond that, it's hard to get fast edges >> with volts of swing. > >Yes, I got the promised double ECL swing from SY100EP89, but the >ADCMP580 was so much faster :-) > >http://www.imagesup.de/bild-_Mgif-119362.htm
No luck on that link so far. I know of a part that will swing 0.8 volts in 40 ps, and it's almost affordable, roughly dinner for two with beer.
>
Hittite and Inphi make really fast flops and gates. Some of the Hittite parts have 12 ps edges... for $500.
>I have played with the ADN2525 laser driver, I would prefer >the 2531, but digikey does not stock them. Should yield >2 * 2V into 2* 50R with tr<30ns, just right for our time >distribution system. It doesn't still work as I want it; maybe >because I drove it single-ended, which is deprecated. >I'll ask our local AD distributor for the 2531 (2*50R).
We are using the ADN2871 as a driver for, umm, other stuff, not a laser. It was hard to convince them that we were worthy of using it. We are DC coupling into its data inputs from 2.5 volts CML. It's a "single loop" part so is somewhat less weird than some of their other drivers. Maxim has stuff, too.
> >> I took a bunch of data on the NE3508/9 if you're interested. > >YES YES YES!!! >
See if this works... https://www.dropbox.com/sh/0ks3yqzwmm0vb7h/24WLWZj7-0
>> >> Bummer that the RF boys usually furnish pitiful DC/time domain specs. > ><sigh> > >Gerhard > >
-- 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
"John Larkin" <jlarkin@highlandtechnology.com> wrote in message 
news:a25o28h9evafmajhm9rh67ldnfqocpu93i@4ax.com...
 A b-e cap on a fast RF PNP, inside my slow opamp loop, is interesting.
> I could get rid of some other parts, maybe. The cap could be pretty > big, because it doesn't do any harm to the opamp loop dynamics. > > How's it gonna oscillate with the base shorted to the emitter? [1] > > [1] wishful thinking
Back in The Day, there was a thing called a Reactance Modulator. This was typically a pentode hung off the local oscillator tank circuit, with a relatively large resistance coupled to the grid. The remaining grid capacitance (which is almost exactly G1-G2 and G-K capacitance; pentodes have G-P capacitance enviable even by PHEMT standards!) acts against the series resistance as an almost exact 90 degree phase shift, thus the tube looks like a reactance (exercise for the student: prove this relation and what phase shift it produces). By varying the gain (by varying bias), the reactance is changed, and therefore the effective inductance (or capacitance) of the tank circuit. It's like that, but with more phase shift ;-) Tim P.S. Had some nice local craft beers this evening. I'm happy ;-) -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
On Wed, 15 Aug 2012 20:20:54 -0500, "Tim Williams"
<tmoranwms@charter.net> wrote:

>"John Larkin" <jlarkin@highlandtechnology.com> wrote in message >news:a25o28h9evafmajhm9rh67ldnfqocpu93i@4ax.com... > A b-e cap on a fast RF PNP, inside my slow opamp loop, is interesting. >> I could get rid of some other parts, maybe. The cap could be pretty >> big, because it doesn't do any harm to the opamp loop dynamics. >> >> How's it gonna oscillate with the base shorted to the emitter? [1] >> >> [1] wishful thinking > >Back in The Day, there was a thing called a Reactance Modulator. This was >typically a pentode hung off the local oscillator tank circuit, with a >relatively large resistance coupled to the grid. The remaining grid >capacitance (which is almost exactly G1-G2 and G-K capacitance; pentodes >have G-P capacitance enviable even by PHEMT standards!) acts against the >series resistance as an almost exact 90 degree phase shift, thus the tube >looks like a reactance (exercise for the student: prove this relation and >what phase shift it produces). By varying the gain (by varying bias), the >reactance is changed, and therefore the effective inductance (or >capacitance) of the tank circuit. > >It's like that, but with more phase shift ;-) > >Tim > >P.S. Had some nice local craft beers this evening. I'm happy ;-)
Happy or hoppy? Most "craft" beers are way too bitter for my taste, and most gourmet red wines are way too tannic. Just got some nice wheat ale that my engineers brewed at home... gotta try it. Hey, how about this? https://dl.dropbox.com/u/53724080/Circuits/Isrc_simpler.JPG 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. If this works, I should have done it years ago. -- 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
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message news:5jmo28h8uq1ilq9najgnnbtkql2bjrcke9@4ax.com...
> Happy or hoppy? Most "craft" beers are way too bitter for my taste, > and most gourmet red wines are way too tannic. Just got some nice > wheat ale that my engineers brewed at home... gotta try it.
I like darker things, there's a lot of good stouts, porters and ales around here. Hops can be good and bad. Something like, say, Heineken, is very hoppy but also light and drinkable. A lot of lagers are excessively hopped and unappealing.
> Hey, how about this? > > https://dl.dropbox.com/u/53724080/Circuits/Isrc_simpler.JPG > > 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.
I don't see a cap helping you there. Ccb puts signal directly on the op-amp, coupling the active node to its nonlinearity, and Cbe in turn couples this to R_E, which puts a (second degree) resistor back in the circuit. Might as well use a ferrite bead, but you already know how sloppy that works out. Better to use a cascode, where the "outer" transistor is as close to the active node as possible, with as little base resistance as possible (just enough to keep it stable), so you get Ccb incorporated as closely as possible into the node. At that point if you still need temp or bias compensation, you won't be any worse off, and you'll most likely be much better off than any other method. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms