Forums

fast ramp follies

Started by John Larkin August 14, 2012
On Tue, 14 Aug 2012 14:53:38 -0700, miso <miso@sushi.com> wrote:

> >And leave the mecca of enlightenment: the SF Bay Area! No thanks.
It's 67F in SF at this instant, 109 in Phoenix. We have better views, too. -- 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
On Tue, 14 Aug 2012 15:48:29 -0500, legg <legg@nospam.magma.ca> wrote:

>On Tue, 14 Aug 2012 11:42:39 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: > >>On Tue, 14 Aug 2012 14:02:15 -0500, legg <legg@nospam.magma.ca> 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 >>>> >>>>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. >>> >>>Maybe it's just timing. >>> >>>Reset always on. >>>Start current source. >>>delay >>>release reset..... >>> >>>RL >> >>What's missing that is clearly obvious to the most casual observer? >> >>Larkin will now invect (my latest invented word :-) >> >>Folks in the Phoenix area are invited to come by (*), and I'll show >>how easy it is to do. But I'll never post it here. I want everyone >>to observe how profoundly ignorant Larkin is ;-) >> >>(*) Not between 12 and 2 today... big pow-wow with the patent >>attorneys. >> >> ...Jim Thompson > >? > >RL
Expert witness in a patent infringement case. ...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.
On Tue, 14 Aug 2012 15:14:22 -0700, John Larkin
<jlarkin@highlandtechnology.com> wrote:

>On Tue, 14 Aug 2012 14:53:38 -0700, miso <miso@sushi.com> wrote: > >> >>And leave the mecca of enlightenment: the SF Bay Area! No thanks. > >It's 67F in SF at this instant, 109 in Phoenix. We have better views, >too.
Really? A bunch of old buildings, occupied by druggies, in Californica yet. I have owls, hawks, coyotes, rabbits, quail, etc, with a beautiful hillside as my back yard. Ask Spehro. ...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.
Well der's yer problem -- the 1k ferrite bead looks like, amazingly enough, 
about 1k ohm for fast edges.  What's C_t and the various C_j's, a few pFs? 
That's a good couple nanosecond time constant right there.  It's relative to 
Q1's capacitance, which makes it more of a pole-zero-pole 
double-time-constant looking thing, but with R1 there, Q1's free to drive 
pretty well as much Miller capacitance as it likes to.

As said elsewhere, cascode Q1, from more voltage if available.  There are 
sub-1pF PNPs available, if not many.  Alternatives: bootstrap (also 
mentioned), invert (PMOS switch, NPN sink), etc.

There, now where's my $20 consulting fee?  ;-)

Tim

-- 
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message 
news:uhfj2895b57gtbmbe77pqc5kt7onadtm5s@4ax.com...
> > > 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. > > > -- > > 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
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. ...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.
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. 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 Aug 14, 5:57=A0pm, dagmargoodb...@yahoo.com wrote:
> On Aug 14, 4:56=A0pm, John Larkin <jlar...@highlandtechnology.com> > wrote: > > > > > > > On Tue, 14 Aug 2012 20:01:38 +0000 (UTC), Frank Miles > > > <f...@u.washington.edu> wrote: > > >On Tue, 14 Aug 2012 09:16:35 -0700, John Larkin wrote: > > > >> On Tue, 14 Aug 2012 16:01:33 +0000 (UTC), Frank Miles > > >> <f...@u.washington.edu> wrote: > > > >>>On Mon, 13 Aug 2012 20:11:09 -0700, John Larkin wrote: > > > >>>> OK, I need to charge a capacitor with a stable constant current. T=
he
> > >>>> 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 B=
CX71
> > >>>> 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. > > > >>>You haven't specified exactly how linear it _has_ to be, nor how > > >>>accurate. > > > >> A couple per cent linearity would be OK. I'd like a percent or so ov=
er
> > >> temperature too, but the beta-50 RF transistor hurts there. There's =
no
> > >> temperature sensor on this board, otherwise I could do some compensa=
tion
> > >> in software, crude shot at beta compensation. I really need some day=
to
> > >> find a fast, low-capacitance PNP with more beta. Mosfets tend to hav=
e a
> > >> lot of capacitance, and they don't make p-channel PHEMTS. > > > >> =A0 What kind of reset restrictions are there (e.g. recovery). > > > >> Well, the fet shorts out the cap, so it discharges pretty fast. That=
's
> > >> not a current source issue. > > > >>>And we're guessing that you don't have any higher voltage supplies. =
=A0All
> > >>>these transistor topologies are fun, but unless we know what's neede=
d...
> > > >> The box runs off 5 volts from USB, which can actually be 4.5 for a > > >> rotten laptop and a cheap cable. I made a little charge pump from an > > >> HC14 and a dual diode, to make +8 or so. That seems OK. > > > >I think some more voltage would help - looking at the Ccb vs. voltage =
for
> > >PNPs looks pretty bad. =A0With a node capacitance of 10pF, 2% linearit=
y
> > >doesn't give you much leeway there. > > > There are junction capacitances (aka varicaps) everywhere in a circuit > > like this. One prays that some of them will sort of cancel. > > > =A0 How far does this sweep have to go? > > > It's driving some comparators to make delays. The ECL comparators have > > a max common-mode voltage spec of +3, and they seem to be serious > > about that. When USB drops to 4.5, it's even less. So, say we want a > > max of +2.5 volts. > > > >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 co=
uld
> > >use NPNs? =A0Yes, you'd have to move your switch but that may be easie=
r
> > >than a positive current source. =A0{I asked about the switch due to ra=
mp-
> > >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: > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Vcc > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0.----+----. > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 [120] > =A0 =A0 =A0 =A0 =A0 =A0 =A0|LM385adj | > =A0 =A0 =A0 =A0 =A0 .-----. =A0 =A0 =A0| > =A0 =A0 =A0 =A0 =A0 | =A0+ =A0|FB =A0 =A0| > =A0 =A0 =A0 =A0 =A0 | =A0 =A0 |------+ > =A0 =A0 =A0 =A0 =A0 | =A0- =A0| =A0 =A0 =A0| > =A0 =A0 =A0 =A0 =A0 '-----' =A0 =A0 =A0| > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 |< > =A0 =A0 =A0 =A0 =A0 =A0 =A0+-------| =A03906 > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 |\ > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0[15K] =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 V > =A0 =A0 =A0 =A0 =A0 =A0 =A0| > =A0 =A0 =A0 =A0 =A0 =A0 =3D=3D=3D > > That gets you an accurate CCS that's slow, all it needs is speed. > > -- > Cheers, > James Arthur- Hide quoted text - > > - Show quoted text -
Maybe hang the fast bits down below the 3906? Or isolate the lm385 from the higher frequencies? George H.
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. -- 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 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? 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 Aug 14, 8:10=A0pm, George Herold <gher...@teachspin.com> wrote:
> On Aug 14, 5:57=A0pm, dagmargoodb...@yahoo.com wrote: > > On Aug 14, 4:56=A0pm, 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? =A0Yes, you'd have to move your switch but that may be eas=
ier
> > > >than a positive current source. =A0{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: > > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Vcc > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | > > =A0 =A0 =A0 =A0 =A0 =A0 =A0.----+----. > > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 | > > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 [120] > > =A0 =A0 =A0 =A0 =A0 =A0 =A0|LM385adj | > > =A0 =A0 =A0 =A0 =A0 .-----. =A0 =A0 =A0| > > =A0 =A0 =A0 =A0 =A0 | =A0+ =A0|FB =A0 =A0| > > =A0 =A0 =A0 =A0 =A0 | =A0 =A0 |------+ > > =A0 =A0 =A0 =A0 =A0 | =A0- =A0| =A0 =A0 =A0| > > =A0 =A0 =A0 =A0 =A0 '-----' =A0 =A0 =A0| > > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 |< > > =A0 =A0 =A0 =A0 =A0 =A0 =A0+-------| =A03906 > > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 |\ > > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 | > > =A0 =A0 =A0 =A0 =A0 =A0[15K] =A0 =A0 =A0 | > > =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 V > > =A0 =A0 =A0 =A0 =A0 =A0 =A0| > > =A0 =A0 =A0 =A0 =A0 =A0 =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. 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. -- Cheers, James Arthur