Forums

fast ramp follies

Started by John Larkin August 14, 2012
On Tue, 14 Aug 2012 16:01:33 +0000 (UTC), Frank Miles
<fpm@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. 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. > >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 over temperature too, but the beta-50 RF transistor hurts there. There's no temperature sensor on this board, otherwise I could do some compensation 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 have a lot of capacitance, and they don't make p-channel PHEMTS. 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. All >these transistor topologies are fun, but unless we know what's needed...
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. -- 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 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? -Lasse
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
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 -- | 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 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
On Tue, 14 Aug 2012 09:16:35 -0700, John Larkin wrote:

> On Tue, 14 Aug 2012 16:01:33 +0000 (UTC), Frank Miles > <fpm@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. 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. >> >>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 over > temperature too, but the beta-50 RF transistor hurts there. There's no > temperature sensor on this board, otherwise I could do some compensation > 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 have a > lot of capacitance, and they don't make p-channel PHEMTS. > > 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. All >>these transistor topologies are fun, but unless we know what's needed... > > 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. With a node capacitance of 10pF, 2% linearity doesn't give you much leeway there. How far does this sweep have to go? A bootstrap "current source" may be better in this instance. 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.}
On Tue, 14 Aug 2012 20:01:38 +0000 (UTC), Frank Miles
<fpm@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 >> <fpm@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. 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. >>> >>>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 over >> temperature too, but the beta-50 RF transistor hurts there. There's no >> temperature sensor on this board, otherwise I could do some compensation >> 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 have a >> lot of capacitance, and they don't make p-channel PHEMTS. >> >> 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. All >>>these transistor topologies are fun, but unless we know what's needed... >> >> 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. With a node capacitance of 10pF, 2% linearity >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. 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 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. -- 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
And leave the mecca of enlightenment: the SF Bay Area! No thanks.

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. 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-be=
ta
> >>>> transistor, BCX71K. The ferrite in the collector is supposed to > >>>> isolate the ramp cap from the transistor capacitance and make the ra=
mp
> >>>> linear. > > >>>> Well, the ramp looked not much better than an R-C curve, and the BCX=
71
> >>>> 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 i=
s
> >>>> 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 over > >> temperature too, but the beta-50 RF transistor hurts there. There's no > >> temperature sensor on this board, otherwise I could do some compensati=
on
> >> in software, crude shot at beta compensation. I really need some day t=
o
> >> find a fast, low-capacitance PNP with more beta. Mosfets tend to have =
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 needed.=
..
> > >> 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 fo=
r
> >PNPs looks pretty bad. =A0With a node capacitance of 10pF, 2% linearity > >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 coul=
d
> >use NPNs? =A0Yes, you'd have to move your switch but that may be easier > >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: Vcc | .----+----. | | | [120] |LM385adj | .-----. | | + |FB | | |------+ | - | | '-----' | | |< +-------| 3906 | |\ | | [15K] | | V | =3D=3D=3D That gets you an accurate CCS that's slow, all it needs is speed. -- Cheers, James Arthur
On Tue, 14 Aug 2012 14:57:24 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

>On Aug 14, 4:56&#2013266080;pm, 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. 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. >> >> >>>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 over >> >> temperature too, but the beta-50 RF transistor hurts there. There's no >> >> temperature sensor on this board, otherwise I could do some compensation >> >> 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 have a >> >> lot of capacitance, and they don't make p-channel PHEMTS. >> >> >> &#2013266080; 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. &#2013266080;All >> >>>these transistor topologies are fun, but unless we know what's needed... >> >> >> 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. &#2013266080;With a node capacitance of 10pF, 2% linearity >> >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. >> >> &#2013266080; 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 could >> >use NPNs? &#2013266080;Yes, you'd have to move your switch but that may be easier >> >than a positive current source. &#2013266080;{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.
That is elegant. A couple more resistors would isolate the fast stuff and maybe keep the PNP from oscillating.
> Vcc > | > .----+----. > | | > | [120] > |LM385adj | > .-----. | > | + |FB | > | |---Re-+ > | - | | > '-----' | > | |< > +-Rb----| 3906 > | |\ > | | > [15K] | > | V > | > ===
and maybe a cap across the LM385? The LM385-adj data sheet has a similar circuit, with a Darlington. -- 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