Forums

fast ramp follies

Started by John Larkin August 14, 2012

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 > >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 thought you said that "resetting a filter" worked? You'd be better off with a simple R-C (or R-L-C), and do the compensation with the following comparator reference. ...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 8/13/2012 8:11 PM, 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. > >
I'd be more inclined to cascode the current source output rather than add an inductor.
On Mon, 13 Aug 2012 20:38:56 -0700, miso <miso@sushi.com> wrote:

>On 8/13/2012 8:11 PM, 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. >> >> >I'd be more inclined to cascode the current source output rather than >add an inductor.
Would that help? The beta error would increase, and cascodes can oscillate too. There's hardly any difference. -- 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 8/13/2012 8:49 PM, John Larkin wrote:
> On Mon, 13 Aug 2012 20:38:56 -0700, miso <miso@sushi.com> wrote: > >> On 8/13/2012 8:11 PM, 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. >>> >>> >> I'd be more inclined to cascode the current source output rather than >> add an inductor. > > Would that help? The beta error would increase, and cascodes can > oscillate too. There's hardly any difference. > > > >
Cascodes are the classic way to isolate circuits, which in this case is basically improving the reverse parameters. [Think of how they kill the MIller effect in input states.] The would also increase the output impedance of the circuit, which would make the ramp straighter. Since the circuit uses feedback, you don't need the precision (i.e. predictability) of a bipolar. Maybe a P-fet can be used, then there is no beta issue.
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. >
Yup. Beads aren't that nice for this sort of job. If you make the BFT92 into a series-shunt pair, i.e. put a resistor in series with its collector and wrap a super-hot NPN around it, 0 | *-----------* BFT92 | | |< | 0---------| | |\ | | | | |/ *--BEAD---| BFP640 | |> R | R | R | | | *-----------* | 0 you get higher collector impedance since the BFT92 runs at lower and constant I_C, and the BFP640's VAF is effectively infinite. Also, since the BFP640 is so fast, you can push the unity gain cross of the feedback pair out far enough for a 10- or 30-ohm bead to do a good job of stabilizing it, without causing your ramp to roll over. Since your barefoot BFT92 is working, there's no reason to do this on this particular board, of course, but it's a cute trick. (I've been investigating all sorts of these kinds of local feedback things recently.) 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, 5:11=A0am, 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
<snip>
> 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.
We found that something between 22R and 33R right up against the base was what it took to stop a BFT92 from oscillating. Is L1 - I presume FB means "ferrite bead" - actually doing anything useful? All it seems to be doing is eating up some of the collector- emitter bias on Q1, increasing the collector-base and collector- emitter capacitances, which isn't in any way a useful contribution. Putting it in series with the emitter of Q1 might make more sense (above the feedback connection to the op amp rather than after it). If you wanted to go nuts, you could replace Q1 with a complementary Darlington, with a BFR92 delivering 9mA of your 10mA current - you'd need to put at least 1mA through the BFT93 to keep it quick, and the BFR92 needs a 22R base-stopper, just like the BFT93 - but it would reduce the base current error by a factor of ten, it it worked. In real life they tend to oscillate, but the parts are cheap so it's easy enough to find out. -- Bill Sloman, Nijmegen
On Aug 14, 12:08=A0am, miso <m...@sushi.com> wrote:
> On 8/13/2012 8:49 PM, John Larkin wrote: > > > > > > > > > > > On Mon, 13 Aug 2012 20:38:56 -0700, miso <m...@sushi.com> wrote: > > >> On 8/13/2012 8:11 PM, 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-bet=
a
> >>> transistor, BCX71K. The ferrite in the collector is supposed to > >>> isolate the ramp cap from the transistor capacitance and make the ram=
p
> >>> linear. > > >>> Well, the ramp looked not much better than an R-C curve, and the BCX7=
1
> >>> 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'd be more inclined to cascode the current source output rather than > >> add an inductor. > > > Would that help? The beta error would increase, and cascodes can > > oscillate too. There's hardly any difference. > > Cascodes are the classic way to isolate circuits, which in this case is > basically improving the reverse parameters. [Think of how they kill the > MIller effect in input states.] The would also increase the output > impedance of the circuit, which would make the ramp straighter. > > Since the circuit uses feedback, you don't need the precision (i.e. > predictability) of a bipolar. Maybe a P-fet can be used, then there is > no beta issue.
Or, damp L1 with a parallel resistor. Right now it looks an awful lot like the classic UHF tuner oscillator: Vcc --- | L1 | +------. | | |/ | Vbias-+---| Q1 --- Cfb | |>. --- --- | | --- +------+ | .-. | =3D=3D=3D | | --- C1 | | --- '-' | | =3D=3D=3D =3D=3D=3D This is a common-base amplifier stage with feedback (parasitics are enough) and tuned loads. Damping L1 kills the gain. -- Cheers, James Arthur
On Aug 14, 6:50=A0am, dagmargoodb...@yahoo.com wrote:
> On Aug 14, 12:08=A0am, miso <m...@sushi.com> wrote: > > > > > > > > > > > On 8/13/2012 8:49 PM, John Larkin wrote: > > > > On Mon, 13 Aug 2012 20:38:56 -0700, miso <m...@sushi.com> wrote: > > > >> On 8/13/2012 8:11 PM, John Larkin wrote: > > > >>> OK, I need to charge a capacitor with a stable constant current. Th=
e
> > >>> 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-b=
eta
> > >>> transistor, BCX71K. The ferrite in the collector is supposed to > > >>> isolate the ramp cap from the transistor capacitance and make the r=
amp
> > >>> linear. > > > >>> Well, the ramp looked not much better than an R-C curve, and the BC=
X71
> > >>> 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'd be more inclined to cascode the current source output rather tha=
n
> > >> add an inductor. > > > > Would that help? The beta error would increase, and cascodes can > > > oscillate too. There's hardly any difference. > > > Cascodes are the classic way to isolate circuits, which in this case is > > basically improving the reverse parameters. [Think of how they kill the > > MIller effect in input states.] The would also increase the output > > impedance of the circuit, which would make the ramp straighter. > > > Since the circuit uses feedback, you don't need the precision (i.e. > > predictability) of a bipolar. Maybe a P-fet can be used, then there is > > no beta issue. > > Or, damp L1 with a parallel resistor. > > Right now it looks an awful lot like the classic UHF tuner oscillator: > > =A0 =A0 =A0 =A0 =A0 =A0Vcc > =A0 =A0 =A0 =A0 =A0 =A0--- > =A0 =A0 =A0 =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0L1 > =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 | > Vbias-+---| =A0Q1 =A0 --- Cfb > =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=3D=3D=3D =A0 | | =A0 =A0--- C1 > =A0 =A0 =A0 =A0 =A0 =A0| | =A0 =A0--- > =A0 =A0 =A0 =A0 =A0 =A0'-' =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =3D=3D=3D > =A0 =A0 =A0 =A0 =A0 =A0=3D=3D=3D > > This is a common-base amplifier stage with feedback (parasitics are > enough) and tuned loads. =A0Damping L1 kills the gain.
Hmmm. L1 + series resistor might work too. That's a little dicier, but still lets L1 act as a super-fast CCS. -- Cheers, James Arthur
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. What kind of reset restrictions are there (e.g. recovery). 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...