On Sat, 3 Oct 2015 11:22:08 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:
>Hello,
>
>I know that there are hordes of nice comparators in SOT-23,
>but just out of curiosity: is it possible to build a small
>voltage (relatively to GND) detector using just one transistor
>and no negative power supplies? If the voltage is >= VBE, i.e.
>0.7V, then the task is a no-brainer, but how about half of that,
>say, 0.3V?
>
> Best reagrds, Piotr
Not just a single transistor, but reasonably temperature stable...
<http://www.analog-innovations.com/SED/SmallVoltageDetect%28SED%29_2015-10-06_08-57-33.jpg>
...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| 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 ●October 6, 20152015-10-06
FET
Reply by Jim Thompson●October 5, 20152015-10-05
On Sat, 3 Oct 2015 11:22:08 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:
>Hello,
>
>I know that there are hordes of nice comparators in SOT-23,
>but just out of curiosity: is it possible to build a small
>voltage (relatively to GND) detector using just one transistor
>and no negative power supplies? If the voltage is >= VBE, i.e.
>0.7V, then the task is a no-brainer, but how about half of that,
>say, 0.3V?
>
> Best reagrds, Piotr
Can the voltage being sensed source AND sink a small amount of
current?
...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| 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 whit3rd●October 5, 20152015-10-05
On Monday, October 5, 2015 at 2:12:52 AM UTC-7, Piotr Wyderski wrote:
> John Larkin wrote:
>
> > Somebody makes a mosfet with a zero volt threshold. That and a drain
> > pullup is the minimal discrete zero-sense comparator.
>
> Google says the company is ALD. Such a part would be very useful
Useful, but there's no free lunch. The transconductance of a MOSFET near
threshold is poor; if you need 1/2 mA of output drive and the threshold
is at 0V, the output goes low at
Vin = 0.5 mA/g_m
which (as I read an ALD datasheet) takes 300 mV swing to move the output,
and it's gotta be biased +4V from the threshold to get THAT sensitive.
Your 'comparator' switches, then, at 4V to 4.3V, not near the 0.0V threshold.
ALD's products are mainly sold in matched-duals and quads, for multitransistor
designs.
By comparison, if you can get the biasing right (10 mA quiescent emitter current)
a plain old NPN transistor will have g_m of 400 mA/V, and switches 0.5 mA
at Vin of 0.600V to 0.6013V...
Either multiple DC supplies, or multiple transistors, are needed to
get a zero-volt switch function that's really clean. I liked the old uA742
for a while (long obsolete, but it ran off AC and made its own regulated power).
Reply by Piotr Wyderski●October 5, 20152015-10-05
John Larkin wrote:
> Somebody makes a mosfet with a zero volt threshold. That and a drain
> pullup is the minimal discrete zero-sense comparator.
Google says the company is ALD. Such a part would be very useful
in my projects, thanks! Another exotica I didn't know about.
Best regards, Piotr
Reply by ●October 4, 20152015-10-04
On Saturday, October 3, 2015 at 5:22:12 AM UTC-4, Piotr Wyderski wrote:
> Hello,
>
> I know that there are hordes of nice comparators in SOT-23,
> but just out of curiosity: is it possible to build a small
> voltage (relatively to GND) detector using just one transistor
> and no negative power supplies? If the voltage is >= VBE, i.e.
> 0.7V, then the task is a no-brainer, but how about half of that,
> say, 0.3V?
>
> Best reagrds, Piotr
Not really, and most of the ckts proposed here are amplifiers and not comparators.
Reply by ●October 4, 20152015-10-04
On Saturday, October 3, 2015 at 3:24:01 PM UTC-4, legg wrote:
> On Sat, 3 Oct 2015 20:52:12 +0200, Piotr Wyderski
> <peter.pan@neverland.mil> wrote:
>
> >legg wrote:
> >
> >> Voltages outputed from the secondary of a power transformer are
> >> typically bipolar
> >
> >After the bridge the transformer's voltage is unipolar
> >and somewhat shifted (by a diode drop if it is a normal
> >silicon Graetz circuit, way smaller if synchronous).
> >
> >> low impedance and large, so your description of the
> >> measurement needing to be made doesn't make much sense.
> >
> >It does, the result indicates which branch of the H bridge
> >should be turned on in a synchronous rectifier. I wondered
> >how would a discrete-only analog controller work if the
> >switch enable threshold is required to be low. My first guess
> >was to clone the input differential pair from an op-amp,
> >but it would be too complex. So I decided to ask the analog
> >gurus here and Jasen's solution needs only 2 transistors,
> >which is much simpler than my idea.
> >
> http://www.magma.ca/~legg/20_Years/1994/1994h.html
LOL- your title had me thinking of some strange diode invented by an individual named Orring....hmmmkay.
Reply by ●October 4, 20152015-10-04
On Sunday, 4 October 2015 06:10:40 UTC+1, Tim Williams wrote:
> "Piotr Wyderski" wrote in message
> news:muo6o0$mu6$1@node2.news.atman.pl...
>
> Hello,
>
> I know that there are hordes of nice comparators in SOT-23,
> but just out of curiosity: is it possible to build a small
> voltage (relatively to GND) detector using just one transistor
> and no negative power supplies? If the voltage is >= VBE, i.e.
> 0.7V, then the task is a no-brainer, but how about half of that,
> say, 0.3V?
>
> Best reagrds, Piotr
> One problem: as the voltage goes down, the required gain goes up (or so
> one would assume).
I don't see how that would be true. Only the gain at & near threshold V matters, and the amount of gain required depends entirely on the circuit the threshold detector drives. Delta V matters, not V.
> This is especially important in a comparator (ideally
> infinite GBW).
>
> So you inevitably need more transistors, or better (GaN maybe??).
No
NT
Reply by Tim Williams●October 4, 20152015-10-04
One problem: as the voltage goes down, the required gain goes up (or so
one would assume). This is especially important in a comparator (ideally
infinite GBW).
So you inevitably need more transistors, or better (GaN maybe??).
Three works pretty well:
http://seventransistorlabs.com/Images/Discrete_Tube_Supply.png
The pair needs to be heavily unbalanced to actually include zero (note the
2.2k is supplying about 6mA, while the gain transistor needs maybe 0.2mA
to turn on), but that isn't usually a problem for this type of
application. Same for the poor turn-off (the gain transistor will take
relatively long storage time here, but that actually helps in this case).
Tim
--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com
"Piotr Wyderski" wrote in message
news:muo6o0$mu6$1@node2.news.atman.pl...
Hello,
I know that there are hordes of nice comparators in SOT-23,
but just out of curiosity: is it possible to build a small
voltage (relatively to GND) detector using just one transistor
and no negative power supplies? If the voltage is >= VBE, i.e.
0.7V, then the task is a no-brainer, but how about half of that,
say, 0.3V?
Best reagrds, Piotr
Reply by John Larkin●October 4, 20152015-10-04
On Sat, 03 Oct 2015 09:42:39 -0400, Boris Mohar
<borism_void_@sympatico.ca> wrote:
>On Sat, 3 Oct 2015 11:22:08 +0200, Piotr Wyderski <peter.pan@neverland.mil>
>wrote:
>
>>Hello,
>>
>>I know that there are hordes of nice comparators in SOT-23,
>>but just out of curiosity: is it possible to build a small
>>voltage (relatively to GND) detector using just one transistor
>>and no negative power supplies? If the voltage is >= VBE, i.e.
>>0.7V, then the task is a no-brainer, but how about half of that,
>>say, 0.3V?
>>
>> Best reagrds, Piotr
>
> Germanium transistor?
Somebody makes a mosfet with a zero volt threshold. That and a drain
pullup is the minimal discrete zero-sense comparator.