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, PiotrNot really, and most of the ckts proposed here are amplifiers and not comparators.
Small voltage detector
Started by ●October 3, 2015
Reply by ●October 4, 20152015-10-04
Reply by ●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 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 usefulUseful, 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 ●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, PiotrCan 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 ●October 6, 20152015-10-06
Reply by ●October 6, 20152015-10-06
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, PiotrNot 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.
