On Tue, 20 Aug 2013 13:10:50 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 8/20/2013 11:44 AM, Jim Thompson wrote: >> On Tue, 20 Aug 2013 11:21:41 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 8/20/2013 2:22 AM, Jan Panteltje wrote: >>>> On a sunny day (Mon, 19 Aug 2013 16:50:36 -0400) it happened Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote in >>>> <-b2dnULzjrWBGI_PnZ2dnUVZ_hidnZ2d@supernews.com>: >>>> >>>>>> That depends, take an RF amp, AGC done by changing Ic, signals are >>>>>> small, no feedback. >>>>> >>>>> But that's a transimpedance effect, not a current-gain effect. If beta >>>>> is constant, changing I_C doesn't change the gain of a current-mode >>>>> amplifier. >>>> >>>> I was always under the impression that beta depends on Ic??? >>>> Confirms your own statement that beta depends on a lot of things :-) >>> >>> Not in a predictable way, it doesn't. >> >> "Predictable"? Sure it does, see... >> >> http://www.analog-innovations.com/SED/BipolarTransistor_FromPSPCREF.pdf >> >> Also some curves from previously posted questions... >> >> http://www.analog-innovations.com/SED/BetaCurves.pdf >> >> http://www.analog-innovations.com/SED/Beta_Curves_SED.pdf >> > >Models do, and variations among devices on the same chip are relatively >small. However, things like defect density and variation of doping vs. >depth vary from wafer to wafer, and those affect the shape of the beta >vs I_C curve, do they not?Not much anymore. You're living in the past ;-) when process variations were much greater. Manufacturer to manufacturer... now that's a different story.> >Besides, the context was Jan's claim that his BJT AGC amps work >primarily by controlling beta via I_C, whereas I claim that it's a >transconductance effect. A purely beta-controlled amplifier might >easily have to use a 100:1 ratio of I_C to get a 10 dB gain range, and >the available range would depend a lot on the selection of the >individual device.Jan doesn't understand that what happens in a typical RF stage, AGC'd that way, is the input impedance pads down the gain.> >OTOH in a transconductance amp it's only a nice repeatable factor of >sqrt(10), corresponding to a delta-V_BE= kT/e*ln(sqrt(10)) ~30 mV >(single-ended). > >Would you build a beta-controlled AGC amp?Not a chance, I don't rely on beta period!> >Cheers > >Phil Hobbs...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 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.
Zener regulator : load and zener current
Started by ●August 18, 2013
Reply by ●August 20, 20132013-08-20
Reply by ●August 20, 20132013-08-20
On Tue, 20 Aug 2013 13:35:44 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 8/20/2013 12:14 PM, John Larkin wrote: >> On Tue, 20 Aug 2013 11:21:41 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >> >>> I was trying to measure both log conformance and beta linearity in one >>> go, to pretty good accuracy. The usual trick of using tee networks in >>> the feedback loop plus really really low offset amplifiers wouldn't work >>> because chopamps all seem to have about 200 pA of input bias current. >> >> Chopamps usually are shooting healthy packets of charge out of both input pins. >> Offset voltage and bias current can depend on what impedances the input pins >> see, especially whether they see capacitance or resistance. > >Some seem to be a lot better than others, e.g. the OPA378 doesn't seem >nearly as bad as the OPA2188. > >Cheers > >Phil HobbsI did a 3-opamp 6-pole SK filter with AD8628s. That seems to be an especially bad configuration... one opamp input has a cap to ground and the other input connects to the output (unity-gain follower.) I got something like 5 uv offset per amp. Had to add a tiny trimpot to tweak the overall offset to zero. This was for some little plug-in filter boardlets. https://dl.dropboxusercontent.com/u/53724080/Circuits/Filters/22A451A1.jpg https://dl.dropboxusercontent.com/u/53724080/Circuits/Filters/22A451A3.jpg Next time, I'll breadboard anything critical that uses a chopamp; the data sheets don't generally mention charge injection. -- 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
Reply by ●August 20, 20132013-08-20
On 8/20/2013 2:13 PM, John Larkin wrote:> On Tue, 20 Aug 2013 13:35:44 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 8/20/2013 12:14 PM, John Larkin wrote: >>> On Tue, 20 Aug 2013 11:21:41 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>> >>> >>>> I was trying to measure both log conformance and beta linearity in one >>>> go, to pretty good accuracy. The usual trick of using tee networks in >>>> the feedback loop plus really really low offset amplifiers wouldn't work >>>> because chopamps all seem to have about 200 pA of input bias current. >>> >>> Chopamps usually are shooting healthy packets of charge out of both input pins. >>> Offset voltage and bias current can depend on what impedances the input pins >>> see, especially whether they see capacitance or resistance. >> >> Some seem to be a lot better than others, e.g. the OPA378 doesn't seem >> nearly as bad as the OPA2188. >> >> Cheers >> >> Phil Hobbs > > I did a 3-opamp 6-pole SK filter with AD8628s. That seems to be an > especially bad configuration... one opamp input has a cap to ground > and the other input connects to the output (unity-gain follower.) I > got something like 5 uv offset per amp. Had to add a tiny trimpot to > tweak the overall offset to zero. This was for some little plug-in > filter boardlets. > > https://dl.dropboxusercontent.com/u/53724080/Circuits/Filters/22A451A1.jpg > > https://dl.dropboxusercontent.com/u/53724080/Circuits/Filters/22A451A3.jpg > > > Next time, I'll breadboard anything critical that uses a chopamp; the > data sheets don't generally mention charge injection.Sometimes they do come clean, e.g. <https://dl.dropboxusercontent.com/u/105025837/sed/OPA2188ChopperNoise.pdf> 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 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●August 20, 20132013-08-20
On Tue, 20 Aug 2013 15:13:10 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 8/20/2013 2:13 PM, John Larkin wrote: >> On Tue, 20 Aug 2013 13:35:44 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 8/20/2013 12:14 PM, John Larkin wrote: >>>> On Tue, 20 Aug 2013 11:21:41 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>> >>>> >>>>> I was trying to measure both log conformance and beta linearity in one >>>>> go, to pretty good accuracy. The usual trick of using tee networks in >>>>> the feedback loop plus really really low offset amplifiers wouldn't work >>>>> because chopamps all seem to have about 200 pA of input bias current. >>>> >>>> Chopamps usually are shooting healthy packets of charge out of both input pins. >>>> Offset voltage and bias current can depend on what impedances the input pins >>>> see, especially whether they see capacitance or resistance. >>> >>> Some seem to be a lot better than others, e.g. the OPA378 doesn't seem >>> nearly as bad as the OPA2188. >>> >>> Cheers >>> >>> Phil Hobbs >> >> I did a 3-opamp 6-pole SK filter with AD8628s. That seems to be an >> especially bad configuration... one opamp input has a cap to ground >> and the other input connects to the output (unity-gain follower.) I >> got something like 5 uv offset per amp. Had to add a tiny trimpot to >> tweak the overall offset to zero. This was for some little plug-in >> filter boardlets. >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Filters/22A451A1.jpg >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Filters/22A451A3.jpg >> >> >> Next time, I'll breadboard anything critical that uses a chopamp; the >> data sheets don't generally mention charge injection. > >Sometimes they do come clean, e.g. ><https://dl.dropboxusercontent.com/u/105025837/sed/OPA2188ChopperNoise.pdf> > >Cheers > >Phil HobbsThanks. The paint can is cool. -- 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
