Limiter + Opamp + Reverse phase on negative swing
in [Design]
|
From: George Jefferson on 15 Jul 2010 04:22 On page 239 of AOE there is a laboratory DC amp. The input uses a limiter. It says that if Vin exceeds V- that the phase will change. I do not understand what it means? The output of the op amp's phase will change sign? Is this a normal property of op amps? e.g., if I put in a voltage that goes below it's most negative rail it will invert the output? Also, what about using fets for limiters instead of diodes? Any worth in doing it or just adding useless complexity?
From: Tim Williams on 15 Jul 2010 05:30 Yes, it is a common feature. Typically, only amps which specifically note "no output reversal" are safe from this. Single supply amps (including LM358, comparators like LM393, etc.) are prone to this, a small sacrifice for getting inputs down to -0.3V. Cue larkin to talk about his new favorite FET current limiters. :) Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms "George Jefferson" <phreon111(a)gmail.com> wrote in message news:i1mggn$ktb$1(a)news.eternal-september.org... > On page 239 of AOE there is a laboratory DC amp. The input uses a limiter. > It says that if Vin exceeds V- that the phase will change. I do not > understand what it means? The output of the op amp's phase will change sign? > Is this a normal property of op amps? e.g., if I put in a voltage that goes > below it's most negative rail it will invert the output? > > Also, what about using fets for limiters instead of diodes? Any worth in > doing it or just adding useless complexity? > >
From: John Larkin on 15 Jul 2010 09:59 On Thu, 15 Jul 2010 04:30:44 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote: >Yes, it is a common feature. Typically, only amps which specifically note "no output reversal" are safe from this. > >Single supply amps (including LM358, comparators like LM393, etc.) are prone to this, a small sacrifice for getting inputs down to -0.3V. LM324/339 types can be even worse. Pull one pin of one opamp a few tenths below ground, and all the opamps on the chip go nuts. You can also sometimes observe a *double* phase reversal as you go negative on an input pin. > >Cue larkin to talk about his new favorite FET current limiters. :) Real men use polyfuses and transzorbs. John
From: Jim Thompson on 15 Jul 2010 10:54 On Thu, 15 Jul 2010 06:59:30 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 15 Jul 2010 04:30:44 -0500, "Tim Williams" ><tmoranwms(a)charter.net> wrote: > >>Yes, it is a common feature. Typically, only amps which specifically note "no output reversal" are safe from this. >> >>Single supply amps (including LM358, comparators like LM393, etc.) are prone to this, a small sacrifice for getting inputs down to -0.3V. > >LM324/339 types can be even worse. Pull one pin of one opamp a few >tenths below ground, and all the opamps on the chip go nuts. You can >also sometimes observe a *double* phase reversal as you go negative on >an input pin. > >> >>Cue larkin to talk about his new favorite FET current limiters. :) > >Real men use polyfuses and transzorbs. > >John One protection scheme... http://analog-innovations.com/SED/SpehroLoopProtect2.pdf dates back 9 years, in answer to an original post by Spehro. ...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 | Since New Yorkers think it appropriate to boycott Arizona over stopping illegal immigration and drug trafficking, I suggest that everyone else support building the Muslim Monument at Ground Zero
From: Kevin McMurtrie on 16 Jul 2010 02:07
In article <i1mggn$ktb$1(a)news.eternal-september.org>, "George Jefferson" <phreon111(a)gmail.com> wrote: > On page 239 of AOE there is a laboratory DC amp. The input uses a limiter. > It says that if Vin exceeds V- that the phase will change. I do not > understand what it means? The output of the op amp's phase will change sign? > Is this a normal property of op amps? e.g., if I put in a voltage that goes > below it's most negative rail it will invert the output? > > Also, what about using fets for limiters instead of diodes? Any worth in > doing it or just adding useless complexity? > http://en.wikipedia.org/wiki/Long-tailed_pair#Long-tailed_pair Consider the case of the 'inv input' being between -v and +v and higher than the 'non-inv input'. The output will go low as expected. Keep raising 'inv input' and it will eventually force the output to an incorrect high level. It gets worse if 'non-inv input' is driven too high - the output will be an open circuit that may receive current from outside the pair. -- I won't see Google Groups replies because I must filter them as spam |