Forums

OpAmp Feedback Resistance vs. Stability

Started by Tim Wescott March 28, 2017
On Wednesday, March 29, 2017 at 7:31:31 PM UTC-4, Jim Thompson wrote:
> On Wed, 29 Mar 2017 22:54:13 +0000 (UTC), Hul Tytus <ht@panix.com> > wrote: > > >Tim - the early opamps (lm707??) required "compensation" so that the loop > >gain was less than unity at the frequency where the phase shift was 180 > >degrees. The first opamp that completely compensated itself, ie could be > >used at unity gain with no weird behavior, was, I think, the single > >version of the lm747. > > Those versions that required compensation had special pins for > >capacitors. But the circuitry between the output and the negative input > >could also be used for compensation, that is to reduce the loop gain with a > >low frequency rc filter. That is probably what you were doing with the > >rail to rail opamp. A 1K resister and 20 pf creates a 3 db point around 8 > >mhz or so. > > > >Hul > > > > [snip] > > My first OpAmp, MC1530/31, early '60's... > > <http://www.analog-innovations.com/SED/MC1530-DataSheet.pdf> > > was like that, external compensation, sliding-class-A output stage, > still being sold by Lansdale (licensed from Motorola), 53-years later > ;-) > > <http://www.analog-innovations.com/SED/MC1530-TeachingExercise.pdf>
Huh!!, I was going to ask what "sliding Class A was". (I first try to be a "class A" kind of guy... ) Thanks for the exercise.. I'm going to have to think about it. George H.
> The military love it... absolutely stable, high-slew-rate for the era, > indefinite-time short-circuit proof. > > ...Jim Thompson > -- > | James E.Thompson | mens | > | Analog Innovations | et | > | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | > | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | > | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | > | E-mail Icon at http://www.analog-innovations.com | 1962 | > > Thinking outside the box... producing elegant solutions.
On Wednesday, March 29, 2017 at 9:30:04 PM UTC-4, George Herold wrote:
> On Wednesday, March 29, 2017 at 7:31:31 PM UTC-4, Jim Thompson wrote: > > On Wed, 29 Mar 2017 22:54:13 +0000 (UTC), Hul Tytus <ht@panix.com> > > wrote: > > > > >Tim - the early opamps (lm707??) required "compensation" so that the loop > > >gain was less than unity at the frequency where the phase shift was 180 > > >degrees. The first opamp that completely compensated itself, ie could be > > >used at unity gain with no weird behavior, was, I think, the single > > >version of the lm747. > > > Those versions that required compensation had special pins for > > >capacitors. But the circuitry between the output and the negative input > > >could also be used for compensation, that is to reduce the loop gain with a > > >low frequency rc filter. That is probably what you were doing with the > > >rail to rail opamp. A 1K resister and 20 pf creates a 3 db point around 8 > > >mhz or so. > > > > > >Hul > > > > > > > [snip] > > > > My first OpAmp, MC1530/31, early '60's... > > > > <http://www.analog-innovations.com/SED/MC1530-DataSheet.pdf> > > > > was like that, external compensation, sliding-class-A output stage, > > still being sold by Lansdale (licensed from Motorola), 53-years later > > ;-) > > > > <http://www.analog-innovations.com/SED/MC1530-TeachingExercise.pdf> > Huh!!, I was going to ask what "sliding Class A was". > (I first try to be a "class A" kind of guy... ) > Thanks for the exercise.. I'm going to have to think about it. >
Oh, I didn't see you gave me all the equations too! GH.
> George H. > > The military love it... absolutely stable, high-slew-rate for the era, > > indefinite-time short-circuit proof. > > > > ...Jim Thompson > > -- > > | James E.Thompson | mens | > > | Analog Innovations | et | > > | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | > > | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | > > | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | > > | E-mail Icon at http://www.analog-innovations.com | 1962 | > > > > Thinking outside the box... producing elegant solutions.
On Wed, 29 Mar 2017 22:54:13 +0000, Hul Tytus wrote:

> Tim - the early opamps (lm707??) required "compensation" so that the > loop gain was less than unity at the frequency where the phase shift was > 180 degrees. The first opamp that completely compensated itself, ie > could be used at unity gain with no weird behavior, was, I think, the > single version of the lm747. > Those versions that required compensation had special pins for > capacitors. But the circuitry between the output and the negative input > could also be used for compensation, that is to reduce the loop gain > with a low frequency rc filter. That is probably what you were doing > with the rail to rail opamp. A 1K resister and 20 pf creates a 3 db > point around 8 mhz or so.
I got out of college just as those weren't being used much any more. -- Tim Wescott Control systems, embedded software and circuit design I'm looking for work! See my website if you're interested http://www.wescottdesign.com
On Thursday, 30 March 2017 04:18:18 UTC+1, Tim Wescott  wrote:
> On Wed, 29 Mar 2017 22:54:13 +0000, Hul Tytus wrote: > > > Tim - the early opamps (lm707??) required "compensation" so that the > > loop gain was less than unity at the frequency where the phase shift was > > 180 degrees. The first opamp that completely compensated itself, ie > > could be used at unity gain with no weird behavior, was, I think, the > > single version of the lm747. > > Those versions that required compensation had special pins for > > capacitors. But the circuitry between the output and the negative input > > could also be used for compensation, that is to reduce the loop gain > > with a low frequency rc filter. That is probably what you were doing > > with the rail to rail opamp. A 1K resister and 20 pf creates a 3 db > > point around 8 mhz or so. > > I got out of college just as those weren't being used much any more.
The 709 was the popular uncompensated one, and the 741 the number 1 unity stable opamp for a long time. There were others of course before & after, eg the 702. And even valve opamps. NT
Jim - I was familiar with the Motorola devices in the late 60's; those and 
RCA's but mostly the Fairchild opamp's, but that was mostly due to 
available purchasing channels.
   Fun times those. If one device was announced with break through 
features in the spring, there'd be a better one in the fall. 
Congratulations on a device that lasted 53 years. That's no mean 
accomplishment.   

Hul


Jim Thompson <To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote:
> On Wed, 29 Mar 2017 22:54:13 +0000 (UTC), Hul Tytus <ht@panix.com> > wrote:
> >Tim - the early opamps (lm707??) required "compensation" so that the loop > >gain was less than unity at the frequency where the phase shift was 180 > >degrees. The first opamp that completely compensated itself, ie could be > >used at unity gain with no weird behavior, was, I think, the single > >version of the lm747. > > Those versions that required compensation had special pins for > >capacitors. But the circuitry between the output and the negative input > >could also be used for compensation, that is to reduce the loop gain with a > >low frequency rc filter. That is probably what you were doing with the > >rail to rail opamp. A 1K resister and 20 pf creates a 3 db point around 8 > >mhz or so. > > > >Hul > >
> [snip]
> My first OpAmp, MC1530/31, early '60's...
> <http://www.analog-innovations.com/SED/MC1530-DataSheet.pdf>
> was like that, external compensation, sliding-class-A output stage, > still being sold by Lansdale (licensed from Motorola), 53-years later > ;-)
> <http://www.analog-innovations.com/SED/MC1530-TeachingExercise.pdf>
> The military love it... absolutely stable, high-slew-rate for the era, > indefinite-time short-circuit proof. > > ...Jim Thompson > -- > | James E.Thompson | mens | > | Analog Innovations | et | > | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | > | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | > | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | > | E-mail Icon at http://www.analog-innovations.com | 1962 |
> Thinking outside the box... producing elegant solutions.
Familiar numbers. Nice to see the correct ones.

Hul

tabbypurr@gmail.com wrote:
> On Thursday, 30 March 2017 04:18:18 UTC+1, Tim Wescott wrote: > > On Wed, 29 Mar 2017 22:54:13 +0000, Hul Tytus wrote: > > > > > Tim - the early opamps (lm707??) required "compensation" so that the > > > loop gain was less than unity at the frequency where the phase shift was > > > 180 degrees. The first opamp that completely compensated itself, ie > > > could be used at unity gain with no weird behavior, was, I think, the > > > single version of the lm747. > > > Those versions that required compensation had special pins for > > > capacitors. But the circuitry between the output and the negative input > > > could also be used for compensation, that is to reduce the loop gain > > > with a low frequency rc filter. That is probably what you were doing > > > with the rail to rail opamp. A 1K resister and 20 pf creates a 3 db > > > point around 8 mhz or so. > > > > I got out of college just as those weren't being used much any more.
> The 709 was the popular uncompensated one, and the 741 the number 1 unity stable opamp for a long time. There were others of course before & after, eg the 702. And even valve opamps.
> NT