Author Topic: Improving the speed response of the LM311's input stage.  (Read 1093 times)

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Offline schmitt triggerTopic starter

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Improving the speed response of the LM311's input stage.
« on: May 07, 2020, 03:28:14 pm »
Many moons ago, I barely remember that in an ancient National Semi app note or datasheet, that one could significantly improve the LM311's input stage slew rate, by tying the both strobe/balance pins to +V.

Analyzing the attached equivalent circuit, it is easy to see why. The PNP input stage Q1,Q2 is fed by a constant current source generated by Q5, with its associated resistors R1 and R2.
Tying up the balanced pins to V+ effectively places R3,R4 in parallel with R1,R2. The end result is that the current fed to the input stage increases about five times, which should have the benefit of a proportional increase in its slew rate.

Unfortunately, the newer datasheets do not mention anything related to any response speed improvement. The datasheets do show that the input bias current increases in this configuration, as it should. But that's it.

So what do you guys think? Is my assessment correct?
 

Offline SiliconWizard

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Re: Improving the speed response of the LM311's input stage.
« Reply #1 on: May 07, 2020, 03:39:49 pm »
Yup. I think so.
 

Offline David Hess

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Re: Improving the speed response of the LM311's input stage.
« Reply #2 on: May 08, 2020, 03:03:33 am »
I remember reading the same thing about increasing the LM311's input stage current but I never had a chance to make use of it.
 

Offline Zero999

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Re: Improving the speed response of the LM311's input stage.
« Reply #3 on: May 08, 2020, 12:00:50 pm »
I wonder how well it would work with capacitors? That should boost the high frequency response, whist leaving the DC bias unaffected.
 
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Offline schmitt triggerTopic starter

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Re: Improving the speed response of the LM311's input stage.
« Reply #4 on: May 08, 2020, 01:29:05 pm »
Wow!

That is one clever idea Zero999.
 

Offline SiliconWizard

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Re: Improving the speed response of the LM311's input stage.
« Reply #5 on: May 08, 2020, 01:51:47 pm »
I wonder how well it would work with capacitors? That should boost the high frequency response, whist leaving the DC bias unaffected.

Worth a shot!
 

Offline magic

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Re: Improving the speed response of the LM311's input stage.
« Reply #6 on: May 08, 2020, 02:01:04 pm »
Wouldn't that merely increase output impedance of Q5,Q6 current sources?

I think the whole purpose of the original circuit was to increase precisely the DC bias of the input buffers, for the sake of beta/fT/whatnot.
 

Offline schmitt triggerTopic starter

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Re: Improving the speed response of the LM311's input stage.
« Reply #7 on: May 08, 2020, 07:42:26 pm »
An alert reader in another forum mentioned  that there is an old National Semi app note (AN-41), originally written by Bob Widlar back in 1970.

It is still available in TI's website, but a little difficult to find as they have renumbered all of the National Semi's app notes.  |O |O

There is a paragraph hidden among the couple thousand word note, which mentions precisely the concept of input stage current rising.

I have attached the app note and highlighted the relevant information.
 
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