Speeding up the LM311 input stage.

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?

The information is in old AN41, still linked on the LM311 literature page. https://www.ti.com/lit/pdf/snoa642

You are talking about slew rate increase which may be misleading. More specifically, the increased input current stage is said to improve the capability to track input common mode transients, not the response speed to differential signals.

Thanks;
I remembered that I had stored somewhere an ancient (1982) National Semi's Linear Applications Handbook.

Searched the book and found it. Indeed, it shows AN41, written by the famous Bob Widlar.

I must confess, that I reviewed the National Semi AN paperback before searching for the link. :smile: