Improving the speed response of the LM311's input stage.

Thread Starter

schmitt trigger

Joined Jul 12, 2010
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?



Joined Jul 11, 2016
the output stage is OC OE which likely sets some limits (i guess the OE was faster ... perhaps)

??? best performance over Vs ≥ ±13.5V (the OE likely at GND and OC pulled to +5V // OC +5 OE pulled to GND)

if you are not overdriving your input the prev post is likely the best option available

i have tested the non-inverting feed-forward biasing/configuration with LM393 where it had a slight effect (in a specific application) but i have not tested it with LM311


K554CA3 - CLM - Test - Q-SR - ON.gif K554CA3 - CLM - Test - Q-SR - OFF.gif
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