Amplifier Comparator LM311P Using Problem

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Markkim
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Amplifier Comparator LM311P Using Problem

Post by Markkim » Wed Aug 25, 2021 1:33 am

Hello, all
For this little project, I'm using an analog differential comparator LM311P(parameter source: https://www.apogeeweb.net/circuitry/lm3 ... alent.html. It is has a collector/emitter output. Because my circuit didn't function the way it should, I started debugging.
With the collector unconnected the emitter (pin 1) still sinks (11mA) and sources (20mA) current, when applying a square wave input. I measured mean dc amperage to either rail, so the max is higher. I cannot explain this behavior.
Doing the current measurement on the collector (pin 7) with an unconnected emitter I get 14mA when sinking, and none when sourcing. But not following the input signal.

Is there any suggestions of this? Many thanks.
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UhClem
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Re: Amplifier Comparator LM311P Using Problem

Post by UhClem » Tue Sep 07, 2021 11:08 am

The TI data sheet includes a schematic which shows that the output is not simply the bare emitter and collector of a transistor. There are other things connected to those pins.
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haklesup
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Re: Amplifier Comparator LM311P Using Problem

Post by haklesup » Wed Sep 08, 2021 2:01 pm

I suppose you are looking at the current path R6 thru D1 thru pin 7 to pin 1 of U2 to ground. Since this is used as a comparator, we can assume it is either fully on or off and not in any active region

Since the CE configuration acts more like a switch it kind of acts like an Inverter. When the transistor is off the "Source" current is limited by the supply voltage/R6 and when the transistor is on you should design R6 to limit the current to the max Sink current of the device. with it on you will need to subtract VCE from your calculation such that Isink=(5V-VCE-Vdiode)/R6 or something like that. maybe I flipped your meaning of source and sink but I hope this follows ok.

If you want the op amp to directly source and sink current from its output pin, you should look for one with a CMOS output stage and bipolar supply that can source from VCC or VEE

With pin 1 open, I think this is not a normal thing, you could be looking at Current flowing through B-C junction or at a parasitic current path resulting from the process the IC was made from (an unexpected PN junction not on the schematic often the Collector Substrate junction which should always be reverse biased, when you float the Emitter, the substrate may be at a different voltage.

To put it another way, with pin 1 open, you have opened the path to the "sourcing" supply which is ground, current can only flow one way through the transistor from C to E (if you see B to E current it is an improper application), the parameter that the op amp is controlling is VCE and its either close to 0V (on) or VCC (off) (idealizing). VCE is also U2 pin 7 to pin 1

Markkim
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Re: Amplifier Comparator LM311P Using Problem

Post by Markkim » Fri Sep 10, 2021 2:07 am

Thank you for you all replies.
I checked the internal schematic of the LM311.
The LM311 is not designed to work with only one of the collector or emitter outputs connected. If the collector is connected to a positive supply, then the emitter output can pull up. If the emitter is connected to a negative supply, then the collector output can pull down.

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