Open Loop Op Amp - Page 1
Forum section: electronics / beginners

author: zrlopez
author_rank: Contributor
timestamp: October 28, 2020, 10:15:19 am
content: Hello all, I'm having trouble observing basic behavior of an open-loop op amp test circuit. I should expect that as I move the voltage input (the blue channel) towards 0 V, the output voltage (the yellow channel) should dither. But I'm not seeing the output voltage budge at all, while I'm sweeping my oscilloscope. I'm using an LM311P op amp and powering it with +/- 15 V from 2 DC power supplies stacked to be symmetrical. I tried swapping out with a new LM311P, but that didn't make a difference.The black clip is GND, the yellow clip is -15 V, the red clip is + 15 VCould it be that I'm using the wrong type of op amp?

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 28, 2020, 10:36:25 am
content: An LM311 is NOT an op-amp, it is a comparator with a floating emitter/collector output not totem pole.Without external circuitry the output will do nothing!I suggest you connect the floating emitter to ground and the floating collector to +15V VIA a 10K resistor, observe the collector voltage, it should SWITCH between 0 & +15V depending upon the applied ofsett voltage between the inputs, only +/-10mV should cause it to switch.

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2020, 10:37:56 am
content: The LM311 is a comparator, not an op-amp.Going from the picture, it's not connected correctly. It has an open collector output. Pin 2 should go to 0V and pin 7 to +V, via a suitable pull-up resistor: try 10k. The output will go high, when the voltage on pin 2 (non-inverting input) is higher than pin 3 (inverting input). In order for it to work, the inputs must both be within +0.5V of the negative rail and -1.5V of the positive rail, so that's between -14.5V and +13.5V, for a +/-15V supply.Refer to the data sheet for more information.https://www.ti.com/lit/ds/symlink/lm311.pdfPlease post a schematic, as well as a photograph, in future, as it's impossible to tell which pin is connected to what voltage, just by looking at a picture.

author: zrlopez
author_rank: Contributor
timestamp: November 02, 2020, 07:57:00 am
content: @Zero999 I managed to find a comparator circuit for the LM311 from my old labs that I could build. But I've tried constructing this circuit several times and my comparator keeps overheating. I'm powering it on pin 4 and 8 with +/- 15 V. I wonder if I should just power it with 3 V because every time it seems to burn out. Is it normal for these comparators to get hot hot with voltage as high as 15 V?My circuit's a little messy, but when I had it hooked up with probes, my power supply and everything, pins 1 and 2 (orange and red on left) were grounded and the white wire to 4 was going to -15 V. I double checked the resistor values. They were fine. And white wire from pin 8 on the right went to + 15 V. Again, I wonder if +/- 15 V is pushing it for this comparator. It just keeps overheating and I don't know why. I checked the pins with my multimeter and sure enough it's getting +/- 15 V on those pins. But it's heating up.

author: Damianos
author_rank: Frequent Contributor
timestamp: November 02, 2020, 10:17:21 am
content: In the last photo, it seems that you have connected the STROBE to the output collector. What you expect from this?Additionally there is not any decoupling capacitor, so the comparator is prone to oscillate. Also don't leave the input floating.

author: zrlopez
author_rank: Contributor
timestamp: November 02, 2020, 07:05:44 pm
content: @Damianos. The output is not connected to the STROBE. It's just at a weird angle. The orange piece of wire connected to the output was just there so I could clip my oscilloscope probe.

author: Wimberleytech
author_rank: Super Contributor
timestamp: November 02, 2020, 09:51:25 pm
content: You sure it is a 311? I don't see markings on either image.

author: zrlopez
author_rank: Contributor
timestamp: November 03, 2020, 05:23:55 am
content: Quote from: Wimberleytech on November 02, 2020, 09:51:25 pmThe IC comparator markings are 81BWQ2Z LM311P