Putting OP AMP -Vcc to ground - Page 1
Forum section: electronics / beginners

author: IanBerry
author_rank: Contributor
timestamp: September 23, 2016, 01:13:36 am
content: I was wondering why connecting the -Vcc of an op amp to ground doesn't work. I have been playing with op amps recently and have had a major issue when trying to use them as a comparator in a DC setting. I connected -Vcc to ground expecting that when I made the non-inverting input voltage lower than the inverting input voltage that the output voltage would drop to 0 volts or ground. That's far from what happened. The output became half of +Vcc (in this case +Vcc was 5 volts). I tried using the offsets to change this, using a potentiometer offset null circuit. But when I turned the potentiometer the output did not change. Is it necessary for op amps to have a negative voltage connected to -Vcc? Are there specific op amps you can and can't do this with? I have been using a UA741CP. I do have a few ideas for some circuits that would use a comparator or an op amp as a comparator but this issue is getting in the way. Should I look into getting specifically a comparator, or would I have the same issue?

author: rstofer
author_rank: Super Contributor
timestamp: September 23, 2016, 01:24:27 am
content: You should first watch Dave's Op Amp video. He starts off talking about using an op amp as a comparator.Second, there is no specific need to have a negative supply but if your op amp won't pull down to the lower rail (and a 741 won't), it might be a problem.Ultimately, you wind up having the create a vcc/2 virtual ground for most op amp circuits.Using an op amp as a comparator is not the best way to do it. You need to deal with feedback if you want any hysteresis.Google for 'single supply op amp comparator' if you want more information. Most of the discussions will use more modern op amps.

author: rdl
author_rank: Super Contributor
timestamp: September 23, 2016, 01:33:11 am
content: Alan (w2aew) has a good video on op-amp power supplies.

author: Audioguru
author_rank: Super Contributor
timestamp: September 23, 2016, 01:53:26 am
content: The 741 opamp was introduced 48 years ago! It was designed to use a +15V and -15V supply but some work from a supply as low as +5V and -5V. Most 741 opamps do nothing if the supply is only +5V. Its inputs do not work if their voltage is within 3V from a supply voltage and its output does not go anywhere near a supply voltage. Your negative supply voltage is 0V so the minimum output voltage is about 2V.A typical comparator is an LM393 dual or LM339 quad. With a single positive supply as low as 2V to 36V the inputs work down to 0V and the output goes down almost to 0V.

author: rstofer
author_rank: Super Contributor
timestamp: September 23, 2016, 06:25:52 pm
content: Quote from: rdl on September 23, 2016, 01:33:11 amThat video has a great demonstration of rail-to-rail op amps. His prototype circuit is just what is needed for that kind of discussion.

author: Towz
author_rank: Contributor
timestamp: September 24, 2016, 02:09:29 am
content: The 741 is certainly far from being a rail to rail opamp, with such a small voltage difference across its rails it will output something between 2ish volts and almost 3ish volts! Modern rail to rail opamps will be able to output 0 to 5 volts with that supply. Also, for a reliable application its better to use a proper rail to rail comparator ic and add a little hysteresis (via positive feedback). To get equivalent results with a 741 will require a split supply.