LM741 as comparator 1.9V problem - Page 1
Forum section: electronics / beginners

author: PerranOak
author_rank: Frequent Contributor
timestamp: May 02, 2018, 06:35:23 pm
content: Hi,I'm experimenting with an LM741 as a comparator.I have a simple circuit with pots to sweep the voltage on both inputs (with meters) and I watch the output with a meter.Powered with 5V and the inv input at 3V all is well as I sweep the non-inv input above and below 3V the output goes high and low as expected.However, if the inv input is below 1.9V the output will not change no matter what the non-inv input.How can this be?Thanks.

author: capt bullshot
author_rank: Super Contributor
timestamp: May 02, 2018, 06:45:36 pm
content: This is pretty normal for a common Op-Amp.Read the datasheet and find the common mode voltage range of the input. If one (or both) inputs exceeds this range, the OpAmp won't work as expected. For a non-RRIO (rail-to-rail In/Out) like this 741, the input common voltage range usually goes from about neg. supply+2V to pos. supply-2V (2V ... 3V with a 5V supply). See the datasheet for exact values. There are some Op-Amps that have a common mode voltage range including the neg. supply (e.g. the LM358), or modern variants that allow from neg. to pos. supply (rail-to-rail input).

author: Audioguru
author_rank: Super Contributor
timestamp: May 02, 2018, 10:40:30 pm
content: The 741 opamp was designed 50 years ago to use a 30V power supply (plus and minus 15V). Many will not work when the total supply voltage is less than 10V but yours is only 5V so of course it does not work.

author: Zero999
author_rank: Super Contributor
timestamp: May 03, 2018, 09:28:19 am
content: As mentioned above, you're exceeding the common mode range of the op-amp.Ideally a comparator IC should be used, such as the LM393 or LM311, rather than a generic op-amp, which will be designed for linear operation.

author: orbanp
author_rank: Regular Contributor
timestamp: May 03, 2018, 12:20:58 pm
content: You got solid advice above: do read the datasheet!Also, here is an app note that gives good info on using opamps as comparators:http://www.analog.com/media/en/technical-documentation/application-notes/AN-849.pdfPeter

author: Lee Leduc
author_rank: Regular Contributor
timestamp: May 03, 2018, 07:58:22 pm
content: Another good paper comparing op amp and comparator.http://www.analog.com/en/analog-dialogue/articles/amplifiers-as-comparators.html

author: David Hess
author_rank: Super Contributor
timestamp: May 04, 2018, 12:38:57 am
content: Quote from: Hero999 on May 03, 2018, 09:28:19 amComparators have common mode input range limits as well although the LM339/LM393 operates down to its negative supply. The LM311 is good down to 0.5 volts from its negative supply and 2 volts from its positive supply.

author: PerranOak
author_rank: Frequent Contributor
timestamp: May 04, 2018, 01:24:23 pm
content: Thanks all.I was just experimenting with comparators and found a circuit online. I had an LM741 so I gave it a go. When I used an LM393 all was well but I just wanted to understand what was happening.I looked on the datasheet for "input common voltage range" but can't find it nor anything that mentions the 2V headroom. I do have difficulty with datasheets though: it says that for my LM741C it says that the supply voltage can be +/-18V but doesn't mention that it only works properly at the higher values.Oh well, new stuff learned and thanks all.

author: David Hess
author_rank: Super Contributor
timestamp: May 04, 2018, 05:33:48 pm
content: Quote from: PerranOak on May 04, 2018, 01:24:23 pmSome operational amplifiers work fine as comparators within their limits and the 741 is one of these.QuoteWhen the 741 was designed, +/-15 volt supplies were assumed but the original datasheet shows dynamic performance and input and output voltage range graphs down to +/-5 volts. It also includes an equivalent schematic from which it is possible to derive the input and output voltage ranges without too much trouble.

author: PerranOak
author_rank: Frequent Contributor
timestamp: May 05, 2018, 01:07:38 pm
content: Quote from: David Hess on May 04, 2018, 05:33:48 pmReally. How is this done?

author: Lee Leduc
author_rank: Regular Contributor
timestamp: May 05, 2018, 01:47:21 pm
content: Wikipedia has a very nice write up of 741 op amp. It walks you through the device explaining how each stage of the circuit works. Very educational.https://en.wikipedia.org/w/index.php?title=Operational_amplifier&direction=next&oldid=436142906

author: PerranOak
author_rank: Frequent Contributor
timestamp: May 05, 2018, 03:07:55 pm
content: Beaut, thanks, I'll have a look.

author: Zero999
author_rank: Super Contributor
timestamp: May 05, 2018, 05:48:47 pm
content: Quote from: PerranOak on May 04, 2018, 01:24:23 pmPage 5, section 6.5, "input voltage range ±13V". It's specified with +/-15V supplies, which implies it's 2V of headroom from either supply rail. Note it also says ±13V typical, ±12V minimum, you you'll need to allow 3V of headroom to ensure it works over the entire temperature range.http://www.ti.com/lit/ds/symlink/lm741.pdf

author: viperidae
author_rank: Frequent Contributor
timestamp: May 05, 2018, 09:19:42 pm
content: Quote from: PerranOak on May 04, 2018, 01:24:23 pmIt says the recommended minimum supply voltage is +/- 10V. All the values in the tables are only valid when running at +/- 15V

author: David Hess
author_rank: Super Contributor
timestamp: May 06, 2018, 07:20:34 pm
content: Quote from: PerranOak on May 05, 2018, 01:07:38 pmJust add up the transistor Vce (about 0.2 volts) and Vbe (about 0.6 volts) saturation values.

author: PerranOak
author_rank: Frequent Contributor
timestamp: May 08, 2018, 07:45:02 pm
content: Excellent, thank you for all the pointers.