Positive and negative reails on LM741 - Page 1
Forum section: electronics / beginners

author: tiofilo75
author_rank: Contributor
timestamp: July 21, 2013, 08:01:45 am
content: So I am using an LM741 to amplify a signal from 0-3.1v to 0-9.1V. Since I only want a positive voltage I did not think about using the negative rail of the amplifier so I took it to ground. It turns out that I do need the negative voltage so that my signal can go to ground, with out it my signal only drops down to about 5.6V. I wanted to know why this is the case. Thanks.

author: AndyC_772
author_rank: Super Contributor
timestamp: July 21, 2013, 08:16:09 am
content: I remember one of my school textbooks referring to the 741 as the "Model T Ford" of op-amps.At the time it was written, I think this comparison was meant as a compliment - the standard, generic op-amp that anyone can afford and lots of people use. Nowadays it remains just as true now as it ever was; the 741 is utterly obsolete, and whilst there may still be quite a few around, they really have historical value only. There's no useful comparison with modern alternatives.Both the input and output voltages are limited compared to the power supply - so, for example, if your're using it as a non-inverting amplifier with a gain of 3 or thereabouts, the 5.6V minimum output you're seeing corresponds to a minimum input voltage of about 1.8V above GND. That looks prettty normal for a 741, the data sheet is rather vague.What you need is an op-amp with input and output voltages that include GND, and there's plenty of them out there. Search for ones that have rail-to-rail input and output capabilities if you want to avoid this problem in future.

author: Zero999
author_rank: Super Contributor
timestamp: July 21, 2013, 08:22:01 am
content: It's because the LM741 won't work when the inputs are too close to either power supply rail and the output also can't go down to the negative or up to the positive rail. The exact values are listed on the datasheet.To do what you want with an LM741, you need a -3V and +12V rail.There are much better op-amps available which will do this with a single +12V supply. The 741 is really old. I don't know if it's still in production. The old LM358 should do the trick if it's just low frequency but I'd recommend something a little newer such as the MC34071 (same pin out as the 741) or MC34072.

author: Paul Price
author_rank: Super Contributor
timestamp: July 21, 2013, 12:24:04 pm
content: The LM741 is still a current product and readily available. It can handle 40V, almost all others drop dead around 36V or less, this has great importance with someone working with 24V AC transformer power supplies.

author: SeanB
author_rank: Super Contributor
timestamp: July 21, 2013, 12:35:07 pm
content: Got a whole lot of them in TO100 cans......

author: Zero999
author_rank: Super Contributor
timestamp: July 21, 2013, 01:54:57 pm
content: Quote from: Paul Price on July 21, 2013, 12:24:04 pmI've never understood why the 741 refused to die.There are many better alternatives, for example, the MC34071 can work up to the same voltage as the LM741.To work at 24VAC you need an op-amp capable of running off 70V and I don't know know of any op-amps which can do that.

author: BravoV
author_rank: Super Contributor
timestamp: July 21, 2013, 02:09:14 pm
content: Quote from: Hero999 on July 21, 2013, 01:54:57 pmLTC6090 - 140V CMOS Rail-to-Rail Output, Picoamp Input Current Op Amp ->http://www.linear.com/product/LTC6090Features :Supply Range: ±4.75V to ±70V (140V)0.1Hz to 10Hz Noise: 3.5?VP-PInput Bias Current: 50pALow Offset Voltage: 1.6mV MaximumRail-to-Rail Output StageOutput Sink and Source 10mA10MHz Gain Bandwidth Product19V/µs Slew Rate11nV/?Hz Noise DensityThermal Shutdown

author: Paul Price
author_rank: Super Contributor
timestamp: July 21, 2013, 02:38:49 pm
content: Hero999, I think you are quite mistaken about requiring 70V op-ampsto be used for 24VAC based power supplies.The highest voltage would be 1.414 x 24V and within the capabilities of the LM741, that's why you will find so many power supplies using commonly available 24V transformers use LM741's.

author: Paul Price
author_rank: Super Contributor
timestamp: July 21, 2013, 02:49:16 pm
content: The +-70V LTC6090 cost $8 US while the +-20V LM741 cost 33 cents quan 1.So for 24VAC powered power supplies it is24 times cheaper.

author: Bloch
author_rank: Unknown
timestamp: July 21, 2013, 03:08:48 pm
content: Quote from: Paul Price on July 21, 2013, 02:38:49 pmI have never seen apower suppliewith a LM741 but i have seen many with a LM723

author: jancumps
author_rank: Unknown
timestamp: July 21, 2013, 03:24:47 pm
content: Quote from: Bloch on July 21, 2013, 03:08:48 pmI have one.

author: Bloch
author_rank: Unknown
timestamp: July 21, 2013, 03:57:06 pm
content: Quote from: jancumps on July 21, 2013, 03:24:47 pmOh i did not mean that they are not used in power supplies.But I have fixed some power supplies from Zetagi >>High Quality<< as we all know is only quality that come from Italy. They used a L146 but that is same as LM723.

author: SeanB
author_rank: Super Contributor
timestamp: July 21, 2013, 06:13:57 pm
content: I have used 741's on +-24V rails, they survived quite well, though to be safe I did give each one it's own little finned push on TO100 heatsink. They ran cool then, otherwise they were barely warm, or warm at a load of 20mA.

author: ignator
author_rank: Regular Contributor
timestamp: July 21, 2013, 06:57:19 pm
content: I did not see anyone answer the OP original question.tiofilo75:Look at the schematic from the spec for the LJ741. The output drive transistors of this device have collector emitter saturation limits, as well as the series resistors. The specification you want to look at is output voltage swing. This will be in the electrical characteristics table. It will typically show several power supply values (Vs), and output load resistor values. From this you can determine the output voltage from the min/max spec.

author: Zero999
author_rank: Super Contributor
timestamp: July 21, 2013, 07:20:10 pm
content: Quote from: Paul Price on July 21, 2013, 02:38:49 pmOh, sorry I thought you were talking about amplifying 24VAC which is a peak to peak voltage of 1.414*24*2 = 68V

author: Vernon
author_rank: Newbie
timestamp: July 21, 2013, 07:21:09 pm
content: Quote from: ignator on July 21, 2013, 06:57:19 pmFinally someone gave an answer! Thanks ignator for reeling in the thread. I would just like to add for tiofilo... A good engineering practice is to evaluate what you have on hand vs. another solution such as a more up to date op-amp design with "rail to rail" capability. For example if you have the LM741 and possibly the parts needed to power it with +/- supplies, maybe stick with that. Otherwise, if you will need to buy more parts it may be more cost effective to design with a different amp.