Function Generator and OpAmp - Page 1
Forum section: electronics / beginners

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 11, 2018, 04:36:32 am
content: Hi guys,I have a LM741 breadboarded in a basic non inverting configuration. I set the resistor to have a gain of 3.It works, when I put 1 volt in, the output reads 3V.Now my issue is how to connect my function generator to it?as Im using a dual supply, my ground is the center tap of the transformer, which is a 24+24 so it is not earth referenced.Do I need to connect the ground of my function generator? If yes, how? because as my FG is earth grounded, I would end up shorting it to the center tap of the transformer, right?

author: Zero999
author_rank: Super Contributor
timestamp: June 11, 2018, 08:19:22 am
content: The centre tap of the transformer is not earthed, so connecting it to the signal generator's ground will not cause a short circuit.My only concern is the supply voltage might be too high for the '741. What voltage do you measure between pins 4 and 7?

author: bd139
author_rank: Super Contributor
timestamp: June 11, 2018, 10:38:01 am
content: Genuine TI 741's are quite robust little opamps. I've accidentally stuffed 80 across both rails and it survived and appeared to work unharmed. Got a bit warm!

author: Old Printer
author_rank: Frequent Contributor
timestamp: June 11, 2018, 12:47:15 pm
content: The data sheet shows a max of +-22V. I devoted a good part of this weekend becoming familiar with op amps, the 741 in particular. Having my troubles too so I am interested in this thread.

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 11, 2018, 06:56:55 pm
content: Quote from: Hero999 on June 11, 2018, 08:19:22 amYou are absolutely right, I didnt explain it right. I made a variable power supply, so the transformer is 24+24 but the voltage I was applying was 10 and -10.

author: Zero999
author_rank: Super Contributor
timestamp: June 11, 2018, 08:56:59 pm
content: Quote from: AngraMelo on June 11, 2018, 06:56:55 pmWell the op-amp will be fine, with that voltage range. It may be even better to go higher, since the '741 is mostly specified at +15V and -15V.My only concern now is the regulators might be receiving too higher voltage: 24-0-24V would give +34V and -34V out, possibly up to 20% more if the transformer is lightly loaded, or the mains supply is on the high end of the tolerance range.

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 11, 2018, 09:48:55 pm
content: Quote from: Hero999 on June 11, 2018, 08:56:59 pmIt was a mistake on my part, when I said about the 24+24 I was only talking about the transformer, I was actually feeding the opamp 10 and -10

author: Audioguru
author_rank: Super Contributor
timestamp: June 11, 2018, 11:42:06 pm
content: What are you using an old 741 opamp for? Its design is 50 years old and there are much better opamps available today.

author: Old Printer
author_rank: Frequent Contributor
timestamp: June 12, 2018, 02:36:46 am
content: Quote from: Audioguru on June 11, 2018, 11:42:06 pmOk. I'll bite. What should beginners studying op amps be using? I was using the 741 as well, mostly because there is so much information on it available.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: June 12, 2018, 03:18:19 am
content: I use them before committing a much more expensive ones to a board. I've learned that from managing to fry the expensive ones (such as $15 precision quads). So, while debugging the board, I use pin-identical 741s.Maybe that's all too passe' and I should junk all my through-the-hole and DIP shit (he-he, I think I'll trademark that).

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 12, 2018, 04:21:39 am
content: Quote from: Audioguru on June 11, 2018, 11:42:06 pmIm a beginner, I dont know much about opamps, I chose the 741 because it is so popular and there are lots of experiments and information about it.Im just trying it out to see if I can make it work correctly.It is just a study.,Also, I live in Brazil, we dont have access to all the ICs you guys from north america have.Besides, even if I get my hands on the new stuff, what is the purpose? If I dont know how it works there is not point.

author: bd139
author_rank: Super Contributor
timestamp: June 12, 2018, 06:45:04 am
content: 741 is absolutely fine to learn with. No nasty surprises and it’s far from ideal so you can learn about the limitations of opamps easily.

author: JS
author_rank: Frequent Contributor
timestamp: June 12, 2018, 06:51:41 pm
content: Here is a list of my conventional opamps to go.NE5532 and NE5534 (the first dual opamps in dip8, the second is the same thing but single, but it has some caveats, it's faster and it must be slowed down if used at gains under 10dB) All audio related stuff as long as it not super high impedance like instrument input. BJT input devices.OP27 for precision, low noise applications. BJT input devices.LM358 for low voltage or single rail applications. BJT input devices.TL07X (71, 72, 74) are quite common and very useful. Low input current which makes it ideal for high input impedance circuits. JFET input devices.CA3140 is MOSFET input, low noise and low input current, reasonable fast, good for high impedance low noise applications.I do have a bunch of LM741 I bought for some reason some time back, when you need an opamp just to work are fine, super stable and reliable. CA1458 is the dual version.For experimentation, 741 are fine, you can learn a lot from them, the only thing you would be missing when you jump to more modern ones are stability issues, as it's slower it doesn't have those, so, you could call it a feature for your situation. The input offset is high but you can trim it, the noise, well, you won't be learning opamps with low noise applications, those are quite bitchy.JS

author: Old Printer
author_rank: Frequent Contributor
timestamp: June 12, 2018, 07:07:02 pm
content: Thank you all to those who responded with suggestions. My question was a bit tongue in cheek. When someone in a beginners forum criticizes a beginners choices, I would hope they would at least include a few suggestions of what that poster's opinion of a good choice would be. Otherwise it is just criticism for criticism sake.

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: June 12, 2018, 07:21:55 pm
content: Quote from: bd139 on June 12, 2018, 06:45:04 amAlso less likely to oscillate at 500MHz on a breadboard with 6 inch long wires everywhere.

author: bd139
author_rank: Super Contributor
timestamp: June 12, 2018, 07:49:03 pm
content: Quote from: In Vacuo Veritas on June 12, 2018, 07:21:55 pmYep that’s a very good point too.My go to opamps are:MCP6201 GP low voltage. Cheap as chips. Does the job.LMC6482 GP high voltage and dual rail.LTC1150 chopper. Low offset apps, integration fun.OP400 low noise quad.Got tubes of LM741, LF356, LF411, LM358 floating around I haven’t touched for a while. They’re good but I don’t want to use designs that use dual rails these days. Too much hassle

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2018, 11:04:33 am
content: Quote from: bd139 on June 12, 2018, 07:49:03 pmAny op-amp can be used with dual rails. One just has to ensure the common mode range isn't exceeded and the OP400 isn't a single supply op-amp, as its common mode range doesn't include the negative rail.The MCP6201 and LMC6482 are only rated to 6V and 16V respectively.The LM358 is fine for single supply, DC applications. Also check out the LM321, which is the single version of the LM358, comes in a tiny package and is extremely cheap.

author: bd139
author_rank: Super Contributor
timestamp: June 13, 2018, 11:12:16 am
content: Sorry my point should have been more clear. I’d rather build a system that has a single supply at low voltage therefore requires the inputs and outputs to get relatively close to the rails. Not a lot of room at 5v. But it keeps the power arrangements simpler.OP400 you’re right. I’ve used exactly two of those and they had 24v available.