Op Amp (uA741CN) Help

[Zero999]

You can use it on audio applications, but how it rolls off high frequencies depending on what gain you set it at and what output voltage you require.  Folks who used the 741 were able to work around those limits 30+ years ago as that opamp was widely known and dirt cheap.

I see the 741 as useless beyond 8kHz because it's the limit to the full power bandwidth and the gain is only 40dB at that point so there's hardly any negative feedback, even at fairy low gains.

Checking the chips you mention I am shocked that the 741 goes for over $1 each from Digikey, who sells the TL072 for 60c, and the LM358 for as low 12c.  There are many better spec'd op amps far cheaper than any of those three today.

I'd be interested to know about those.

I just had a quick look through Farnall/RS to see if there's anything but couldn't find it.

I asked the question: what do we want from a general purpose op-amp and came up with the following list of minimum requirements:

• Single supply operation, preferably 5V to 18V, so it can be used from a 5V power source, a 9V battery or a bipolar 9V supply.
• >40dB@20kHz plenty of gain at the upper audio band.
• Full power bandwidth >20kHz we don't want any nasty slew rate distortion on audio signals.
• Frequency compensated to unity gain.
• Can be used as a low frequency comparator without becoming unstable.
• Low noise.
• Low THD.
• Low offset.
• Capable of sourcing/sinking at least 10mA so can be used to light an LED.
• Nice through hole DIL package.
• Available in both dual and quad packages and single would be nice for lower power consumption/offset trimming.
• And lastly cheap and widely available.

[Mechatrommer]

thanx for the tip hero. now i see some amplification with previous circuit. it will distort at 30KHz (slight noticable at 20KHz) and attenuate from there (amp.png)
now its time for the fun part! 741 abuse! connect the output to various load (resistor)! pop quiz, why it can only maintain its output shape at 1K ohm or greater? (load.png)
this proven to be working ac amplifier deserve its place in the next 3rd edition of AoE (aoe.jpg!)

[scrat]

I asked the question: what do we want from a general purpose op-amp and came up with the following list of minimum requirements:

• Single supply operation, preferably 5V to 18V, so it can be used from a 5V power source, a 9V battery or a bipolar 9V supply.
• >40dB@20kHz plenty of gain at the upper audio band.
• Full power bandwidth >20kHz we don't want any nasty slew rate distortion on audio signals.
• Frequency compensated to unity gain.
• Can be used as a low frequency comparator without becoming unstable.
• Low noise.
• Low THD.
• Low offset.
• Capable of sourcing/sinking at least 10mA so can be used to light an LED.
• Nice through hole DIL package.
• Available in both dual and quad packages and single would be nice for lower power consumption/offset trimming.
• And lastly cheap and widely available.

I don't see the THD to be really an opamp parameter, but an amplifier parameter, since it depends on the use you make of that opamp. The opamp itself doesn't have to be so linear, it needs to have a high gain (and GBWP), and the feedback will make the amplifier behave linearly.

[Zero999]

You're right so you could trade THD for gain and bandwidth so there's more negative feedback to get rid of the distortion.

[scrat]

BTW, I was searching for an opamp for a simple measurement (even less than the audio band, and gain <1), and found that AD gives THD+Noise figures, too, for their opamps, even if they precisely specify the circuit (so they're indeed considering an amplifier). So you were partially right.

[tecman]

A lot of op amps, especially older designs like the 741, have crossover distortion in the output stage.  There is generally enough gain to minimize the effect, but at higher frequencies it becomes more of a factor.

paul

[saturation]

As there are so many op amps today, you can find one with specific performance for your design, say audio, than simply getting a 'general purpose' one, so you needn't pay more than you need.

However, the MCP601 from Microchip is closer to better GP needs for <= price of the 741, its typically $0.50-.70 each.  I think its easily available too worldwide.

http://uk.farnell.com/microchip/mcp601-e-p/op-amp-cmos-single-2-7v-8-dip/dp/1852038

I have to correct my LM358 price, its not 12c but 60c too.  My error.

Op amp noise is usually 10-30nV/sq rt Hz.  If you need something less than that, you'd have to find an op amp that specifies "low noise."

For high performance 'gp' op amps, consider this, at $6 each.

http://uk.farnell.com/linear-technology/lt1226cn8-pbf/ic-op-amp-high-speed-dip8-1226/dp/1330624

You can use it on audio applications, but how it rolls off high frequencies depending on what gain you set it at and what output voltage you require.  Folks who used the 741 were able to work around those limits 30+ years ago as that opamp was widely known and dirt cheap.

I see the 741 as useless beyond 8kHz because it's the limit to the full power bandwidth and the gain is only 40dB at that point so there's hardly any negative feedback, even at fairy low gains.

Checking the chips you mention I am shocked that the 741 goes for over $1 each from Digikey, who sells the TL072 for 60c, and the LM358 for as low 12c.  There are many better spec'd op amps far cheaper than any of those three today.

I'd be interested to know about those.

I just had a quick look through Farnall/RS to see if there's anything but couldn't find it.

I asked the question: what do we want from a general purpose op-amp and came up with the following list of minimum requirements:

• Single supply operation, preferably 5V to 18V, so it can be used from a 5V power source, a 9V battery or a bipolar 9V supply.
• >40dB@20kHz plenty of gain at the upper audio band.
• Full power bandwidth >20kHz we don't want any nasty slew rate distortion on audio signals.
• Frequency compensated to unity gain.
• Can be used as a low frequency comparator without becoming unstable.
• Low noise.
• Low THD.
• Low offset.
• Capable of sourcing/sinking at least 10mA so can be used to light an LED.
• Nice through hole DIL package.
• Available in both dual and quad packages and single would be nice for lower power consumption/offset trimming.
• And lastly cheap and widely available.

[Mechatrommer]

ok since this has become op-amp selection (which i dont mind, infact i'm looking forward to it). what i have in mind is an op-amp/amplifer that can work as voltage follower (buffer) non inverting amplifier for my hantek 3x25, work up to 100MHz, something like 12Vpp or ±6V 24Vpp, acceptable(*1) noise and distortion, that can give some reasonable(*1) power to a load. i dont have any clue, but this 741 exercise proved that building a voltage follower for that is not as easy as theory said. if any pointer, i'll thank you.

(*1) acceptable and reasonable terms are subjective matter as i'm not expert on this, you may define whats acceptable and reasonable based on quality vs price, performance, usability, availability etc.

[Mechatrommer]

http://uk.farnell.com/linear-technology/lt1226cn8-pbf/ic-op-amp-high-speed-dip8-1226/dp/1330624
just saw this, just like what i spec'ed above, can it handle high load? or do i have to create extra circuit (stage) for that?

[Zero999]

That's far too expensive fot general purpose.

The Microchip op-amp looks good but it's limited to a power supply voltage of only 6V.

It seems like a reasonable general purpose op-amp doesn't exist, the semiconductor companies have seemed to have gone for op-amps designed for specific applications, rather than general purpose.

[saturation]

The relationship of slew rate, output voltage and maximum output frequency is:

Slew Rate in V/uS ~= 2*pi*Vp*MHz

~ 3770 V/uS.

Vp = 6V
MHz = 100

You can use a lesser slew rate, but the output voltage or max frequency  has to be dropped to compensate.

ok since this has become op-amp selection (which i dont mind, infact i'm looking forward to it). what i have in mind is an op-amp/amplifer that can work as voltage follower (buffer) for my hantek 3x25, work up to 100MHz, something like 12Vpp or ±6V, acceptable(*1) noise and distortion, that can give some reasonable(*1) power to a load. i dont have any clue, but this 741 exercise proved that building a voltage follower for that is not as easy as theory said. if any pointer, i'll thank you.

(*1) acceptable and reasonable terms are subjective matter as i'm not expert on this, you may define whats acceptable and reasonable based on quality vs price, performance, usability, availability etc.

[Mechatrommer]

That's far too expensive fot general purpose.

£6.20, i think its worth it rather than $100++ on another dedicated FG that can do higher voltage. not for production, but personal use. and i believe shipping cost to my country will render this figure negligible, i might want to get a couple or dozen of this in single package (order 10 or more will dramatically reduced to half priced), will try on ebay if i have luck. 1GHz BW is too much though for my FG, but maybe usefull for another project, could be a RF Amplifier?.

The relationship of slew rate, output voltage and maximum output frequency is:
Slew Rate in V/uS ~= 2*pi*Vp*MHz
~ 3770 V/uS.
Vp = 6V
MHz = 100

LT1226CN8 spec:
Supply Voltage Range:± 2.5V to ± 15V
Bandwidth:1000MHz
Slew Rate:400V/µs

so there its confusing. for 1GHz it should be ~94KV/us. at 1GHz, 400V/us... it will be only 0.06Vp (-24db the 15Vp), why on earth it is spec'ed as 1GHz BW?

edit: found it! LT1226 in Ebay, the same hongkong seller that i found earlier selling bunches of fancy op-amps, you may want to check his store. but the listing got no item description whatsoever. so i think he expects the buyer to know what they want. but he's the only one selling this kind of stuff, no other option.

[saturation]

hi mecha,

May I suggest look for another op amp, I haven't time right now to persuse Digikey but roughly:

reduce the output voltage from 6V to 0.6Vp or 1.2Vpp at 100 MHz, uses a slew rate of 377V/uS.
reduce the frequency to 10 MHz you can use 6Vp or 12Vpp for the slew rate of 377V/uS.

So the LT1226 is limited but its a 'general purpose' op amp for MHz frequencies, but still not the specs you required.

I can try to spec one out later but I can't right now.

The 1 GHz isn't bandwidth, its bandwidth product, its frequency x gain.  So its 1 GHz at unity gain, but still limited by slew rate so the maximum output is ~ 60mVp.

I was looking for such an op amp such months ago to use as a voltage follower for the Hantek, but I got side tracked and didnt' buy it.  I have the unit data somewhere.

The relationship of slew rate, output voltage and maximum output frequency is:
Slew Rate in V/uS ~= 2*pi*Vp*MHz
~ 3770 V/uS.
Vp = 6V
MHz = 100

LT1226CN8 spec:
Supply Voltage Range:± 2.5V to ± 15V
Bandwidth:1000MHz
Slew Rate:400V/µs

so there its confusing. for 1GHz it should be ~94KV/us. at 1GHz, 400V/us... it will be only 0.06Vp (-24db the 15Vp), why on earth it is spec'ed as 1GHz BW?

edit: found it! LT1226 in Ebay, the same hongkong seller that i found earlier selling bunches of fancy op-amps, you may want to check his store. but the listing got no item description whatsoever. so i think he expects the buyer to know what they want. but he's the only one selling this kind of stuff, no other option.

[Mechatrommer]

its ok saturation, i'll work it out. your advice on technical part is much more important. take your time

[jahonen]

If you want slew rate, try THS3091.

Regards,
Janne

[Mechatrommer]

thanx janne for the pointer. i've made a little exercise, (reading from another source about things like rise/fall/settling time and some calculation on slew rate needed, comparing both Hantek 3x25's op-amp (LMH6702) and the fast slew rated THS3091. here is the outcome (picture). seems tough on this one (requirement), with compromises, THS3091 still can be used as 2nd stage amplifier (G = 2, ie 1.2Vmax or 2.4Vpp out) and at cheaper price compared to LT1226CN8 mentioned earlier. will be keep looking, at least i've gain knowledge in op-amp parameters. thanx guys.

[saturation]

Great mecha.  The reason I'm reluctant to recommend an op amp is there are 29,000 listed on Digikey. almost 200 with slew rate over 3800V/uS.  If you can deal with non-DIP packaging, some spectacular op amps are under $3, because you can still optimize other parameters like you spreadsheet shows.

For a DIP, I was looking at this, which fits the Hantek nicely, but I didn't delve into the non-DIP competitors:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM7171BIN-ND

The LM7171 for $3.

Also, some chips maybe easier to get in Malaysia that would also work, that would not be my 1st choice because of availability here or its final price or that I prefer DIP for prototyping.  

thanx janne for the pointer. i've made a little exercise, (reading from another source about things like rise/fall/settling time and some calculation on slew rate needed, comparing both Hantek 3x25's op-amp (LMH6702) and the fast slew rated THS3091. here is the outcome (picture). seems though on this one (requirement), with compromises, THS3091 still can be used as 2nd stage amplifier (G = 2, ie 1.2Vmax or 2.4Vpp out) and at cheaper price compared to LT1226CN8 mentioned earlier. will be keep looking, at least i've gain knowledge in op-amp parameters. thanx guys.

[tecman]

I am not sure of your end game.  There was some mention of audio work earlier in the post.  Look at an NE5532.  Low cost and excellent for audio applications.

paul

[Mechatrommer]

I am not sure of your end game.  There was some mention of audio work earlier in the post.  Look at an NE5532.  Low cost and excellent for audio applications.
paul

this is for 100MHz signal, as much as Vpp we can squeeze out of it, but less stringent requirement on THD, imo. Audio is vice versa (lower KHz freq but very low noise and THD)... AFAIK

[Mechatrommer]

20V-100V supply? well thats really something. thanx for your suggestion, it will be very good for HV job.
edit: and errr... the price... $328.48?

[saturation]

I think the thread shows easily everyone has their favorite op amp.

I would suspect that the uA741 is mostly purchased by schools and students.  It has all the features of the ideal op amp [ but less ideal performance!]  As the 741 is old bipolar technology, its suited to student abuse, and for running with low end gear found in schools.  Also, if the basics of op amps have remained unchanged for years, why update the paperwork to accommodate a different chip and give the instructor more work?

[Mechatrommer]

i would like to extend this "off topic" matter of op-amp selection...
my previous parametric


since THS3091 is out of stock in ebay, forced me to browse more... at last i found the luxury that i thought was impossible.
THS3202, 2GHz, 9KV/us, ±7.5Vss
THS3061, 300MHz, 7KV/us, ±15Vss

my intention is just to buffer and amplify my 100MHz (0.6Vpp - 3.5Vpp) signal out of a function generator.
both opamp have strength and weakness (Vss vs GBW) so i think i will get both. in process negotiating with seller.
luxurious doesnt come cheap. both of them priced USD10 each. so if you have a clear view that what i'm doing is wrong, pls stop me. thanx.

[DaveW]

It's probably not an issue for a simple lab application like that, but in general good design practice is to not have more bandwidth than you need for a given application, and if you do, to limit the bandwidth. Should help reduce noise

[Mechatrommer]

thanx for the advice Dave. so maybe for proper design, the THS3061 is better? since its GBW 300MHz, i can crank a bit more 2-3x gain at 100MHz, because thats my intention (buffer and amplifier). for the 2GHz GBW THS3202, maybe not a good use for 100MHz amplification, but maybe i can extend its use for higher frequency amplifier. i will see if i can use it as 434MHz RF amplifier (3-4X gain?).

[scrat]

Remember not to push the GBWP to the limit, since there you will see a behaviour quite far from the ideal one. I think something around 2x or 3x should be good, best if a small capacitance limits the BW.
Moreover, look at the minimum GBWP spec!

BTW: which is the situation about samples in your country? Don't TI, Analog, Linear and National send them?