Op Amp Vs and Vo

dl324

Joined Mar 30, 2015
12,807
Then you would love the next exercise, Amplifier Design Using Op Amps: A Sound System. Using an LM741 as a sound amplifier, as does all of the op amp exercises in this ancient book. :eek:
Designs using LM741 will continue to work correctly as long as the part is available. Where you can get into trouble is using it inappropriately. Schools are still having students use it. Any issues are more likely to be caused by the user, not the opamp.
 

Thread Starter

SamR

Joined Mar 19, 2019
3,256
Set it up as a +/-10V noninverting amp with a gain of 2 and it started clipping the +rail at ~8.9V and bottom rail ~9.3V. Still gives an edge to the bottom rail but not quite reaching it so the -5.03V @rail number I was getting as a comparator is questionable. OK, I learned a bit more so thanks for the input guys!
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BobaMosfet

Joined Jul 1, 2009
1,653
You're interpreting the datasheet incorrectly (and, as others have mentioned, this isn't an offset voltage).

Excerpts from Fairchild LM741 datasheet:
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The offset voltage is the deviation of the opamp from ideal input characteristics. What you're talking about is output voltage swing.

The output voltage swing isn't a percentage of the power supply voltage. With a 560 ohm load, the opamp is "heavily" loaded. The datasheet only gives parameters for as low as 2k. So the worst case voltage swing with +/-5V supplies is 0V; typical would be +/-3V. It would be even worse in your case.
You're violating the common mode input voltage range. You can't count on the opamp to work under those conditions.

From the datasheet, the maximum guaranteed input range with +/-10V rails is +/-7V (3V below the rails). Typical would be +/-8V.
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Just adding to this- 2 important things from the datasheet:

Notice the statement immediately under 'Electrical Characteristics'. What Fairchild is secretly telling you here is that it expects you to use Vcc = 15, and Vee = -15 for this OpAmp. Manufacturers state their test conditions (which is actually the optimal condition for the device). This is a marketing ploy that many vendors use to make their product appear better than it is. So, while they imply 'anywhere between +15 and -15V is okey-dokey... at they are really saying is- run it at +15 and -15.

Next, notice the 'Input Voltage Range... _TYPICAL: +/- 13V. Given the above, the 13V is your guaranteed range.

Best to choose an OpAmp suited for almost rail to rail +5 to -5V for your needs. LM741 was a general purpose OpAmp, probably largely meant for use in 12 or 24Vpp systems, I'm guessing.
 

Audioguru again

Joined Oct 21, 2019
3,008
The datasheet for the 741 opamp says to never feed its input with voltages higher than its supply voltages (like you did).
You are lucky that your signal generator limited the overload current.
 

Audioguru again

Joined Oct 21, 2019
3,008
Most of us heard sounds up to 20kHz before we played with guns. The lousy old 741 opamp has slew rate limiting that causes serious trouble above 9kHz as shown on its datasheet. Also it is very noisy (hissss).
Audio opamps work perfectly up to 100kHz. They have low noise.
 

Thread Starter

SamR

Joined Mar 19, 2019
3,256
You are lucky that your signal generator limited the overload current.
I realized that at the time but wanted to push the limit on the 741 and didn't care if I burned it up doing so. Was actually surprised that it survived the overloaded input. I have noticed that these chinese SGs do not have much power after trying to use it as a low voltage AC power source.

As I said, I am just starting to learn about op amps here so taking baby steps and any ole op amp will do and 741s are cheap. Plus I already had some from back in the 70-80s that I never got around to using. I have other better op amps but unless the example calls for a specific model I'll throw in a cheap 741 and see what happens. At least for now.
 

OBW0549

Joined Mar 2, 2015
3,566
I have other better op amps but unless the example calls for a specific model I'll throw in a cheap 741 and see what happens. At least for now.
Rather than just seeing what happens, try consulting the 741 data sheet first and seeing if you can predict what will happen; then verify your prediction by experiment. Much more educational.
 

Thread Starter

SamR

Joined Mar 19, 2019
3,256
I did and it said it wouldn't reach the -rail but I was seeing -5.03V on the comparator output which was dead-on the -rail voltage. So I tried to replicate that using a voltage follower which provided no gain and the negative swing of the SG was not hitting the rail. So I did it again as a non-inverting amp with a gain of 2 and there it was. What sorta threw me off was the exercise calling for a 741 with +/-supply and being told that was not a good idea as the minimum supply voltage is not specifically spelled out in the PDF. But I'm learning more as I go.
 

dl324

Joined Mar 30, 2015
12,807
the minimum supply voltage is not specifically spelled out in the PDF.
All of the data is given for supplies of +/- 15V. You can operate above or below that as long as you know what your input and output voltage ranges are. If you have an input and/or output that swings +/- 7V, you can use +/- 10V supplies.
EDIT: removed redundant word.

That tracking supply circuit I gave you requires an input or output of +8V to -16.7V. I specified supplies of +10V and -20V; taking into account the common mode input range and output swing. That's also why I started out with a divide by 2 voltage divider and then decided to use divide by 3 so I could reduce the positive supply rail voltage from the initial design.

Incidentally, I liked that design so much that I did a toner transfer for a couple boards so I'll have an opamp power supply too.
 
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