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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 2:41 pm 
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anchorman wrote:
I'm just a rank amateur here, but it seems that the input couldn't swing +/- if it wasn't riding on a DC offset too, given that the inverting input is ground referenced. Wouldn't that DC offset be amplified by the opamp along with the AC portion of the signal, thus preserving the DC offset in the output and preventing any clipping of the AC waveform?

It took me a while to figure this out, but after removing the double negatives, yes, if the input signal had a positive DC offset it also would be amplified and there would be no half wave clipping at the output.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 2:49 pm 
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anchorman wrote:
Scott wrote:
I am not going to burn my head on this one. My best guess is that there is a DC offset and the AC coupled probe shows it dropping below zero. How the DC offset was achieved remains up in the air.


I'm just a rank amateur here, but it seems that the input couldn't swing +/- if it wasn't riding on a DC offset too, given that the inverting input is ground referenced. Wouldn't that DC offset be amplified by the opamp along with the AC portion of the signal, thus preserving the DC offset in the output and preventing any clipping of the AC waveform?

There is a differential input---IOW, the device responds to the voltage difference between the + and - input terminals. The data sheet gives the allowable voltages on these inputs based on the supply voltage set.

The most common setup for an op-amp working down to DC is to have both positive and negative supplies, with a common reference (return or "ground")). This node is also used as the reference point for the inputs and feedback networks. When using a single supply, either supply line could be designated as the common reference. However, for DC operation, the inputs would need to be biased somehow to put them at or near the midpoint. If the input is AC coupled (on both terminals), then the issue goes away)

For your reading pleasure:
http://www.ti.com/lit/ds/symlink/lm358.pdf

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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 3:41 pm 
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LM386 wrote:
Dennis H. wrote:
I do not remember exactly which scale each was on but I remember that they showed a 10 times increase. I did not make any changes in the original schematic.


Is there any chance that the ground lead to your oscilloscope was not properly connected (floating) and the two traces are actually nothing more than power line hum?

Do you have 10X probes on the oscilloscope and was one of the scope probes switched to 10X and the other to 1X?


Nope, grounds were connected and probes set to 1X.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 3:46 pm 
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anchorman wrote:
pauls.ironhorse wrote:
We have come a long way since this OP_Amp was offered as the advanced State of the art..


If anyone knows where I can get one of those for cheap... I'd love to have in my "collection"/hoard.


If you're talking about and LM358 they are available on amazon.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 4:19 pm 
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I think he was referring to my quiz question about the uA709...

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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 4:43 pm 
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anchorman wrote:
pauls.ironhorse wrote:
We have come a long way since this OP_Amp was offered as the advanced State of the art..


If anyone knows where I can get one of those for cheap... I'd love to have in my "collection"/hoard.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 10:23 pm 
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anchorman wrote:
Scott wrote:
I am not going to burn my head on this one. My best guess is that there is a DC offset and the AC coupled probe shows it dropping below zero. How the DC offset was achieved remains up in the air.


I'm just a rank amateur here, but it seems that the input couldn't swing +/- if it wasn't riding on a DC offset too, given that the inverting input is ground referenced. Wouldn't that DC offset be amplified by the opamp along with the AC portion of the signal, thus preserving the DC offset in the output and preventing any clipping of the AC waveform?


Yes. I addressed 2 issues with my statement. The OP is getting a clean sinewave out because the input has a DC offset and the signal is not being driven into the rails. Is it coming from the generator or a mis-wiring, I don't know. Either way, a DC offset is in play. Second item is why the OP was showing the output dropping below zero on his O-scope, and that is because he is using AC coupling on the scope.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Fri 12, 2019 2:50 am 
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Here is the same circuit I posted last evening the only difference is the input is now 600mV pkpk with a 300mV DC offset from my function generator. Scope setting are the same and both the input and output have shifted upward as would be expected. As can be seen the output is now within the 0 to 12V supply rails (3.25V min to 9.938V max). If the DC offset is lowered the bottom will clip, likewise if the offset is increased the top will clip. This is due to the non rail-to-rail characteristics of this device.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Fri 12, 2019 12:34 pm 
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flyboy71 wrote:
Here is the same circuit I posted last evening the only difference is the input is now 600mV pkpk with a 300mV DC offset from my function generator. Scope setting are the same and both the input and output have shifted upward as would be expected. As can be seen the output is now within the 0 to 12V supply rails (3.25V min to 9.938V max). If the DC offset is lowered the bottom will clip, likewise if the offset is increased the top will clip. This is due to the non rail-to-rail characteristics of this device.


Yes this is exactly the case.
I started this discussion because I wasn't get ANYTHING at all. That was traced to a lead from my power supply not making connection.
Some of my confusion on my interpretation was due to my inexperience at reading a scope. Up to this point I've only used one to find signals not measure them. I do have a lot to learn.

Nice scope by the way. I just picked up the 100 mhz version of that scope, a 54600, yesterday. It seems to work perfectly and passes all of the self tests. I'm printing the manual this weekend.

Again thanks to all who posted it has been enlightening.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Fri 12, 2019 12:56 pm 
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Dennis H. wrote:
Yes this is exactly the case.

But this shouldn't be the case if using the original circuit which you posted with a single supply, which now leads to the question, what are you using for a signal generator for the input signal?


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 Post subject: Re: Op Amp problems
PostPosted: Apr Fri 12, 2019 1:57 pm 
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LM386 wrote:
Dennis H. wrote:
Yes this is exactly the case.

But this shouldn't be the case if using the original circuit which you posted with a single supply, which now leads to the question, what are you using for a signal generator for the input signal?

IOW---It isn't right until <<someone>> understands why it was wrong.

Seriously, the best plan is get enough insight to avoid similar issues in the future.

this might help in working out what happened:
The typical Op-Amp produces an output based on the difference between the signals at the + and - inputs. This could imply that the absolute value does not matter--but, as with most circuits--the allowable
range of voltages depends on the power supply connections. (one exception is circuits that are transformer-coupled)
A device data sheet will specify the allowable range of voltages for both input and output. These are given with reference to one of the power supply "rails".
So, the basic input/output relationship is valid only when the DC voltages are within the specified limits.
When either input or output is AC-coupled, the DC level is set by the internal circuitry and/or the feedback network.
With all this in mind, a complete diagram of the test setup, including the reference point (return) for all power supplies and signal sources, will allow explanation of how the circuit will operate.

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Last edited by pixellany on Apr Fri 12, 2019 2:27 pm, edited 2 times in total.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Fri 12, 2019 2:20 pm 
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pixellany wrote:
LM386 wrote:
Dennis H. wrote:
Yes this is exactly the case.

But this shouldn't be the case if using the original circuit which you posted with a single supply, which now leads to the question, what are you using for a signal generator for the input signal?

IOW---It isn't right until <<someone>> understands why it was wrong.

Seriously, the best plan is get enough insight to avoid similar issues in the future.


I'm using an HP 3311A that I bought in the classified. I believe the offset it set on 0 but probably not exactly. I am probably getting just enough DC to make it look right.
I do understand why I am getting what I got and not what the simulation showed.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Fri 12, 2019 2:28 pm 
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I just edited my last post while you were posting---sorry for any confusion.

it sounds like things are converging....

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 Post subject: Re: Op Amp problems
PostPosted: Apr Sat 27, 2019 9:28 pm 
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The LM358 is marketed as a single supply op-amp.

The input transistors are PNP bipolar transistors, biased so the inputs continue to function slightly below the negative supply pin voltage. ( "Slightly" means hundreds of mV. Less than one diode drop.)

The LM358 output will go very close to the negative supply pin voltage, with a light load.

The PNP input devices require some base bias current. This causes the input pins to pull-up, toward the positive supply pin.

The 10k resistor in series with the signal source, combined with the input bias current, will establish some slight positive bias voltage for the non-inverting input. At low enough signal levels, the original circuit will function with the op-amp operating within its linear range.

Since the circuit has a DC voltage gain close to 10X, the needed positive bias for the non-inverting input pin is only about +600mV. ( We presumably want the no-signal output close to one half the 12V supply.)

If the signal source has a high DC output resistance, that would contribute to the needed positive input bias voltage.

As already pointed-out, the original circuit is not really safe to use, even if one breadboard example appears to work OK. It may not work tomorrow, particularly if a different LM358 is used. It may work in the sun, but not in the shade.

The simulations seen above may use device models that don't accurately model the particular LM358 used in the physical breadboard.

Another consideration entirely is that the LM358 is not a low-distortion audio amplifer. The output stage is low power, but generates crossover distortion. If you can stand the extra power, a pull-up resistor can force class-A operation to cure this.

The LM358 (Dual) and LM324 (Quad) op-amps are very useful for many things, even if they are not a new design. One plus is that they are slow and stable enough to work well in the white "poke the part leads in the holes" breadboards. If TI has stripped the functional schematic from the datasheet, go find an old National datasheet that includes a schematic of the IC's insides. The application data in the older datasheet may be more complete, as well.

Ted


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 Post subject: Re: Op Amp problems
PostPosted: Apr Sun 28, 2019 6:03 pm 
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I just came across this application book, pretty complete.
http://www.ti.com/lit/an/sboa092b/sboa092b.pdf

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