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 Post subject: Re: Op Amp problems
PostPosted: Apr Wed 10, 2019 7:13 pm 
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pixellany wrote:
LM386 wrote:
flyboy71 wrote:

I was amazed when reading through this thread of the number of individuals who responded who admitted knowing little about Opamps.
I couldn't understand why anyone would respond to a post in which they knew little about the subject.

I just skimmed back and did not spot anyone making an inappropriate post.....It is not uncommon here to get a bit of banter where someone is simply guessing at the answer while admitting their lack of in-depth knowledge. If we outlawed that, there'd be just about no-one left.

Just for the halibut, how about we try some questions:

1. What's a uA709 and why was it so difficult to work with?

2. Who was Robert (Bob) Widlar?

3. Before CMOS took over everything, what was the key trick to get low input currents on the early op-amps?

4. Why is it often important to have a very high open-loop bandwidth?

1. The first widely used successful integrated op-amp. It was difficult to work with mainly because it was uncompensated requiring not only the design of the basic circuit to (as in the OP's question), but also required the design of the external frequency compensation to insure stability.

2. Bob Widlar was the (eccentric) analog design genius behind many integrated circuits such as the LM109 voltage regulator and later the LM10 op-amp. See https://www.autodesk.com/products/eagle/blog/bob-widlar-life-engineering-legend/

3. A JFET front end, either integrated onto the chip, as part of a hybrid IC or externally.

4. There are several reasons why a very high open-loop bandwidth.
a. First and most obvious is the requirement that the circuit operate at high frequencies.
b. Then there is the requirement that the op-amp still behave as expected even at lower frequencies. A good example of this is a low pass filter. It should have an open-loop bandwidth high enough that the higher frequencies that the circuit is designed to filter out ARE filtered out. If this condition isn't met the filter may have the desired attenuation at ten times the corner frequency, but a less attenuation at higher frequencies.
c. While the op-amp may be used to condition low frequency baseband signals it must have sufficient bandwidth to faithfully reproduce complex waveforms. A good example of this is a pulse amplifier where the pulse repetition rate may be 1000 Hz., but to amplify the pulses faithfully would require a 10MHz. bandwidth. this is also a function of slew rate which, in general, increases with open-loop bandwidth.

John

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 Post subject: Re: Op Amp problems
PostPosted: Apr Wed 10, 2019 7:29 pm 
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Yeah!!

nittypicky:
--when did "super-beta" junction transistors come in? (I seem to remember hearing Widlar discussing this.)

--One reason for high bandwidth is so it can be made stable at high loop gains, n'est-ce pas?

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 Post subject: Re: Op Amp problems
PostPosted: Apr Wed 10, 2019 7:45 pm 
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Here is a photo of my power supply. 12V pos. and ground. Nothing connected to 12V neg.
And a photo of my scope. The upper trace is the input and the lower one is the output.
The lower trace is centered on 0 volts and is symmetrical above and below 0. The upper trace was not centered on 0v.
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. I do not know how it is going below 0 volts. Maybe the LM358 op amp has an internal voltage divider. How else would a single supply op amp work?


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power supply.jpg
power supply.jpg [ 159.56 KiB | Viewed 246 times ]
trace.jpg
trace.jpg [ 145.45 KiB | Viewed 246 times ]

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 Post subject: Re: Op Amp problems
PostPosted: Apr Wed 10, 2019 10:06 pm 
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Is your scope AC or DC coupled? If it is AC coupled, the output waveform could appear to go below 0 Volts.

If your scope is switched to DC coupling, what is the DC voltage on the output?


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 Post subject: Re: Op Amp problems
PostPosted: Apr Wed 10, 2019 10:19 pm 
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If the Op-Amp is powered with only +12 and "ground", then its output cannot swing negative!! How is that power supply connected internally?---eg is "ground" the return for both +12 and -12?

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 Post subject: Re: Op Amp problems
PostPosted: Apr Wed 10, 2019 10:58 pm 
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Switch the scope input to GND and see where the horizontal trace is.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Wed 10, 2019 11:24 pm 
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Scott wrote:
Switch the scope input to GND and see where the horizontal trace is.

Centered on 0.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Wed 10, 2019 11:26 pm 
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LM386 wrote:
Is your scope AC or DC coupled? If it is AC coupled, the output waveform could appear to go below 0 Volts.

If your scope is switched to DC coupling, what is the DC voltage on the output?


If the trace is centered on 0 without any input, how can it appear to go below 0 volts if it's not really going below 0 volts? I don't understand that.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Wed 10, 2019 11:50 pm 
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AC coupling removes the DC component from the waveform.

Again, is your scope AC or DC coupled? It is likely AC coupled and the output of the LM358 is actually at some DC potential above zero volts with an AC component superimposed on the DC.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 12:08 am 
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LM386 wrote:
AC coupling removes the DC component from the waveform.

Again, is your scope AC or DC coupled? It is likely AC coupled and the output of the LM358 is actually at some DC potential above zero volts with an AC component superimposed on the DC.


You are probably right on that. But again this will work just fine for my use. Thanks all I have really learned a lot.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 12:34 am 
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LM386 wrote:
AC coupling removes the DC component from the waveform.

Again, is your scope AC or DC coupled? It is likely AC coupled and the output of the LM358 is actually at some DC potential above zero volts with an AC component superimposed on the DC.


That is exactly it. Dennis states that the trace is on zero with no input, so the AC coupling will cause the signal to swing above and below the zero centerline once the DC component is removed.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 12:44 am 
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So I wired the circuit as was originally published and if DC coupled it cant go below ground. Top trace, CH1, DC coupled, gnd ref two graticles from the top. Bot Trace, CH2, DC coupled, gnd ref two graticles from the bottom. If what you have is working then no arguments but its still puzzling.

As a note to the 62mV level shown: The clipped portion of the output cannot get to ground in this opamp becuase the LM358 is not a rail-to-rail device since it is an early op-amp circa 1970s. There are a slew of rail-to-rail parts today with the advent of consumer devices being single supply.

Another note is that it is more difficult to use a virtual ground on a non-inverting design unless the virtual ground has enough current and is low impedance since the ground in that configuration has current flow whereas the inverting design would have minuscule current flow to the virtual ground point. There are "rail splitter" ICs which provide a 1/2 Vcc virtual ground specifically for this purpose if you want to explore further.

.......and thats all I have to say about that.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 1:42 am 
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Although I can explain how the output waveform appears to go negative because of AC coupling, I can't explain the waveforms that Dennis H. posted from his oscilloscope when using the original schematic.

flyboy71 has demonstrated what the waveforms should look like.

Dennis H., I would like to ask you again if your scope is AC or DC coupled. There is a switch on every modern scope which changes the coupling. Also are you certain that the waveforms you posted were derived from an accurately constructed circuit? I am having difficulty in understanding how the waveforms which you posted can exist if the circuit was constructed according to the original schematic.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 1:57 am 
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LM386 wrote:
Although I can explain how the output waveform appears to go negative because of AC coupling, I can't explain the waveforms that Dennis H. posted from his oscilloscope when using the original schematic.

flyboy71 has demonstrated what the waveforms should look like.

Dennis H., I would like to ask you again if your scope is AC or DC coupled. There is a switch on every modern scope which changes the coupling. Also are you certain that the waveforms you posted were derived from an accurately constructed circuit? I am having difficulty in understanding how the waveforms which you posted can exist if the circuit was constructed according to the original schematic.


It was set on AC coupling.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 10:36 am 
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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?


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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 12:50 pm 
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There are several questions the OP never answered....

I hate to keep saying "Op-Amp 101", but the normal expectation is that the DC operating point of both input and output is halfway between the rails. Thus, if the circuit **appears** to be working correctly, the either input or output is not referenced where we think it is.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 12:50 pm 
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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.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 12:55 pm 
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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.

If you are referring to the output, the center point of the (DC) waveform will be halfway between the rails. When using AC coupling on the scope, that point appears to be at zero.
The question is why there is no clipping----I suspect that has to do with the input coupling / referenece point.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Thu 11, 2019 2:19 pm 
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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 2:27 pm 
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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?

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