# 741 OP Amp Lab

Discussion in 'Homework Help' started by chrisjsmith, Feb 26, 2017.

1. ### chrisjsmith Thread Starter Member

Nov 12, 2016
36
0
I recently did a Lab using a 741 Op Amp. I was asked to write my observations of what's happening with the output signal relating to the input.
In my pictures I have different scenarios during the experiment. I'm unsure if in my observations i'm explaining what's happening correctly or covering everything that's taking place. Please read the scenarios taking place in the pictures and critique my observations.

One scenario the Rf is replaced by a 10K Potentiometer and the resistance is varied. Please assist in explaining to me what's taking place as i'm also unsure.

Thanks for the help all

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2. ### crutschow Expert

Mar 14, 2008
22,530
6,604
It's better if you tell us what you think might be happening.
Does any of what happened make sense?
Do you know what that circuit does?

3. ### #12 Expert

Nov 30, 2010
18,076
9,686
It all looks very normal to me except you forgot to declare (-) when describing an inverting gain.

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4. ### chrisjsmith Thread Starter Member

Nov 12, 2016
36
0
So i know it's an inverting Op Amp. The input will if +ve will be inverted to -ve. A sine wave will be inverted by 180 degrees
Rf is forms the feedback loop to the inverting input
I know the gain is equal to -Rf/Ri and I know the parameters of an ideal op amp
I'm just unsure if i'm missing anything in my observation or if my description of what's happening is technically correct

5. ### Papabravo Expert

Feb 24, 2006
12,194
2,687
The behavior is what you might expect at "relatively" low frequencies. It quickly "runs out of gas" at higher frequencies and there is a substantial delay between input and output due to "slew rate limitations". You might also observe that the output cannot get very close to either power rail, leading to the sine wave displaying a "clipped" appearance at high gain settings. Knowing what these behaviors look like is important to understanding why this obsolete part is not recommended for "new" designs. Just like a microprocessor from 50 years ago should no longer be used, the same is true for this venerable part.

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6. ### crutschow Expert

Mar 14, 2008
22,530
6,604
But as someone noted, it's great for learning about real circuits since it has so many characteristics that are far from the ideal.

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7. ### chrisjsmith Thread Starter Member

Nov 12, 2016
36
0
Thanks for the explanation. I have some research to do regarding some of the terms you've mentioned but it's a pointer in a good direction

8. ### WBahn Moderator

Mar 31, 2012
24,360
7,613
Do you have a reference (from some kind of official source) that says that this part is obsolete and not recommended for new designs?

I'm not being combative, I'm quite curious about this. The fact that some unknown large number of millions of these devices are still manufactured every year by several different vendors makes it hard to claim that it is an obsolete part that is not intended for new designs. TI's data sheet, in fact, categorizes it's status as "active" and specifically states that it is intended for new designs. Other manufacturers' data sheets generally don't say one way or the other (and, despite being curious, it isn't worth it to me to make the effort to research it further, so if you happen to have information to indicate that the part is truly considered obsolete and not to be used for new designs, by at least some manufacturers, I would love to hear it).

I think the part has some advantages over many newer parts, including cost as well as some performance advantages such as input/output overload protection and no latch-up when the input common mode range is exceeded, that make it useful for non-demanding designs. Certainly the types of circuits that we can design and implement today have narrowed the domain of applications that the 741 can be used for, but the evidence would seem to indicate that even that narrowed domain is still more than rich enough to justify continuing its production.

9. ### Papabravo Expert

Feb 24, 2006
12,194
2,687
Well from a current datasheet (Jul 2016) it looks as if a PDIP part they acquired from National is obsolete (Non ROHS compliant). I didn't see any NRND.

U9T7741393 OBSOLETE PDIP P 8 TBD Call TI Call TI 0 to 70 LM 741CN

What seems to be current and in production are surface mount parts.

10. ### WBahn Moderator

Mar 31, 2012
24,360
7,613
That's what I'm seeing -- but it appears that the LM741CN marking is still active, it's just now on the lead-free version.

11. ### WBahn Moderator

Mar 31, 2012
24,360
7,613
A general comment is that you need to be more complete in your writing and, while qualitative observations are fine, try to also be quantitative where you can. A good way to approach it is to assume that you are writing your report so that someone taking this lab next semester could use your report to reproduce your results. So you need to describe what your input signals are, something like "The input signal was a 1 Vpp sinusoid with no DC component at nominally 1 kHz." Then when you modify the circuit, present the modified schematic. You say you replaced the feedback resistor with a potentiometer. But the former is a two-terminal device while the latter is a three-terminal device. Draw a schematic showing how you hooked it up. If nothing else, this would let the grader spot potential problems if you hooked it up incorrectly. On the one where you are varying the magnitude of the input signal, comment on the maximum amplitude that the input can be before clipping occurs, whether clipping for positive and negative outputs at the same time, what the clipping level is, and how this compares to the datasheet specifications. You might also measure the small signal gain and the gain just prior to clipping (say at an amplitude that is 90% of the signal at which clipping begins). Similarly for the scenario in which you are changing the supply rail voltages -- mention what the supply rails are at the time you took the scope trace and compare the observed results to what the datasheet would lead you to expect. Also, in the large-input signal scenario, consider whether the part of the waveform in the active region matches the nominal gain. If not, consider the observed rate of change to the part's slew-rate limit.

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12. ### chrisjsmith Thread Starter Member

Nov 12, 2016
36
0
Thanks for your in dept feedback. Yes, I could have been more detailed i'll definitely change this for future post so it'll be more helpful for another forum user. Thanks again man