OT: Is this question too challenging for a BSEE graduate?

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Q is for Qan't Quite Qalculate the Quiirky parts of the Quantity Qorrectly

You trust the applications sections on datasheets?

I think I'm a little older than you. ;-) The 709 was available but very few used them at the time. The first classes were just RLC circuits, no active components at all.

I had a great prof for my more advanced circuits classes. He was a full professor, a friend of the family (my father was an EE prof), and my (academic) boss when I was a tech. I also had him for well over 10% of my credits (which wasn't supposed to happen). I took the "special problems" classes (8 semester hours, IIRC, another no-no) with him, too.

He was a great guy, though not all liked him. He gave miserable exams, but then curved them so simple mistakes weren't a disaster. He planned the exams so he could finish them in an hour, so the average was 50-60. One transfer student got pissed because he had an 80 on the first exam. He studied his ass off for the second and got everything right. The prof *never* had a perfect exam, so took a point off for penmanship. The guy went ballistic. ...much too serious.

This prof retired and went became a lawyer for his second career. :-/

op-amp tries to force both inputs to the same voltage. Since it was stipulated the op-amp is a 'classic, ideal' op amp, we can assume it has none of the defects found in the real world. As a result the voltage across R3 will also be 1/4 of VDC. The only way that can happen is if the effective resistance of Q1 is 3 times the resistance of R3, or 120 ohms.

It would seem to me you aren't familiar with a customized BOM or SIT. I worked with both in manufacturing.

On 10/26/2010 12:05 PM John Fields spake thus:

No; what article? (I take it this is the real John Fields, right?)

Imagine the interview when a director of engineering asks "Can you design an Op Amp" when applying for a job as a component engineer when he meant, "Can you design a circuit using an op amp." A few days prior to that he refused an ECO to correct a problem with an OpAmp that i had submitted. Not because it was wrong, but because it was one of his designs. I had to go to management to get it signed off.

How the same director of engineering giving a 'class' in the lunchroom and using a microphone in his 'Power Point' presentation as a video source for a telemetry system?

How about his proposed new inventory system that listed passive components by their value. We had over a dozen 10K resistors in inventory, but his system would have assigned a single part number to all of them. The interview was over when I pointed out the flaws in his system, nd his decision that there only be one stock number for an IC available in different speeds, packages and multiple vendors.

Ok, I get it now. It was for a cost plus government job.

tm

In fact, _it most certainly does_. Without external connection - the feedback - it will fail to make the inputs equal. But it will do all it can do until those voltages are equal.

You seem to have missed the fact that my entire post was discussing feedback. Sigh.

Ed

On 10/26/2010 8:06 PM ehsjr spake thus:

You mean the little op amp will huff and puff and turn blue? Not sure what you're getting at here. Without feedback, the inputs will simply be whatever they are.

See the remainder of my reply that you posted below but didn't reply to.

No, you don't get it. Customized BOM allows lower costs by reusing most of a design for different customer needs. SIT (Select In Test) allows equipment to be built that exceeds the normal specifications of the current state of the art. HP and Tektronix used to do it in a lot of their products, as well. There was no 'cost plus' jobs for anyone. We would get a request for bid, or inquiry to see if we could supply what a customer needed. We would bid it, based on a current base model and all required customization to meet their needs. If they agreed to the price we built it and shipped it on schedule and on budget. It didn't matter who the customer was. NASA, NOAA, the ESA or numerous other customers in the Aerospace industry. Some of that equipment was in use 24/7 for over 30 years with no repairs.

If you think that you can build state of the art microwave receivers with off the shelf parts, you need help.

I really do get it. I see you never worked on any "black" projects.

BOM and SIT are EE-101 so don't try to be so impressive with what you think you know.

And by the way, I am not trying to piss you off. I was just trying to be a bit sarcastic. By making a simple "resistor" using five parts and calling it a "programmable" load sure sounds like someone was selling a design at cost plus.

Regards (really), tm

Sigh.

Regarding the subject line: When I first read this question and looked at the schematic, it was not clear to me what the programmable load that was asked about actually was. If it was the FET, that should have been stated. There were many responses that treated the entire circuit as the 'programmable load' hence the 'in parallel with 4K ohms' type of answers. Furthermore, if we assume the question was about the FET being the load to calculate, that resistance has a DC and AC component and they are very different. A question like this is very poorly worded and should in no way be a judge of the person attempting to answer it. David

Do you understand that the maximum output voltage that op amps can produce differs between devices? Some go to the rail(s) (or very close) and others go toward the rail(s) but can end up well more than a volt away.

Assuming unequal input voltages: If there is no feedback, the op amp responds to the unequal input voltages and drives the output to whatever the maximum voltage is for that specific op amp, in whichever direction the voltages on the inputs dictate. It continues responding and driving forever, or until the voltages are made equal by some external means.

If there is feedback, the output will go to whatever voltage causes the input voltages to be equal, or will go to maximum and keep on responding and driving the output to maximum if the circuit feedback is insufficient to equalize the voltages.

In all cases, the op amp has done all that it can. In the first case what it can do is limited by its maximum output voltage. In the second case, where the feedback allows the voltages to become equal, the op amp has done all that it can, because the equal input voltages limit the Vout to the specific voltage that creates equal inputs. In the third case, the op amp is once again limited by whatever the maximum (or minimum) Vout it can produce.

"An op amp does all that it can" does not mean or imply that it somehow internally adjusts the input voltages. It can't do that. It does what it _can_. It does everything that the inputs allow and that the op amps own internal design allows.

I hope that clears it up for you. You're likely to encounter the phrase again and again. It would seem that the smart course is to try to understand it, rather than engage in what you termed a semantic quibble.

Ed

On 10/27/2010 2:45 AM John Fields spake thus:

Thunderbird knows not what to do with that address, just opens up a new "compose" window.

Thanks, but as you can see, my email address is (intentionally) munged. Post it to web-space and give us a link?

Anyhow, thanks for the effort.

Like John said, it is a message ID, not a URL. It will open the related message on most newsreaders, if you have access to the group it was posted to.

On 10/27/2010 4:52 PM Michael A. Terrell spake thus:

I understand that. I clicked on it only half-expecting it to do anything useful.

Well, I certainly have access, as I'm reading s.e.r. using Thunderbird. But I don't know how to search by message ID. Do you, using Tbird? I assume it has this capability, but the online help is, as usual, useless.

I still use Netscape 4.78 to access newsgroups so I'm no help with Thunderbird. I tried it last year and didn't like it.

On 10/27/2010 6:28 PM Michael A. Terrell spake thus:

Well, I just "subscribed" to the Mozilla Tbird newsgroup, news://news.mozilla.org:119/mozilla.support.thunderbird (only took about a goddamn half-hour to download all 107,000 headers); no help there. Apparently Thunderbird, for all its geeky greatness, doesn't have the capability of searching my message ID. They say there's an "extension" you can download that does this; but other people say the extension can't be installed.

So much for the vaunted superiority of open-source software projects ...