# Op-amp gain question -- should be easy

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If I have a non-inverting amplifier, the gain is normally 1+Rf/R1.
So if I had 9k for Rf and 1k for R1, the gain would be 10.
Now how do I calculate it if I put a capacitor with an equal Xc value to R1
in series with R1????
I thought I just calculate Xc, add it to R1. Then I would divide Rf by that
answer. But it doesn't seem to be the case, my gain is higher. I'm using
18k for Rf, 2k for R1, 5uF for C1 and 17Hz input. At 17Hz, my gain is 7.3.
Can someone shed some light on this?? I've spent a few hours on this
problem and I don't know what to search for because I don't know what this
type of circuit is called.
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according to my texts its Av = (Rf+Rin) / Rin for a noninverting op-amp
I guess that would make it R1 times 2 ....at the frequency of interest
are you measuring a real circuit or doing a computer simulation?
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The gain of the non-inverting op amp is 1 + Zf / Zi. Now, Xc = - j 1872 so Zi = 2000 - j 1872 and Zf = 18000. The magnitude of the gain becomes 7.57.
Herbert
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I'm running a computer simulation. I got the answer in the next message, but I'm trying to understand polar cordinates right now, so the answer is a bit confusing for me. But the gain he calculated is correct.
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j being the square root of negative one used to be terribly important to me but then I read the Hitchhikers Guide to the Galaxy. Now I'm even more confused but happier :)
if you really want to have some fun run down to radio shack and get a few 741's and a variety pack of resisters and caps. you will discover you have to use standard values for parts. resistors have a 5% tolerance (or worse) electrolytic have a 20% tolerance (if they are new). your meter probes load the circuit and affect the measurement.
but its ever so much fun to make something that actually does something :)
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Gee, Tim, when was the last time you bought any parts? 1% is common for resistors today, and .1% is gaining popularity. 1% SMD capacitors are also showing up in more equipment. 741? That's the poster child for how not to make an op-amp. ;-)
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You guys should just drop all that this instant & go straight to therapy };-) =AEoy
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Paul: I really hope you get some resolution to this gain your interested in obtaining from your Op Amp., I gave up on trying the hard way years ago after i saw actual equipment designed by Phd's (Doctors:-) that can take you were no mind has gone before.... but don't give up...yet., i used to be good at it for a playful time, but other more pecune things got in the way };-) so i chose Sanity in a box instead of burnt neurons & fallen eyelashes on worksheets}:-0 =AEoy
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gold band is still what i see most of in axial lead resistors.
1% SMD capacitors
the \$5 multipack at the shack is easiest to come by
its real hard to plug surface mount devices into an experimenters board
741? That's the poster child
hey i started out with 709's.
to learn the "real world" as opposed to "ideal world" problems you have to blow stuff up sometimes. get whatever is easy to come by and cheap. sometimes you can get free "boat anchors" at hamfests and grab the parts.
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I use mostly 1%, although I still have a stock of 5% & 10% from 2W down to 1/8 W in Carbon Comp.
I don't buy parts there. If I don't already have what I need I buy from Digi-Key, MCM or Mouser.
Not really. A couple pieces of component lead and a little solder will take care of that, and I still have a 20,000 foot roll on the shelf.
I didn't get my hands on an opamp until the early '70s and it was a 709. I wasn't impressed. BTW, I still have a couple tube op amps.
Take a look at my website (link is in the header) and you will see that I have a lot of parts. The site only lists maybe 5% of what's on hand. I still have seven work benches and a 1200 sq. ft workshop here at home. I planned to restore old electronics when I had to retire, but my health doesn't let me spend much time in the shop. :(
Tim, I started with used 20% resistors and 10% capacitors salvaged from old radios and TV sets in the early '60s. When I was forced to stop working full time (I'm 100% disabled) I was hand soldering 288 pin chips in high end telemetry equipment as an engineering tech. As far as scrapping true boat anchors, i prefer to rebuild them and scrap modern garbage. I have a National NC-183R and a TS-382 on the bench right now.
Here is a link to the last electronic product I was involved with getting to the production floor:
which is a dual telemetry receiver and combiner with an optional spectrum display, all in a compact EIA rack mount package. It was one of the first production receivers that could handle FQPSK modulation.
I had to get the design ready to release to the production floor and work with the Manufacturing Engineers to eliminate the bugs from their assembly processes with the smaller SMD packages and closer lead spacing. We had to test a half dozen different types of paste solder, and switched to .015" Ersin solder for rework. That's not an "Ideal world", its the real world for today's modern electronics.
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Mike - tried to E you to let you know I sent the memory parts today - but the *^%#&%\$@ spam blocker wouldn't let me, even though it worke 2 days ago. Ed
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I have you white listed but it blocked you anyway. Just ignore the spam blocker message, I usually check that account several times a day, but I've been gone all day. I added you to the white list again, and got the message. Thanks for the memory, again. ;-)
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what advise would you give to a young electronics student who probably hasn't even touched a soldering iron, who is studying electronic theory and using idealized computer simulations that will prepare him for the shock of the real world?
alas Heathkit is no more... there are still a few kit type things you can get if you know the right places to look.
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By my senior year I was designing complex tube superhetrodyne general coverage receivers and building my own radios. I was drafted two years out of high school and after a couple arguments over what my duty would be I was tested for, and awarded 26T20 MOS which was a military broadcast engineer. I was told that I was the only person to ever pass the test at Ft Knox without spending three years of study at Ft Monmoth, Nj. first.
It would be nice if Heathkit was still in the commercial kit business, but they still sell educational electronic kits to schools.
Its my understanding that some of the kits are only available to schools and have to be bought in quantity, but there are dozens of small companies selling electronic kits even though the manuals are not up to Heathkit standards.
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Im not sure logical thinking can be tought. It may be something that just happens by living a while.
A pale shadow of its former self. I visited the benton harbor plant in 1974. There were women putting parts in boxes with one hand and knitting with the other. There were numeric controlled robot "waldo" devices for moving and stacking invantory. Rows of repair benches. In all a pretty cool operation.
so you are saying the same as me... get some real parts and see what you can make with it. (although with much better prose)... only the vast majarity of students only want to make the minimum effort effort to pass the tests.
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In today's "Instant gratification" world, its a sad fact of life. Too many cool electronic toys are on the shelves, and are too easy to toss when they get tired of them. Its hard to find someone who wants to learn it because they wanted to. The kids either saw bug money, or were forced into by their family. One kid told me he didn't need to know how to solder, because he was going to build robots, and was telling his teacher that no one used an oscilloscope anymore. The teacher saw me walking into the class and said, "He does!" and asked what I had on the bench. At that time I had a Tektronics 350 Mhz 4 channel scope, and described about 20 other pieces of equipment on the bench to him and the other kids.
Most was used daily, and if I didn't use something at least one day a week I told our in house Cal lab to give it to another tech. You should have seen the look on the other techs faces when I was offered the first of the new HP RF network analyzers for my bench and passed on it. Jaws dropped as I asked another tech if he wanted it, on the condition that I could "Borrow" it if I ever needed it. I was doing more analog and digital at the time and he needed to align tubular filters so he would use it several days a week and it tripled his output.
Several of the EEs at Microdyne wanted to know why I was working as a tech instead of going back to school to get my degree. I told them it was because of my health. Four years later, I'm 100% disabled. :( When I found a problem, I dug up the data sheets, did my research and not only told them, "We have a problem", I gave them the answer. I also designed and built some of the test fixtures, I wrote and updated test procedures. When engineering wrote them they were too close to the design. Some steps were out of logical order. Descriptions were poor because they wouldn't describe how to set up the test equipment for some critical tests. You were supposed to "Know" what they were thinking. When I wrote, or updated an older test procedure I would toss the proof copy on a bench and ask one or two techs to try it that didn't normally do that board or module to make sure I didn't miss anything. I took the test and alignment time on some boards from 2.5 hours down to 18 minutes. I wrote software to recalculate the set of resistors for sets of low pass Sallen Key video filters to tweak the -3 db point. Some of the original calculations had too much ripple and there were 14, 1% parts in the two stage filter with a 10% output spec. What I'm trying to say is, Don't be afraid to learn new things in any area of electronics, because you'll use it to build other skills on.
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Michael:
You impress me !!!.
Colin G...
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}:-) very cool ..............................
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Thanks. I always felt that it was important to do good work for whatever pay you got. I was the guy that got all the "Shit" work at Microdyne, but it was because I wanted a challenge. I enjoyed tackling what was considered unworkable and making it as easy to do, as possible. Then someone else would demand that they should get "Those easy jobs that Mike always gets!" :)
As far as passing on test equipment I didn't need: It would have been in my way 99% of the time and it never hurts to make friends out of your co workers. The guy I passed the network analyzer to didn't have many friends there, but people treated him a lot better because he had the equipment they wanted to borrow.
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Has anyone recently taken the FE exam? What resources prepared you the most?
Thanks Engineer

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