op-amp circuit with gain of 100

[unknown_entity]

For my electronics class we are supposed to design an amplifier with a gain of 100 using a single op-amp. It has to be non inverting and the output voltage must not drop below ground.In other words a positive output. I kind of have an idea of how it works. Like how varying the feed back changes the gain. But i'm not sure how to figure out the gain? Could somebody show me an example?<p> Thanks,
James

[billdar]

gain is simply vout/vin or Iout/Iin <p>But with op amps, there are two ways to do it. There is the difficult theoretical way, and the easy empirical way.<p>*** warning, the below is from a rusty memory ***<p>Theoretical, if I remember correctly, you can do a nodal analysis using the equvilent model for an opamp. You've seen the circuit, the inputs have a voltage but do not sink current. The output is a voltage controlled voltage source with a small resistance and an open circuit gain Av. You throw all of that in the mix with your feedback circuitry. Your vin is the voltage at the non-inverting pole, your vout is at the output of the op-amp.<p>You'll end up discovering that with a two resistors(one on the input, other in feedback) you get a gain of 1+(Rfb/Rin). Thats the simple empirical way.<p>Then again, you should pull out your text and check all the above, but it will give you a start.

[Edd]

James:
In case your textbook is a bit abbreviated on the
topic I believe I can offer a viable alternative.
Gain is just the appropriate scaling of the
ratios of two resistors in a feedback loop.
Use this reference , it gets down to your topic in as soon as 8 pages. Its site is:
http://www.web-ee.com/primers/files/sboa092a.pdf <p>73's de Edd<p>[email protected](Interstellar~~~~Warp~~~~Speed)
[email protected](Firewalled*Spam*Cookies*Crumbs)

[unknown_entity]

Thanks for the help, i figured it out.<p>(Feedback )
(-------- ) +1 =
( Imput ) <p>(9900 )
(---- ) +1 =
(100 )<p>(99)+1 = <p>Gain of 100

What unit is gain measured in? I heard it was unitless. But can't it be measured in dB (decibles)?<p>[ September 25, 2002: Message edited by: unknown_entity ]</p>

[bodgy]

Decibels are a logarithmic ratio measurement and are 1/10 of a BEL.<p>Normally referred to a standard voltage which depends on whether we talking Audio,Sound,or Power.<p>your gain of 100 in voltage terms is 20dB.<p>bodgy<p>Perhaps I should add that at RF and some AF you'll see dBm as the reference. This means 0dBm= 1 milliwatt into 50 ohms.<p>A rough rule of thumb working in dB's is take the most significant digit of the dB figure and that's how many noughts you need for magnitude. Eg:. 10dB =10, 20dB=100, 30dB=1000 etc.<p>[ September 25, 2002: Message edited by: bodgy ]</p>

[Dean Huster]

James, gain is a dimensionless quantity (i.e., no unit of measure) since it is derived from a ratio. If you find the logarithm of the gain, then it can be converted into decibels, but Bodgy's numbers were incorrect if working with voltage or current gain, which you are. The correct equation for that is:<p>dB = 20logAv = 20log(Vo/Vi) or<p>dB = 20logAi = 20log(Io/Ii)<p>So your circuit with a gain of 100 has an equivalent gain of 40dB (20 times the base 10 logarithm of 100).<p>Bodgy was working with power gain. If you know the power in vs. the power out, then the equation would be:<p>dB = 10logAp = 10log(Po/Pi)<p>If the output isn't to go negative, then connect the V- supply pin to ground.<p>Dean<p>[ September 30, 2002: Message edited by: Dean Huster ]</p>

[MrAl]

Hi there,<p>Slightly off topic Dean:<p>Many years ago i enjoyed reading Popular Electronics, and it was my fav mag.
I had two year subscriptions back then
that i always renewed. When i moved
i had stacks and stacks of the mags <p>I didnt know the two mags merged either,
which sounds interesting.<p>Dont subscribe to any now, but where can i
find a copy of the Poptronics mag to check out?<p>Good luck with your circuits,
Al

[bodgy]

Bodgy will now take his pills and lie down.<p>Then remember to answer the actual question.

[Bernius1]

ALL EXCELLENT !! BUT ISN'T THE LM358 AUDIO AMP
CAPABLE OF A GAIN OF 200 ?? I SUGGEST READING
"PRACTICAL ELECTRONICS FOR INVENTORS"(AVAILABLE
@ BORDER'S, OR THROUGH NUTS-VOLTS BOOKSTORE).
IT SHOWS A SCHEMATIC.
I ALSO SUGGEST "THE HISTORY OF PI" BY PETR BECKMANN. GOOD READING FOR THE ERUDITE.
GOOD LUCK !!!!

[Dean Huster]

Bodgy, at least you have pills you can take. I'm not that "fortunate"! <p>No_vice (novice?) -- I bought Petr's "History of Pi" back in 1972 when it was first published. Inside the front and back covers had pi listed out to 100,000 decimal places, something with which you could always impress your nerd friends. It also provided an interesting history of how the accuracy of pi got better and better and then got worse and then got better again. My favorite story is how one state legislature -- some state south of the M-D line -- legislated pi to the value of 22/7.<p>Al, you've been out of the loop, haven't you? Like you, I was a fervent reader of Popular Electronics, Radio-Electronics, Radio-TV Experimenter, Electronics World and Electronics Illustrated starting somewhere around 1963 or so and have yet to stop.<p>Popular Electronics turned into Computers and Electronics and then ceased publication under Ziff-Davis. Died altogether.<p>Radio-Electronics changed its name to Electronics Now. Gernsback began publishing Hand-On Electronics as Computers and Electronics died (it was a justifiable death) and then procured the old Ziff-Davis title Popular Electronics and began republishing the magazine under their umbrella, merging Hands-On into it. I'm starting to get tired of inserting these italics, folks. Then Gernsback, in January of 2000 merged their two titles, Popular Electronics and Electronics Now into a single magazine, Poptronics, which is what most of us called Popular Electronics back in the 1960's and 1970s. It was between PE and RE as to which was the best hobbyist electronics magazine back in the 1960's and 1970's. They were both excellent.<p>You can find their main website at http://www.gernsback.com where you can access subscription information or one of their four forums. The "old" forums are still the most active while the "new" forums provide easy posting of illustrations in .bmp or .jpg format and tend to be used a lot just for that.<p>Although it isn't distributed to magazine racks today like it was in 1967, you can usually buy Poptronics at Hastings or Barnes & Noble. Don't look for it at Wal-Mart or your grocery store magazine rack anymore.<p>On a sad note, there used to be a lot of hobbyist magazines out there, as noted above, in the earlier years. The U.S. has only two remaining such general electronics hobbyist publications, Poptronics and the relatively younger Nuts and Volts Magazine. There are still a few amateur radio magazines and audio magazines (the "real" ones like Glass Audio and The Audio Amateur) around, but electronics as a hobby is dying if you use the magazines as a gauge. Europe and Australia still have a few titles such as Everyday Practical Electronics, Silicon Chip, Electronics World (used to be Wireless World) and Elektor Electronics, but they cost a bit of change to have a subscription here in the States.<p>Dean<p>Well, duh, who would have thought I'd need to make changes after all that junk up there? <p>[ October 02, 2002: Message edited by: Dean Huster ]</p>

[bodgy]

On a remembrance note, my first Electronics Magazine was Practical Wireless 1973(still going just), followed by Wireless World(then had actual projects, rather than EE's slanging each other), Radio Constructor (RIP),Hobby Electronics and ETI all(RIPed),Maplin Magazine gone. <p>The rise of computer magazines seem to have taken up all the shelf space in newsagents and of course the electronic gadgets have come down in price. In the UK it is very hard to find the Electronic magazines in newsagents even in WH Smith (the main distributor and retailer in the country). <p>Here in Ozland there is now only one magazine published, and in Queensland not every newsagent stocks it on a consistent basis.<p>bodgy

[unknown_entity]

Thanks for the help guys, It turned out that the formula and answer i posted was right. This was just a small problem he told us to look at. Only a few people out of the class even bothered to do the problem besides me.