OpAmp datasheet gains: GAIN(dB) vs. G? - Page 1
Forum section: electronics / beginners

author: raff5184
author_rank: Regular Contributor
timestamp: November 21, 2017, 07:02:44 am
content: Hi all,I am studying the datasheet of this AD811 opAmphttp://www.analog.com/media/en/technical-documentation/data-sheets/AD811.pdfWhat is the difference inFigure 6between the GAIN (dB) on the y axis and the G = +2?Similarly inFigure 29there are 3 different gains: GAIN (dB), G = +1 (top left) and Gain = + 10 (caption)Finally, in the table 3 (page 12) I need to choose RFBfor the closed loop gain. Do the positive values for the gain correspond to the non-inverting configuration and the negative values to the inverting configuration? I mean, what's the difference between +1, +10, and -1, -10 etc?Thank you

author: ogden
author_rank: Super Contributor
timestamp: November 21, 2017, 07:27:07 am
content: Quote from: raff5184 on November 21, 2017, 07:02:44 amFigure 6 shows GBW (Gain Bandwidth) for amplification factor 2. Gain Bandwidth (beasured in dB) is defined by opamp's frequency characteristics, amplification gain/ratio is defined by feedback resistors, their ratio. Hope it helps. If not - do not hesitate to ask further, thou after reading doc belowFurther reading:http://www.ti.com/lit/an/sloa011/sloa011.pdfQuoteYes, exactly.

author: raff5184
author_rank: Regular Contributor
timestamp: November 21, 2017, 07:51:06 am
content: Quote from: ogden on November 21, 2017, 07:27:07 amFirst of all thank you.Since this opamp is a current feedback I thought that it is incorrect to use the concept of GBW. By GBW I mean gain-bandwidth product

author: ogden
author_rank: Super Contributor
timestamp: November 21, 2017, 03:22:57 pm
content: Quote from: raff5184 on November 21, 2017, 07:51:06 amBriefly looking into datasheet I missed that this is current feedback opamp, but anyway GBW for such apply. After all General Description says "The -3 dB bandwidth of 120 MHz at a gain of +2" which indeed is GBW at -3dB compression point, specified.

author: raff5184
author_rank: Regular Contributor
timestamp: November 21, 2017, 07:58:13 pm
content: Another question about this device. I build the circuit on Figure 31, with RFB=442 and RG=44 and the 1uF two capacitors.My input signal is a 700kHz 1Vp_p sinusoid. In output a see a 50-60MHz sinusoid, and it is not amplified. What do you think I might be doing wrong?Indeed I see the 60MHz output even though there is no input. So if the input is there or not, doesnt change the outputI'm attaching a sketch of the circuit

author: ogden
author_rank: Super Contributor
timestamp: November 21, 2017, 11:21:37 pm
content: Quote from: raff5184 on November 21, 2017, 07:58:13 pmSeems, you are not reading datasheet well enough. That amprequiresbipolar supply _and_ proper decoupling. You cannot create ground for such amplifier just using resistor divider and 1uF(!) capsNo wonder that thing gets feedback and generates. Why you need 140MHz video amp for 0.7 MHz signal? Maybe try LM324N or similar for starters? If you really need this amp for some reason running - find some AD high speed amp reference design PCB and replicate it's layout, follow all the hi-speed amplifier design rules.

author: raff5184
author_rank: Regular Contributor
timestamp: November 21, 2017, 11:29:28 pm
content: I don't have experience with this type of opamp. So I was reading general technical documentation from AD but nothing was mentioned about decoupling, bipolar supply... And I was following the circuit on the datasheet and don't know how to generate a bipolar power supplyThank you though

author: rstofer
author_rank: Super Contributor
timestamp: November 22, 2017, 07:02:44 am
content: Fig 25 and 31 in the datasheet show connections to +Vs and -Vs as well as bypass capacitors to ground. From this you can deduce that a bipolar power supply is needed. This can be as low as +- 5V up to +- 15V. The table on Page 4 only shows data for those two voltages but notice that in every case, the power supply is bipolar.You need a +- 15V supply. This is pretty common for dual supply op amps.

author: ogden
author_rank: Super Contributor
timestamp: November 22, 2017, 07:28:07 am
content: Quote from: raff5184 on November 21, 2017, 11:29:28 pmThose require hi-freq experience. Because fast amplifiers tend to feedback in case of mistakes.QuoteWhen you read specified for +/- 5V to +/- 15V operation - it means bipolar supply. Decoupling is *basic* rule for fast circuits. Run some internet search about decoupling. It's basically 0.1uF ceramic capacitor on each power pin as close you can get it to the pin connected to huge ground plane which runs all over the board and below IC too. Better you use 2-layer board with whole bottom layer as ground plane... There's lot to learn if you want to use such fast amps properly. I advise to put this one aside and take some "generic" amp which is not so fast - you won't have to fight with feedback so badly.Did you put this amp on breadboard?QuoteYou just *have* supply with output of +5V *and* -5V (or other *two* outputs of both positive/negative voltages up-to +/- 15V accordingly). Hey, do you know what internet search is?http://www.bristolwatch.com/ele/power_supplies.htm

author: raff5184
author_rank: Regular Contributor
timestamp: November 22, 2017, 08:19:30 am
content: Quote from: ogden on November 22, 2017, 07:28:07 amAnd I did put the two 0.1uF but you said it can't be done as I did.Quote from: ogden on November 22, 2017, 07:28:07 amYes, I have the breadboard version of the opAmpQuote from: ogden on November 22, 2017, 07:28:07 amSorry I was imprecise, I know what bipolar supply is, and that's what I tried to do with the resistor divider, but apparently it is incorrect

author: Zero999
author_rank: Super Contributor
timestamp: November 22, 2017, 09:09:21 am
content: I don't see why this shouldn't work from a single supply. AC couple both the input and output signals.A breadboard is the most likely cause of the problems you're experiencing. The parasitics (capacitance between the tracks and the long inductive component leads) will begin to have a significant effect at these frequencies. Use proper ground plane construction.

author: ogden
author_rank: Super Contributor
timestamp: November 23, 2017, 10:22:46 pm
content: Quote from: Hero999 on November 22, 2017, 09:09:21 amSure it will work -ifvirtual ground is made correctly. However if hi-speed opamp can pull lousy ground all over the place - then as we can see, it does not work.Quote from: raff5184 on November 22, 2017, 08:19:30 amForget to make 140MHz amp work on the breadboard. It is close to impossible. You can possibly succeed using "dead bug" style.Better just pick some 1MHz GBW opamp and end your struggle.