Op Amp (uA741CN) Help - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 21, 2011, 12:19:37 am
content: well its a shame but i have to put it here. i never actually played with an op amp. last night i tried, but a simple thing is much of a discouraging. i tried closely by the book, but its not what i'm getting. Here's an example of a simple non inverting amp that i tried...R1=100, R2=100, V+=3.3V, V-=0V, the signal that goes into +input is from my FG.the gain should be (1+R1/R2) = 2, should be output = 2 * input right?this is what i'm getting (yellow=signal on non inverting input, blue is the op amp output)...top: 0 (no) signal from input, note the output is 0.9V offsettedbottom: input of sine wave, the output? WTF?!pls help noob hereps: i triedR1=10M, R2=1K, the gain should be 10,001, even a slight voltage difference in the input should show up in output right?but none, all i get is some random distorted noise.

author: Afrotechmods
author_rank: Unknown
timestamp: April 21, 2011, 12:35:58 am
content: You might want to power it up with more than 3.3V, and use a dual rail power supply if you want to use that exact circuit. Keep in mind that the 741 is one of the most outdated POS op amps in existence. Also R1 and R2 of 100 ohms is a bit low - try 1k and 1k.And with R1=10M, R2=1K you would be attenuating the signal, not amplifying it. You've got the resistors in the gain equation backwards. Even if you did reverse them a gain of 10k is too much - try to keep it around 10 or 20 to start with.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 21, 2011, 01:08:42 am
content: i reversed the 10M and 1K resistor, afaik it will turn into something like comparator? saturated at V+ when the input difference is positive, and saturated at V- when the input difference is negative. thats what i learnt, at least.

author: mkissin
author_rank: Regular Contributor
timestamp: April 21, 2011, 05:12:53 am
content: If you check the datasheet (http://focus.ti.com/lit/ds/symlink/ua741.pdf) that particular opamp is only spec'd to swing to within 3V of the rails (page 5, Vom). That's worst case, but you should always design with worst case in mind. So, with you only having 3.3V rails, your opamp might not have any output at all, by design! You also have input voltage concerns at such a low rail voltage, as the input common mode voltage is also only spec'd to within 3V of the rails.On another note, as Afrotechmods said, the 741 is pretty much the most useless opamp money can buy. Try an LM358, which is much easier to use, is spec'd for 3V rails, and allows input and output voltages that swing to the nagative rail (though not the positive rail).

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 21, 2011, 05:49:54 am
content: i know 741 is the most useless op-amp today. i'll consider getting the latest op-amp such as LM358 during my next order.Quote from: mkissin on April 21, 2011, 05:12:53 amso what i suppose to do?dont blame me man, i'm mechanical!

author: mkissin
author_rank: Regular Contributor
timestamp: April 21, 2011, 06:43:57 am
content: Quote from: Mechatrommer on April 21, 2011, 05:49:54 amRealistically, you're going to need higher voltage rails. 3.3V simply isn't enough for this opamp. If you can, use +/- 5V (or 0-10V, depending on how you want to look at it) or even higher.You also want to avoid either your input or your output going to within 3V of any of your rails. That means, with a 0V and 10V rail, keep your input and expected output within a 3V to 7V range. Depending on your input signal source, and how you connect it, you might have to AC couple it using a capacitor.

author: saturation
author_rank: Super Contributor
timestamp: April 21, 2011, 11:36:17 am
content: The 741 is as historical a linear chip as the 555 timer is; and they are both obsolete by today's standards but they can be used for frequencies under 1 MHz.http://en.wikipedia.org/wiki/Operational_amplifierA problem you have is your power source, it needs positiveandnegative supplies. You cannot use a single supply such as 0-10V to give +/-5V unless you use a voltage divider to split the supply, which will create quite a design headache for a newbie.It wasn't until the late 1970s that single supply op amps changed the face of using op amps, and that didn't become popular until a decade later, as most folks designed with dual supplies; that's why tracking dual supply bench PSU became popular and are not as popular today.Second, how far the output swings to the input is a function of gain, and if you exceed the specified rail limits, it just clips, so reduce the gain. Since there is no output on mecha's scope, thats probably not the problem. Rail limit was later overcome in later designs, but it will still clip if gain > V+, as expected.Why play with the 741 or 555? Because they are very forgiving for errors and good for experimenting, like what is in mecha's design; chances are it will still work if you put the right supplies in after an design error.If you don't have a negative supply, take 2 9V batteries, connect them together to make 18V from the combined + and - end and use the joint were the 2 batteries meet as your ground.

author: tjw
author_rank: Contributor
timestamp: April 21, 2011, 11:49:16 am
content: Hi,I would follow mkissin's advice.Specifically, power the opamp from +10v and bias the non-inverting input at ~ 1/2 the supply voltage. This is easily achieved by connecting a 10k resistor from the +Vsupply to the non-inverting input and another 10K resistor from the non-inverting input to gnd. Theory tells us that since no current flows into the non-inverting input (not strictly true in real life) the quiescent voltage at the non-inverting input will be :Vsupply*(10k)/(10k + 10k) = Vsupply/2 and the output will also be at approximately Vsupply/2.If you then apply a 1v p-p sinusoidal signal to the input, the output will be a 1v p-p sinusoidal but the Vmax and Vmin will be :(Vsupply/2) +0.5 v and (Vsupply/2) -0.5 v = approximately 4.5 to 5.5 volts if Vsupply=10v.I think you are doing yourself a disservice by listening to advice like "....Keep in mind that the 741 is one of the most outdated POS op amps in existence. (Afrotechmods)" This is clearly incorrect. Read chapter 4 of "The Art of Electronics", and page 211 in particular.The web is awash with information about the 741, so go to it.The LM358 is quite similar to the 741 - yes, it is designed to be used in single supply circuits and can swing it's output closer to the supply rails but other than that works much like a 741.I would stick with the 741 and learn everything you can about it. For example :- why won't the output swing to the supply rails ?- what is the importance of the 741's slew rate ?- how do the input offset specifications affect the performance ?- what is output offset ?- what is latchup ?- what is the bandwidth ?All these questions apply equally to the latest opamp with gold-plated specs so you may as well start at the beginning with an easy to use opamp like the 741. One day you may be called on to repair a piece of equipment that uses 741's.Regards, tjw.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 21, 2011, 12:09:55 pm
content: thanx guys. i'll keep experimenting esp on the V+ V- thing. i also played with 324 that worked quite differently.why i played with 741 (and 555?) because a have a few in my stock. so i think i'll miss the game if i dont play with em.

author: scrat
author_rank: Frequent Contributor
timestamp: April 21, 2011, 12:13:14 pm
content: You can make it work from a single supply (or, better, the IC doesn't know wether supply you are using, since there is no 0 V pin, just two +,- ones). The problem is just it doesn't work if the inputs are too close to the supply voltages.So if you need to amplify just the AC part of the signal, then adding an offset will make it possible even from a single supply. tjw forgot to mention that, if the amplification is >1 (= R1<Inf) and you don't add a second capacitor in series with R1, the opamp will likely go into saturation, or at least will work around an offset that is 1+R2/R1 times the input offset (Vsupply/2). See below.You know the impedance of C is 1/(j*2*pi*f*C). Then the amplification factor for the DC component (offset on n.inv pin) is: 1 + Z2/Z1 = 1 + R2/(R1+1/(j*0*C)) = 1. For frequencies above 1/(2*pi*(R1+R2)*C1) and above 1/(2*pi*(R3||R4)*Cin) the AC input to output gain will be about 1+Z2/Z1 = 1+R2/R1 (since the caps are approximatively a short), which is the expected value.

author: saturation
author_rank: Super Contributor
timestamp: April 21, 2011, 12:21:00 pm
content: I agree with tjw fix but while that works for pulling the 741 input to 5V using a 10V supply you'd need to be aware that you are putting 5+V into the output of your signal source, which is referenced to 0V. Most function generators will tolerate this, some will not, so rather than risk overloading the signal source's input consider truly splitting the supply. One safety feature is adding a blocking capacitor to reduce currents flowing into this source's output; connect it between the signal source output and the 741 input.

author: tjw
author_rank: Contributor
timestamp: April 21, 2011, 12:51:50 pm
content: Excellent advice - shows how much can be learnt from experimenting with such a simple circuit !Again, I'd like to mention Chapter 4 of "The Art of Electronics". This explains the effects of adding capacitors to opamp circuits and what it does to the frequency response. Essential information from authors that really know what they are talking about.A reason for my suggestion of biasing the input in the single supply mode is that over the years I have built hobbiest-grade test equipment from published magazine articles that use this technique and it performs very well. These designs most often use an external transformer (plug pack) and a simple single supplyregularregulator circuit. Advantage - low cost, simplicity and safety since no mains wiring required.Regards, tjw.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 21, 2011, 01:21:21 pm
content: Quote from: scrat on April 21, 2011, 12:13:14 pmi tried the circuit in the picture... not working. this thing is crazy. at idle (no signal, or 0 volt at non.inv input) the output stays at 4.6V level. gave it a 1Vpp sine, nothing, the output stays at 4.6V, nuts! its like some short circuit happened inside, but i tried different chip earlier, they all behave the same.R3=R4=100Ohm, confirmed 2.66V at node connecting them.Cin=1nF, so impedance at 1MHz not far from 50Ohm.i dont put Ci and Cout, and i think it doesnt matter and wont change anything.R1=R2=10M Ohm.

author: scrat
author_rank: Frequent Contributor
timestamp: April 21, 2011, 01:38:45 pm
content: If you don't put C1, the cap in series with R1, of course it will not work as you want: the DC component will be multiplied by 2, hence ~5V output (it's a typical error).R3 and R4 should be higher (let's say at least one order of magnitude): otherwise the output impedance (50 Ohm usually) of the FG + Cin impedance in series will divide the input with R3||R4.It's not a good rule to use high (~10MOhm) resistors on the feedback, unless you want the power consumption to be very low: every small parasitic capacitance will limit the bandwidth, and every noise will easily couple, not to mention the resistance itself generates voltage noise proportional to sqrt(R) (http://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise).

author: tjw
author_rank: Contributor
timestamp: April 21, 2011, 02:35:06 pm
content: Hi,try R3=R4=10k, R1=R2=10k to 100K, Cin=100nF to 1uF and C1=1uF to 10uF - polarised electrolytic for C1 ok.This circuit is explained on p178/179 in The Art Of Electronics.Keep at it, it will work !

author: Zero999
author_rank: Super Contributor
timestamp: April 21, 2011, 05:44:56 pm
content: Quote from: saturation on April 21, 2011, 11:36:17 amEven that's optimistic.The old 741 isn't much good at frequencies much over 8kHz because the open loop gain and slew rate are too low so the 741 is useless for audio.The LM358 also suffers from the same problems but is more useful because the inputs work down to 0V and will work down to 3V.The TL072 is a better choice for audio because it's low noise and has a high enough slew rate but it has similar power supply requirements to the uA741.

author: saturation
author_rank: Super Contributor
timestamp: April 21, 2011, 07:26:37 pm
content: Its not that terrible but its certainly obsolete given newer op amps available with better responses.At least 3 major items limit its performance substantially: gain bandwidth product, output impedance and its slew rate.I found this link which summarizes my experience with 741, it was written over 10 years ago:http://sound.westhost.com/highspeed.htmYou can use it on audio applications, but how it rolls off high frequencies depending on what gain you set it at and what output voltage you require. Folks who used the 741 were able to work around those limits 30+ years ago as that opamp was widely known and dirt cheap.Checking the chips you mention I am shocked that the 741 goes for over $1 each from Digikey, who sells the TL072 for 60c, and the LM358 for as low 12c. There are many better spec'd op amps far cheaper than any of those three today.Nevertheless, if you have any old op amps around, you should play with either it to see what they do, moreso to know op amps rather than trying to design something operational in the 21st Century using 30-40 year old technology, its also not cost effective given those prices.I remember when the LM741 was 20c each! I guess these $1 is the "antique roadshow" prices [ this thread triggered my interest in this old chip and dust out the cobwebs!]Quote from: Hero999 on April 21, 2011, 05:44:56 pm

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 22, 2011, 04:45:13 am
content: @tjw and scrat: thanx for the circuit advice, i somehow manage to get some signal on the 741's output, the annoying thing is the dc offset (~2V) from the 741 output. Cout is needed to block it, yet the clipped signal remained (as in ua741.png ch2, except graph moved down to origin with Cout presence). i also played with another op-amp/amplifier with minimal effort/parts (shown below, ch1 yellow=input signal, ch2 blue=output), esp the tda1517 stereo amplifier i soldered out of an old soundcard, i managed to get 12Vpp out of it last night and produced the sound on the speaker, but now its gone i think, connect the wrong pin earlier, whats left is the distorted output at 1517.png. and i stumbled/interrupted with transmission line issue that i posted on another thread, luckily i didnt toss my FG to smoke. i can see all of them are not able to catch up with greater than 1MHz signal, so now i know the limitation. thanx all guys for the help.pictures setup:signal: 2Vpp 5KHz sine.V+: 5V V-: gnd = 0V

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 22, 2011, 05:01:25 am
content: anyway. also am reading AoE (chapter 4) esp the golden rule. There is however on page 183 that i dont quite understand...Quote

author: oPossum
author_rank: Super Contributor
timestamp: April 22, 2011, 05:20:21 am
content: no feedback == comparator == output always at lowest or highest voltagedc feedback (resistor) == amplifier == output clips with too much inputac feedback (capacitor) == integrator == output always moving toward lowest or highest voltage (at rate determined by input current) - will get there eventually

author: allanw
author_rank: Frequent Contributor
timestamp: April 22, 2011, 05:31:49 am
content: There must always be a path for DC current to flow.http://www.opamp-electronics.com/tutorials/practical_considerations_bias_current_3_08_15.htm

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 22, 2011, 07:03:57 am
content: o ok, misunderstanding, i thought "feedback at dc" means either inv or non.inv input must be tied to V+i know the basic/ideal model of an op-amp (hi Z inp, low Z out, inf gain) and what it does. but as i said, what i learnt in theory is not what i get in practice. just try a simple voltage follower with 741 (-ve feedback shorted to ouput), does it follow the voltage on +ve feedback? not gonna happen i think with 741 (a voltage "not" follower), tried that.

author: scrat
author_rank: Frequent Contributor
timestamp: April 22, 2011, 07:56:28 am
content: uA741 is singing (cfr ):"I will follow in(put)ever since you put me on I knewthere is an input too deepan input so high it can keepkeep me awayaway from work"It works if the input stays between the two output limits (gnd+3V, +Vcc-3V). So give it a DC offset input, and a properly high supply. It must work!

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 22, 2011, 08:32:20 am
content: Quote from: scrat on April 22, 2011, 07:56:28 amthis is the golden rule i'm looking for! simply put, i was violating this rule (due to my FG limitation ±3.5V max, and err... knowledge). to be specific from my test (GND-Vcc supply), the limit is (GND+2V)-(Vcc-0.3V) for the particular voltage follower circuit, distortion will be prominent working near those limit. so now, yes, its working! thanx scrat.i also was told earlier by mkissin, but i did not quite get it.Quote from: mkissin on April 21, 2011, 06:43:57 am

author: scrat
author_rank: Frequent Contributor
timestamp: April 22, 2011, 09:20:42 am
content: Attention to details (which is something I really lack of) is what usually drives not understanding simple errors.I always blame there's something unexplainable happening, just to find myself wrong after a while (most times, not always..).

author: Zero999
author_rank: Super Contributor
timestamp: April 22, 2011, 11:35:12 am
content: Quote from: saturation on April 21, 2011, 07:26:37 pmI see the 741 as useless beyond 8kHz because it's the limit to the full power bandwidth and the gain is only 40dB at that point so there's hardly any negative feedback, even at fairy low gains.QuoteI'd be interested to know about those.I just had a quick look through Farnall/RS to see if there's anything but couldn't find it.I asked the question: what do we want from a general purpose op-amp and came up with the following list of minimum requirements:Single supply operation, preferably 5V to 18V, so it can be used from a 5V power source, a 9V battery or a bipolar 9V supply.>40dB@20kHz plenty of gain at the upper audio band.Full power bandwidth >20kHz we don't want any nasty slew rate distortion on audio signals.Frequency compensated to unity gain.Can be used as a low frequency comparator without becoming unstable.Low noise.Low THD.Low offset.Capable of sourcing/sinking at least 10mA so can be used to light an LED.Nice through hole DIL package.Available in both dual and quad packages and single would be nice for lower power consumption/offset trimming.And lastly cheap and widely available.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 22, 2011, 11:45:01 am
content: thanx for the tip hero. now i see some amplification with previous circuit. it will distort at 30KHz (slight noticable at 20KHz) and attenuate from there (amp.png)now its time for the fun part! 741 abuse! connect the output to various load (resistor)! pop quiz, why it can only maintain its output shape at 1K ohm or greater? (load.png)this proven to be working ac amplifier deserve its place in the next 3rd edition of AoE(aoe.jpg!)

author: scrat
author_rank: Frequent Contributor
timestamp: April 22, 2011, 04:11:38 pm
content: Quote from: Hero999 on April 22, 2011, 11:35:12 amI don't see the THD to be really an opamp parameter, but an amplifier parameter, since it depends on the use you make of that opamp. The opamp itself doesn't have to be so linear, it needs to have a high gain (and GBWP), and the feedback will make the amplifier behave linearly.

author: Zero999
author_rank: Super Contributor
timestamp: April 22, 2011, 04:59:14 pm
content: You're right so you could trade THD for gain and bandwidth so there's more negative feedback to get rid of the distortion.

author: scrat
author_rank: Frequent Contributor
timestamp: April 22, 2011, 05:13:44 pm
content: BTW, I was searching for an opamp for a simple measurement (even less than the audio band, and gain <1), and found that AD gives THD+Noise figures, too, for their opamps, even if they precisely specify the circuit (so they're indeed considering an amplifier). So you were partially right.

author: tecman
author_rank: Frequent Contributor
timestamp: April 22, 2011, 05:55:12 pm
content: A lot of op amps, especially older designs like the 741, have crossover distortion in the output stage. There is generally enough gain to minimize the effect, but at higher frequencies it becomes more of a factor.paul

author: saturation
author_rank: Super Contributor
timestamp: April 22, 2011, 06:34:46 pm
content: As there are so many op amps today, you can find one with specific performance for your design, say audio, than simply getting a 'general purpose' one, so you needn't pay more than you need.However, the MCP601 from Microchip is closer to better GP needs for <= price of the 741, its typically $0.50-.70 each. I think its easily available too worldwide.http://uk.farnell.com/microchip/mcp601-e-p/op-amp-cmos-single-2-7v-8-dip/dp/1852038I have to correct my LM358 price, its not 12c but 60c too. My error.Op amp noise is usually 10-30nV/sq rt Hz. If you need something less than that, you'd have to find an op amp that specifies "low noise."For high performance 'gp' op amps, consider this, at $6 each.http://uk.farnell.com/linear-technology/lt1226cn8-pbf/ic-op-amp-high-speed-dip8-1226/dp/1330624Quote from: Hero999 on April 22, 2011, 11:35:12 am

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 22, 2011, 07:26:37 pm
content: ok since this has become op-amp selection (which i dont mind, infact i'm looking forward to it). what i have in mind is an op-amp/amplifer that can work as voltagefollower (buffer)non inverting amplifier for my hantek 3x25, work up to 100MHz, something like12Vpp or ±6V24Vpp, acceptable(*1) noise and distortion, that can give some reasonable(*1) power to a load. i dont have any clue, but this 741 exercise proved that building a voltage follower for that is not as easy as theory said. if any pointer, i'll thank you.(*1) acceptable and reasonable terms are subjective matter as i'm not expert on this, you may define whats acceptable and reasonable based on quality vs price, performance, usability, availability etc.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 22, 2011, 07:46:39 pm
content: http://uk.farnell.com/linear-technology/lt1226cn8-pbf/ic-op-amp-high-speed-dip8-1226/dp/1330624just saw this, just like what i spec'ed above, can it handle high load? or do i have to create extra circuit (stage) for that?

author: Zero999
author_rank: Super Contributor
timestamp: April 22, 2011, 08:45:01 pm
content: That's far too expensive fot general purpose.The Microchip op-amp looks good but it's limited to a power supply voltage of only 6V.It seems like a reasonable general purpose op-amp doesn't exist, the semiconductor companies have seemed to have gone for op-amps designed for specific applications, rather than general purpose.

author: saturation
author_rank: Super Contributor
timestamp: April 22, 2011, 08:51:07 pm
content: The relationship of slew rate, output voltage and maximum output frequency is:Slew Rate in V/uS ~= 2*pi*Vp*MHz~ 3770 V/uS.Vp = 6VMHz = 100You can use a lesser slew rate, but the output voltage or max frequency has to be dropped to compensate.Quote from: Mechatrommer on April 22, 2011, 07:26:37 pm

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 22, 2011, 09:54:43 pm
content: Quote from: Hero999 on April 22, 2011, 08:45:01 pm£6.20, i think its worth it rather than $100++ on another dedicated FG that can do higher voltage. not for production, but personal use. and i believe shipping cost to my country will render this figure negligible, i might want to get a couple or dozen of this in single package (order 10 or more will dramatically reduced to half priced), will try on ebay if i have luck. 1GHz BW is too much though for my FG, but maybe usefull for another project, could be a RF Amplifier?.Quote from: saturation on April 22, 2011, 08:51:07 pmLT1226CN8 spec:Supply Voltage Range:± 2.5V to ± 15VBandwidth:1000MHzSlew Rate:400V/µsso there its confusing. for 1GHz it should be ~94KV/us. at 1GHz, 400V/us... it will be only 0.06Vp (-24db the 15Vp), why on earth it is spec'ed as 1GHz BW?edit: found it!LT1226 in Ebay, the same hongkong seller that i found earlier selling bunches of fancy op-amps, you may want to check his store. but the listing got no item description whatsoever. so i think he expects the buyer to know what they want. but he's the only one selling this kind of stuff, no other option.

author: saturation
author_rank: Super Contributor
timestamp: April 22, 2011, 11:07:25 pm
content: hi mecha,May I suggest look for another op amp, I haven't time right now to persuse Digikey but roughly:reduce the output voltage from 6V to 0.6Vp or 1.2Vpp at 100 MHz, uses a slew rate of 377V/uS.reduce the frequency to 10 MHz you can use 6Vp or 12Vpp for the slew rate of 377V/uS.So the LT1226 is limited but its a 'general purpose' op amp for MHz frequencies, but still not the specs you required.I can try to spec one out later but I can't right now.The 1 GHz isn't bandwidth, its bandwidth product, its frequency x gain. So its 1 GHz at unity gain, but still limited by slew rate so the maximum output is ~ 60mVp.I was looking for such an op amp such months ago to use as a voltage follower for the Hantek, but I got side tracked and didnt' buy it. I have the unit data somewhere.Quote from: Mechatrommer on April 22, 2011, 09:54:43 pm

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 22, 2011, 11:25:26 pm
content: its ok saturation, i'll work it out. your advice on technical part is much more important. take your time

author: jahonen
author_rank: Super Contributor
timestamp: April 23, 2011, 07:19:09 am
content: If you want slew rate, tryTHS3091.Regards,Janne

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 23, 2011, 01:30:53 pm
content: thanx janne for the pointer. i've made a little exercise, (reading from another source about things like rise/fall/settling time and some calculation on slew rate needed, comparing both Hantek 3x25's op-amp (LMH6702) and the fast slew rated THS3091. here is the outcome (picture). seems tough on this one (requirement), with compromises, THS3091 still can be used as 2nd stage amplifier (G = 2, ie 1.2Vmax or 2.4Vpp out) and at cheaper price compared to LT1226CN8 mentioned earlier. will be keep looking, at least i've gain knowledge in op-amp parameters. thanx guys.

author: saturation
author_rank: Super Contributor
timestamp: April 23, 2011, 01:56:37 pm
content: Great mecha. The reason I'm reluctant to recommend an op amp is there are 29,000 listed on Digikey. almost 200 with slew rate over 3800V/uS. If you can deal with non-DIP packaging, some spectacular op amps are under $3, because you can still optimize other parameters like you spreadsheet shows.For a DIP, I was looking at this, which fits the Hantek nicely, but I didn't delve into the non-DIP competitors:http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM7171BIN-NDThe LM7171 for $3.Also, some chips maybe easier to get in Malaysia that would also work, that would not be my 1st choice because of availability here or its final price or that I prefer DIP for prototyping.Quote from: Mechatrommer on April 23, 2011, 01:30:53 pm

author: tecman
author_rank: Frequent Contributor
timestamp: April 23, 2011, 02:21:15 pm
content: I am not sure of your end game. There was some mention of audio work earlier in the post. Look at an NE5532. Low cost and excellent for audio applications.paul

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 23, 2011, 02:41:12 pm
content: Quote from: tecman on April 23, 2011, 02:21:15 pmthis is for 100MHz signal, as much as Vpp we can squeeze out of it, but less stringent requirement on THD, imo. Audio is vice versa (lower KHz freq but very low noise and THD)... AFAIK

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 23, 2011, 09:25:30 pm
content: 20V-100V supply? well thats really something. thanx for your suggestion, it will be very good for HV job.edit: and errr... the price... $328.48?

author: saturation
author_rank: Super Contributor
timestamp: April 24, 2011, 11:11:52 pm
content: I think the thread shows easily everyone has their favorite op amp.I would suspect that the uA741 is mostly purchased by schools and students. It has all the features of the ideal op amp [ but less ideal performance!] As the 741 is old bipolar technology, its suited to student abuse, and for running with low end gear found in schools. Also, if the basics of op amps have remained unchanged for years, why update the paperwork to accommodate a different chip and give the instructor more work?

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 05, 2011, 07:14:23 pm
content: i would like to extend this "off topic" matter of op-amp selection...my previous parametricsince THS3091 is out of stock in ebay, forced me to browse more... at last i found the luxury that i thought was impossible.THS3202, 2GHz, 9KV/us, ±7.5VssTHS3061, 300MHz, 7KV/us, ±15Vssmy intention is just to buffer and amplify my 100MHz (0.6Vpp - 3.5Vpp) signal out of a function generator.both opamp have strength and weakness (Vss vs GBW) so i think i will get both. in process negotiating with seller.luxurious doesnt come cheap. both of them priced USD10 each. so if you have a clear view that what i'm doing is wrong, pls stop me. thanx.

author: DaveW
author_rank: Frequent Contributor
timestamp: May 05, 2011, 08:30:28 pm
content: It's probably not an issue for a simple lab application like that, but in general good design practice is to not have more bandwidth than you need for a given application, and if you do, to limit the bandwidth. Should help reduce noise

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 05, 2011, 08:48:48 pm
content: thanx for the advice Dave. so maybe for proper design, the THS3061 is better? since its GBW 300MHz, i can crank a bit more 2-3x gain at 100MHz, because thats my intention (buffer and amplifier). for the 2GHz GBW THS3202, maybe not a good use for 100MHz amplification, but maybe i can extend its use for higher frequency amplifier. i will see if i can use it as 434MHz RF amplifier (3-4X gain?).

author: scrat
author_rank: Frequent Contributor
timestamp: May 05, 2011, 10:30:30 pm
content: Remember not to push the GBWP to the limit, since there you will see a behaviour quite far from the ideal one. I think something around 2x or 3x should be good, best if a small capacitance limits the BW.Moreover, look at theminimumGBWP spec!BTW: which is the situation about samples in your country? Don't TI, Analog, Linear and National send them?

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 05, 2011, 11:39:04 pm
content: Quote from: scrat on May 05, 2011, 10:30:30 pmi will figure out the GBWP limit thing, this is new to me. i believe something to be learnt.thanx for reminding me about the sample program. i just filled the form. it seems successfull, but i dont know... we'll see.

author: joelby
author_rank: Frequent Contributor
timestamp: May 06, 2011, 01:58:15 am
content: If you're just buffering or amplifying a fixed frequency signal, why not use an RF transistor as an amplifier instead of an op amp?

author: allanw
author_rank: Frequent Contributor
timestamp: May 06, 2011, 04:06:32 am
content: Extra bandwidth isn't a big deal. You can always slow the op-amp down by putting a capacitor in parallel with the feedback resistor.You should simulate extensively but sometimes it doesn't actually match up to real life.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 06, 2011, 05:41:09 am
content: Quote from: joelby on May 06, 2011, 01:58:15 ambuffering/amplifying non fixed signal shape and frequency, it could be arbitrary signal, the FG in question is Hantek 3x25 Arbitrary Function/Wave Generator.i'd love to use transistor as well. but since my knowledge is very limited, a simple link(s) info on "transistor as a high frequency amplifier" could be very helpful.and last time while looking for audio amp circuit, to be able to generate +ve to -ve signal (pure sine?), we need several setup and stages of transistors and have to deal with clipping/distortion at "near 0V" region.thanx allanw for the advice. isnt an oscilloscope enough for that simulation? ie make the real circuit and debug?ps: and i have this in mind... auto balancing bridge for impedance analyzer using the high speed op amp (i'm reading the agilent 5968-4506E.pdf)

author: allanw
author_rank: Frequent Contributor
timestamp: May 06, 2011, 05:55:26 am
content: Doing your own discrete design won't be trivial if you have such high specs (why do you think there's very few op-amps that have 7k slew rate and 2GHz GBW?)Use findchips instead of ebay. The TI parts are available on Digikey but they're non-stock.Also they're current-mode, which is different than traditional op-amp topologies. Since it's so fast, parasitics will cause it to oscillate easily unless you lay it out carefully. Even something like having a ground plane underneath the op-amp is discouraged because it adds too much capacitance at the input nodes to ground. So you'll also need to build your own board and do it right. There's plenty of readings online for this though, look up high-speed op amp layout.For simulation: use LTspice. You should figure out the circuit before you actually build it.Anyway, I don't think going from using the LM741 to the fastest current-mode op-amps available on the market is a very good transition. Maybe you should start with something in the middle.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 06, 2011, 07:19:31 am
content: Quote from: allanw on May 06, 2011, 05:55:26 amthis is a helpful keyword, thanx. i have Dave's PCB pdf somewhere in my HDD. will be reading it.Quote from: allanw on May 06, 2011, 05:55:26 amhttp://www.findchips.com/avail?part=high+slew+ratemany parts, good! gotta download all the spec so to know whats the slew rate figure is. past few days, i was downloading bunch of datasheets just to find out the definition of high slew rate can be something from just a few hundred V/us, and high speed can be as little more than 10MHz. deleted many of the pdf so not to fuzzy my brain. only TI provides KV/us at GHz BW (but in the form of CFB opamp sigh). even LT or any other VFB cannot defeat.Quote from: allanw on May 06, 2011, 05:55:26 ami dedicated a new thread for this.Quote from: allanw on May 06, 2011, 05:55:26 ammaybe i'm just heading to an evil in the dark.

author: scrat
author_rank: Frequent Contributor
timestamp: May 06, 2011, 08:40:37 am
content: Quote from: Mechatrommer on May 06, 2011, 07:19:31 amUsing CFB and low value resistors is the way to go at the highest frequencies. Just think of what a few pF parasitic can do if in parallel with the high input impedance of a VFB opamp. The feedback input pin of a CFB opamp is ideally shorted to gnd, thus minimizing the R*C time constant at that node.Quote from: Mechatrommer on May 06, 2011, 07:19:31 amI agree with allanw about the gradual moving towards higher frequencies. Even 10MHz at high gain can be a challenge...

author: joelby
author_rank: Frequent Contributor
timestamp: May 07, 2011, 05:52:31 am
content: If it's for a lab solution, you could see ifMini-Circuitshave an amplifier module that suits your needs. They have all sorts of things that don't show up in Digikey etc. searches and can provide modules (metal boxes with BNC/SMA connectors) as well as components.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 07, 2011, 07:13:50 am
content: thanx joelby. 1st impression on the main page is they sell high speed products, i havent browse very detail yet. just looking for the chip (hi-speed amp), put in search box... BW 1-100MHz, gain 1-3, result none = "Please go Back and review / change your requirement or Contact Us.", i havent put the slew rate requirement yet. thanx for the link anyway. and to scrat for the advice.Quote from: joelby on May 07, 2011, 05:52:31 am

author: joelby
author_rank: Frequent Contributor
timestamp: May 07, 2011, 07:18:31 am
content: They sell pretty much everything.Try not specifying a maximum gain. Perhaps amplifiers with less than 3dB gain aren't very popular!

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 07, 2011, 07:43:38 am
content: now i see they sell very strange amplifier (picture). it is from my 100MHz BW amplifier search (i didnt specify the gain).

author: joelby
author_rank: Frequent Contributor
timestamp: May 07, 2011, 08:51:34 am
content: Could be good! They're usually very easy to use, and you can set gain through the bias current/voltage.

author: Wim_L
author_rank: Regular Contributor
timestamp: May 07, 2011, 03:06:58 pm
content: Those cross-shaped packages are fairly common for high frequency components.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 07, 2011, 03:58:17 pm
content: some sort of darlington arrangement? cant we just build using its equivalent circuit? (npn transistors)

author: Zero999
author_rank: Super Contributor
timestamp: May 07, 2011, 04:59:50 pm
content: It certainly doesn't look like anything new, as you say, just a Darlington pair with some biasing resistors.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 07, 2011, 06:11:14 pm
content: i certainly need to go back in time. all this hi-tech stuff hi-tech "hot" opamp blinding my eyes

author: saturation
author_rank: Super Contributor
timestamp: May 08, 2011, 04:40:30 pm
content: Hiya mecha,Well yes, the device you linked is just a darlington but the main reason for using the op amp voltage follower is megaohm input impedance and low output impedance. So effectively, it doesn't load the Hantek.The darlington RF amp you linked has an input impedance of 50 ohms, which is fine too. But for isolating the 3x25 I prefer the voltage follower.Quote from: Mechatrommer on May 07, 2011, 06:11:14 pm

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 08, 2011, 05:50:04 pm
content: Quote from: saturation on May 08, 2011, 04:40:30 pmhi. yup, alot to catch up. even this weird looking darlington turned out to be a norm. thanx for your advice about the impedance. have you look at the earlier posts? i found the THS3062/1, THS3202/1, THS3095/1 (suggested earlier). those are candidates for hi-speed hi-slew amplifier. maybe you can look into it better than i can. unfortunately, i've made an order before completing my lesson here. THS3062 & 3202 from free sample ti program. 3095 bought from ebay (with other stuffs combined), am waiting here...ps: where have you been?!

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 25, 2011, 10:30:16 pm
content: not so bad for a one night dead bug soldering. i was scared, but this one seems easy, maybe just because i dont have a spectrum analyzerhere the pictures of a working but "not yet" optimized 100MHz, G=5 CFB THS3095 TI Operational Amplifier.thanx to all people who gave encouragement, esp credit to allanw for suggesting simulation. the design is "sim approved" before the opamp arrived!cheersfrom ths3095 to uA741... i hear them dancing!ths3095: "hi 741. i respect you as the great ancestor. i hope you will be peace in the afterlife"uA741: "hi 3095. i respect you for your speed and ease of use. serve your 'makers' well"

author: saturation
author_rank: Super Contributor
timestamp: May 25, 2011, 11:54:07 pm
content: Awesome work mecha, will review this; its looks fantastic. I was away on holiday, I think I posted a bit on another thread. Sorry I missed this post, it was the culmination of all your op amp inquiries!Quote from: Mechatrommer on May 08, 2011, 05:50:04 pm

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 26, 2011, 06:04:59 am
content: err, i think there's mistake in the schematic, the PD pin should be higher or floated compared to Vref (grounded). pls take note and read datasheet carefully if you are into it. just got back from work after rushing in the morning after posting this quick report. still havent claim my sleep yet. until then.

author: qinaan
author_rank: Newbie
timestamp: December 21, 2011, 12:16:49 pm
content: Mech Can u fwd me the schematic of THS3091 based simple amplifier which can boost up signals for Hantek DDS-3X25 function generator from 1 Hz to 25 MHz. thanks

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 21, 2011, 02:11:43 pm
content: http://www.soasystem.com/eng/goltek/amplifier.htm