Looking for a high gain LNA at acoustic/ultrasonic frequencies - Page 1
Forum section: electronics / beginners

author: raff5184
author_rank: Regular Contributor
timestamp: September 19, 2019, 01:48:04 pm
content: Hi all,I am looking for an LNA OpAmp that can give me 60dB of gain at relatively low frequencies - about 150kHz - narrow bandwidth signals, and powered by 5V battery in single supply mode.I have input signals of 5mV in the 140-160 kHz range.I found some LNAs but they are mainly for higher frequencies >100MHz. Are there OpAmps that can satisfy my requirements?Or can I use "normal" OpAmps and design the appropriate circuit? And in this case what are the parameters that I need to consider for low noise amplification?

author: magic
author_rank: Super Contributor
timestamp: September 19, 2019, 02:08:28 pm
content: 60dB at 150kHz is 150MHz gain-bandwidth product. That's on the fast side of audio opampsA two stage solution would enable the use of slower amps and only one would be critical in terms of noise performance.For a single stage, AD797 with decompensation capacitor is one easy bet and it has very low voltage noise albeit a bit of current noise. Lists "sonar" and "ultrasound" among recommended applications. Expensive.You need to know your source impedance to correctly choose between voltage noise and current noise. You also need to know your target SNR / tolerable nV/rtHz or some other number specifying how much noise you can put up with. Perhaps even an LM358 would meet your specThere is also the question of gain accuracy and distortion. If you need it low, you don't want to run close to GBW limits so two stages it is.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2019, 02:22:04 pm
content: 60 dB gain at 150 kHz is already requires a rather fast amplifier. I would prefer 2 stages for this.The OPs usually give the GBW, that is the product of frequency and gain. Normally one some >20 dB or so of extra OP gain to compensate errors. So a single stage would need an > 1.5 GHz.Another important factor is the source impedance - there are different LNAs for high and low impedance.5 V supply and 60 dB gain to a 5 mV does not go together well. A question is also how much accuracy / stability of the gain is needed.

author: Zero999
author_rank: Super Contributor
timestamp: September 19, 2019, 02:46:43 pm
content: The bandwidth is only 20kHz, so I wonder how well a discrete amplifier would work?

author: raff5184
author_rank: Regular Contributor
timestamp: September 19, 2019, 04:15:05 pm
content: Quote from: magic on September 19, 2019, 02:08:28 pmThanks, I'll try it, AD sends free samples, even if cost is not a problem.Quote from: magic on September 19, 2019, 02:08:28 pmThe source is an ultrasonic transducer - high capacitance. I'll get the measurement soon.Quote from: magic on September 19, 2019, 02:08:28 pmSNR = 15dBQuote from: Zero999 on September 19, 2019, 02:46:43 pmWhy is a narrow band a problem? Shouldn't it make things easier? Or is it the bandpass filtering the problem?Quote from: magic on September 19, 2019, 02:08:28 pmSounds like a good solution. But, do I need to match the output impedance of the 1st opamp with the input of the cascaded one, or can I connect them directly?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2019, 04:47:01 pm
content: For a 2 stage design, there is not yet need to have impedance matching between the stages. 150 kHz is still relatively low frequency.A discrete solution could be an option if accurate gain is not important and some filter function is needed. AT 150 kHz one may already use inductors and LC filters.60 dB from 5 mV would be 5 V at the output. At such a large amplitude one also has to watch out for the maximum output level / slew rate.15 dB SNR does not sound like really low noise.

author: magic
author_rank: Super Contributor
timestamp: September 19, 2019, 06:52:51 pm
content: Sorry, forget AD797, it's only rated for high voltage dual supplies and isn't rail to rail. I missed your 5V supply requirement.The good news is that your noise spec isn't really demanding by audio frequency standards.Except for low frequencies where 1/f noise dominates and very high frequencies where feedback factor is insufficient to correct noise of gain/output stages, opamp noise is generally determined by the input stage, flat with frequency and rated in nV per root of Hz. You multiply the nV/rtHz datasheet rating by square root of bandwidth and get RMS voltage of noise.The super-lousy jellybean LM358 is 40nV/rtHz, giving a few µV RMS over 20kHz bandwidth. Add some more because you will also amplify out-of-band noise due to imperfect feedback filters. That's still perfectly within your demands, it seems. And that's the lousiest, noisiest opamp you could possibly buy (not that you should, it's way too slow).Current noise could become a concern with bipolar opamps only if source impedance is more than several kΩ. Is it that bad?So it seems your main points of concern are R-R output and enough bandwidth. Perhaps distortion, you said nothing about that.I really have zero clue about the kind of opamps fast enough to do all of that in one stage. But you can get a dual and make two stages of 30~33x gain, particularly if gain accuracy isn't critical. Out of "audio" parts, perhaps OPA1612 or AD8397. I'm sure they are both overkill and some cheap-ass 200~500MHz chip may exists which could be even better, but I'm completely unfamiliar with that market. Parametric search is your friend, or wait for other folks' suggestions.As for slew rate to reach 5Vpp in a few µs period signal, I think a few V/µs will suffice, which is typically met by any fast opampeditActually, AD8397 is a "video" opamp, although some people use it for audio too because of supposedly decent performance. Which perhaps is a good hint: look for analog video opamps. These are fast and quite accurate at high-kHz to low-MHz frequencies.

author: raff5184
author_rank: Regular Contributor
timestamp: September 19, 2019, 08:58:29 pm
content: Thanks I'll read your answer carefully. And what about the OPA625?

author: Zero999
author_rank: Super Contributor
timestamp: September 19, 2019, 09:20:35 pm
content: Quote from: raff5184 on September 19, 2019, 04:15:05 pmI didn't say it was a problem and you're right, a narrow bandwidth makes things easier. It's definitely possible to make an amplifier which works at that frequency with a single transistor. The parasitic capacitances can be tuned out using an LC circuit. The gain won't be accurate and a input/output buffer may be necessary.It might be easier to use an op-amp or two cascaded op-amp stages.See thread linked below:https://www.eevblog.com/forum/projects/lf357-opamp-replaced-by-an-ad797-subsititude-must-work-better-but-it-dosn't!!!/msg1505074/#msg1505074EDIT:Quote from: Kleinstein on September 19, 2019, 02:22:04 pmOh, I missed this. Yes, presumably the original poster means 5mV peak-to-peak? A gain of 1000 would of course give 5V peak-to-peak out, which would mean there would be some distortion with a 5V supply, even with a rail-to-rail output op-amp.Flicking quickly through Mouser, two TSX9292 cascaded will probably do.https://www.mouser.co.uk/datasheet/2/389/tsx9291-957366.pdf

author: magic
author_rank: Super Contributor
timestamp: September 20, 2019, 08:28:55 am
content: 16MHz is a bit slow. 160kHz and 30x gain is almost 5MHz GBW, leaving you with only 3x feedback factor. Gain accuracy won't be great and hell knows about distortion. Unless you meant two chips and four stages.Two OPA625 might work. This reminded me about one relatively cheap and fast SOT23 opamp I have heard about: LMH6601.Using full output range is indeed inviting clipping problems, I hope OP can tolerate gain of 900x or something like that if 5mV input and 5V supply are hard requirements.As important as the choice of opamp is the feedback network. A capacitor should be included in series with the gain resistor to reduce DC and low frequency gain to unity. Otherwise amplified offset voltage and 1/f noise may dominate the output, driving it into rails all the time. Choose cutoff point a bit below the band of interest.