Speaking of high speed opamps

The faster amps are usually current-mode, and have awful DC specs. And input impedance drops as frequency goes up. They steal a lot of power from their inputs.

TI makes some nice fast current-mode amps with decently large supply voltages.

TBH I'm a little afraid of fast amps.. Dang thing will oscifllate and I'll never see 'em on my slowish 'scope.

Right. I've only used one CFB opamp. How do you slow those things down if they are going too fast? (more FB resistance?)

If I can get ~5MHz overall that would be fine.

George H.

The x-Chapters, Tables 4x.2 (VFB, 4-pages), and 4x.3 (CFB), with accompanying discussions, pages 304-323.

I'm much less familiar with this field than most here, but if you want high gain and in the front end you need low noise, isn't that what instrumentat ion amps are about?

My brother inherited a design from the US Army for a biomonitor which uses the signal from fish coughing as an indicator of pollution in the water. O ne version of the system uses standard rack equipment for the ADC. A versi on that is intended to be used in the field with the cage being a bit remov ed from the rack has an instrumentation amp board located on the rack. Whe n I looked into replacing the obsolete amp in that design I found there are a number of amp chips available all of which seem to meet your requirement s... well, except for maybe the overall bandwidth, and can replace all thre e front end chips. I don't know if they work up to a few MHz. BTW, how "f ew" are you looking for? Is 2 enough, 6 too many? Extra points if you can tell me what that question is from.

coughing fishes ?!? How do they cover their mouths to prevent getting all the other sea creatures wet ?

Wow ! Who woulda' thunk it ?!?

First Google hit....

"A fish does indeed cough, just not the way you think it coughs. ... No, a fish only coughs when their ventilation cycle is interrupted for some reason. The only time this occurs is when a fish has to cleanse its gills."

igh gain and in the front end you need low noise, isn't that what instrumen tation amps are about?

es the signal from fish coughing as an indicator of pollution in the water.

Yeah, seems fish are uniquely sensitive to pollutants. If they start cough ing, you may not know what is bothering them, but you know it's time to sto p drinking the water.

They use the system in New York and at least once the fish detected somethi ng in the water at about the same time a report showed up that there had be en a petroleum spill some miles upriver.

I think my brother will do very well with this system. Not only is there m oney to be made in selling the systems, but the fish have to be swapped out periodically and there is a continuing revenue stream on this service aspe ct of the business. Sell them the razors AND the blades.

But what did the Army have to do with it? Testing foreign water sources for soldiers in the field to drink?

Crayfish have been "working" in St.-Petersburg since December 2005.

The US Army did the initial development and prototype design. They wanted someone to commercialize and produce the device. The documentation was not in a good state and some key portion of the code were lacking source. I t hink most of that is straightened out now as they are shipping systems.

The Army also has need for such systems to verify they are not polluting wa ter ways at their US bases. So it's for our protection too. Do you know w hat they do at Fort Detrick? I recently read the commander of Fort Detrick gave a press conference on Corona Virus. Kinda like in prisons. You don' t want anything unauthorized getting in or out.

Yeah my table 4x.2 has all sorts of pencil marks on it.

So here's thing I see. All the low noise parts (v_n

gh gain and in the front end you need low noise, isn't that what instrument ation amps are about? Yeah this is not the front end. 1 k ohm R_in inverting amp so it starts off with 4 nV/rtHz of noise.

George H. Oh I always think of instrument amps, when I've got some differential signal and common mode stuff (at DC or low freq.) that I don't want.

s the signal from fish coughing as an indicator of pollution in the water. One version of the system uses standard rack equipment for the ADC. A ver sion that is intended to be used in the field with the cage being a bit rem oved from the rack has an instrumentation amp board located on the rack. W hen I looked into replacing the obsolete amp in that design I found there a re a number of amp chips available all of which seem to meet your requireme nts... well, except for maybe the overall bandwidth, and can replace all th ree front end chips. I don't know if they work up to a few MHz. BTW, how "few" are you looking for? Is 2 enough, 6 too many? Extra points if you c an tell me what that question is from.

High slew rate does suggest big voltage swings! So noise doesn't matter much. In what I think is your case, only the last stage needs big swing and high slew. Keep its gain moderate.

Current-mode opamps are inherently cruddy for noise and DC performance. They are deceptive in that they steal a lot of power from the input signal as speeds go up; that's one way to get slew rate!

They get hot too, which doesn't help.

In classical op amps with pole-splitting compensation, the GBW and slew rate are related by

SR ~= 0.3*GBW.

IIRC this is in National AN4, which talks about the op amp as the universal analogue circuit.

In that architecture, improving the slew rate requires reducing the transconductance of the front end, e.g. by putting in emitter resistors. This contributes additional noise in fast bipolar devices. The naturally lower transconductance of JFETs allowed a better tradeoff of noise vs. slew BITD.

Modern architectures are more complicated, and I don't know too much about the details, but you still have a diff pair in the first stage!

Cheers

Phil Hobbs

For applications that aren't too noise sensitive, I'm a fan of LT6171/6172.

Cheers

Phil Hobbs

Some bipolar opamp has the usual diff pair, but if the input error gets too big, hundreds of millivolts or something, some extra transistors kick in to help. Like a current-mode amp, that loads the input signal more to get current to process. That in turn opens up possibilities for creative spec-sheet writing.

LM6171, LM6172.

Bravo! Take care of the Earth's creatures and the Earth will take care of you.

Nature taught me that it takes a spider to fight a spider. Here's a description of my shop's symbiotic spider "workforce:"

And here's a new page about coping with COVID-19 (Coronavirus):

Thank you,

Bravo! Take care of the Earth's creatures and the Earth will take care of you.

Nature taught me that it takes a spider to fight a spider. Here's a description of my shop's symbiotic spider "workforce:"

(Note: link fixed.)

And here's a new page about coping with COVID-19 (Coronavirus):

Thank you,

The page says one thing that is not correct. "Numbers are low in the US be cause of the public health measures we?ve implemented". Numbers ar e low in the US be cause like in many places we were not prepared for this and we are not testing very many people. The Federal government isn't allo wing testing for COVID-19 unless you meet strict criteria which means they aren't really looking for community based spreading. So we have some signi ficant number of infected that we can't report because we aren't testing th em. These people continue to spread the disease raising our infection numb ers in spite of our efforts to contain the disease.

We had a chance to stop this thing early on. But we failed to take appropr iate measures. Now it is moving quickly enough we may not be able to get i n front of it.