Author Topic: What's the use of OpAmps?  (Read 7005 times)

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Offline IDEngineer

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Re: What's the use of OpAmps?
« Reply #25 on: April 29, 2019, 04:48:08 pm »
I still have some hardbound copies of books like that. National did a bunch of linear books that are near-priceless (like the original Audio Applications books, which I still have!). Analog Devices did a bunch on converter technology too. Over the years I've shed more and more of my books and other flotsam and jetsam, but those books are likely to pass on to my son (who leaves for CalPoly this fall to study EE).

In a tangental response to the OP: I have long emphasized to my son that he include a LOT of analog in his studies. We've had many a kitchen table discussion with pen and paper on the topic of opamps. Digital is nice, digital is important, but analog is the basis of everything and as digital frequencies keep going up a lot of the design rules are more analog than purely digital. I promise the more you study opamps, the more you will appreciate and respect them. It's stunning what a simple three terminal black box (OK, triangle!  :)) can do.
 
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Online tggzzz

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Re: What's the use of OpAmps?
« Reply #26 on: April 29, 2019, 05:26:48 pm »
I still have some hardbound copies of books like that. National did a bunch of linear books that are near-priceless (like the original Audio Applications books, which I still have!). Analog Devices did a bunch on converter technology too. Over the years I've shed more and more of my books and other flotsam and jetsam, but those books are likely to pass on to my son (who leaves for CalPoly this fall to study EE).

In a tangental response to the OP: I have long emphasized to my son that he include a LOT of analog in his studies. We've had many a kitchen table discussion with pen and paper on the topic of opamps. Digital is nice, digital is important, but analog is the basis of everything and as digital frequencies keep going up a lot of the design rules are more analog than purely digital. I promise the more you study opamps, the more you will appreciate and respect them. It's stunning what a simple three terminal black box (OK, triangle!  :)) can do.

Yes to all of that, except everything is analogue except femtoamp circuits and photon counting applications. Think signal integrity, and understand that 10Gb/s signals are analogue :)

I too have saved the NS books, and many more
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Offline rstofer

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Re: What's the use of OpAmps?
« Reply #27 on: April 29, 2019, 05:31:38 pm »
Over the years I've shed more and more of my books and other flotsam and jetsam, but those books are likely to pass on to my son (who leaves for CalPoly this fall to study EE).

Congratulations to you and your son!
 

Offline IDEngineer

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Re: What's the use of OpAmps?
« Reply #28 on: April 29, 2019, 05:34:16 pm »
Yes to all of that, except everything is analogue except femtoamp circuits and photon counting applications.
Well, if we dig deep enough everything goes digital at the quantum mechanics level, I suppose. {grin} And I'd argue that those femtoamp circuits (really, electron counters!) and photon counting applications had better adhere to proper analog design techniques too! {bigger grin}

The point I think we're both making is that folks ignore analog at their own peril, and these days opamps are a big part of the analog world.
 

Offline IDEngineer

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Re: What's the use of OpAmps?
« Reply #29 on: April 29, 2019, 05:36:18 pm »
Over the years I've shed more and more of my books and other flotsam and jetsam, but those books are likely to pass on to my son (who leaves for CalPoly this fall to study EE).
Congratulations to you and your son!
Thanks, very proud Daddy here. He still rolls his eyes at me when I drag him over to the oscilloscope for something, but I know data is seeping in through the cracks.  ;D 
 

Offline rstofer

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Re: What's the use of OpAmps?
« Reply #30 on: April 29, 2019, 05:57:27 pm »
Thanks, very proud Daddy here. He still rolls his eyes at me when I drag him over to the oscilloscope for something, but I know data is seeping in through the cracks.  ;D

I know the feeling!  My grandson is wrapping up what is effectively his 2d year of an ME program.  It's a 5 year program so he has a long way to go.  Then there is grad school...  I told him that BS degrees are pretty common around Silicon Valley and if he wants to stand out, get an MS in something.  Or an MBA if he would rather be a manager than an engineer (typically pays better).  An MBA with an engineering undergrad should land him a job somewhere!  Or even an MSES (Engineering Science - kind of a generalist program).

He has a little less than no interest in electronics or programming.  The programming thing changed a little bit as he wandered through a MATLAB course this semester and I think he began to understand and appreciate the thought process behind problem solving in code.  We'll see...
 

Offline 0culus

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Re: What's the use of OpAmps?
« Reply #31 on: April 29, 2019, 06:12:23 pm »
If he wants to be a manager at all, make sure he gets the technical skills too. Managers who have no clue about how things work "in the trenches" are the worst. IMO the ideal technical manager is someone you could replace one of the engineers with and not notice the difference. [edit] that's not to say that everyone with the technical background makes a great manager...that is certainly not true!
« Last Edit: April 29, 2019, 06:14:23 pm by 0culus »
 

Offline rstofer

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Re: What's the use of OpAmps?
« Reply #32 on: April 29, 2019, 06:55:43 pm »
If he wants to be a manager at all, make sure he gets the technical skills too. Managers who have no clue about how things work "in the trenches" are the worst. IMO the ideal technical manager is someone you could replace one of the engineers with and not notice the difference. [edit] that's not to say that everyone with the technical background makes a great manager...that is certainly not true!

BSME
MSES
MBA

In that order - technical stuff first.  There's a LONG way to go.  We'll see...
 
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Online tggzzz

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Re: What's the use of OpAmps?
« Reply #33 on: April 29, 2019, 06:57:27 pm »
Over the years I've shed more and more of my books and other flotsam and jetsam, but those books are likely to pass on to my son (who leaves for CalPoly this fall to study EE).
Congratulations to you and your son!
Thanks, very proud Daddy here. He still rolls his eyes at me when I drag him over to the oscilloscope for something, but I know data is seeping in through the cracks.  ;D

Yeah! Deal with it.

Kids: who'd 'ave 'em :)
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Offline lordvader88

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Re: What's the use of OpAmps?
« Reply #34 on: April 29, 2019, 07:28:59 pm »
A super high gain function block
 

Offline DarkwingTopic starter

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Re: What's the use of OpAmps?
« Reply #35 on: May 01, 2019, 08:23:03 pm »
Apart from the hijackers: thank you very much guys!  :)  ;D

The linked PDF book is a very good read, I like  that! Must read more.  8)

I understand for now, that such an OpAmp is a very flexible signal amplifier and in terms of "manufacturing tolerance" and reliability and signal quality to prefer, instead of simple BJTs.

But what I don't understand is: can they drive loads? Are they suitable for "the power stage" of a project? It doesn't seem so. I would be interested, if someone could point me to a part what I would then call an "OpAmp Driver", that can handle bigger loads.

(Maybe that's why I never saw a use for them in any of my projects so far, because I understand 'amplification' as being able to 'amplify voltage AND current'. [A MOSFET with 1.4mΩ Rdson is not even slightly impressed, when there's a current of 5A or so ... would an OpAmp be?])


However, you guys helped me a lot! Thanks!  :-+
« Last Edit: May 01, 2019, 08:28:00 pm by Darkwing »
 

Offline IDEngineer

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Re: What's the use of OpAmps?
« Reply #36 on: May 01, 2019, 08:47:37 pm »
can they drive loads? Are they suitable for "the power stage" of a project? It doesn't seem so.
There used to be "power opamps" available. Apex made some that could drive multiple amps, came in suitable heaksink-able packages, etc. I don't know if Apex is even still around anymore.

Generally opamps are used for the small-signal part of the signal chain. Then it's common to see a discrete power transistor stage driven by an opamp. Many opamps have enough output current to drive power transistors, at least the first stage, and if you need even more power you cascade another discrete power amplifier stage after the first one, with the latter acting as a preamp for the former.

Stated differently: You'd use opamps to do small-signal amplification, filtering, and other signal conditioning. Then, once the signal has been manipulated the way you like, you use power transistors as a final to get the voltage and/or current you need to drive the load.

EDIT: Apex is still around! https://www.apexanalog.com/  I see they have output currents to 50A and voltages exceeding 350V. Most likely they are simply encapsulating my above description, a traditional opamp with a discrete power final. But it is nice that they make them available in a ready-to-go package.
« Last Edit: May 01, 2019, 08:50:32 pm by IDEngineer »
 

Offline DarkwingTopic starter

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Re: What's the use of OpAmps?
« Reply #37 on: May 01, 2019, 09:02:35 pm »
Ok, thanks for clarification!  :)


Btw: Apex PA75CD Power Op Amp 40 bucks!?  :wtf:

Oh boy ... and that's their entry level ... This must be a nery niche product I promise to never use!  :-DD
 

Offline IDEngineer

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Re: What's the use of OpAmps?
« Reply #38 on: May 01, 2019, 09:42:57 pm »
Btw: Apex PA75CD Power Op Amp 40 bucks!?  :wtf: Oh boy ... and that's their entry level ... This must be a nery niche product I promise to never use!  :-DD
Yep, they're very proud of them, but Apex has been around for a very long time so they're selling them to someone.

They'd be useful for one-offs where you didn't know how craft your own, or your focus was on some other part of the device and you just wanted a quick solution, etc. For example, I've always thought they'd be a quick fit for a galvanometer amplifier. I'm sure there are other applications, and their continued existence seems to confirm that.
 

Offline free_electron

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Re: What's the use of OpAmps?
« Reply #39 on: May 01, 2019, 10:08:43 pm »
there's many more power opamps. open any kind of surround sound amplifier these days and you will find plenty of TDA , STK  and other series opamps.

i personally like the ICEPOWER opamps. :) no analog muckery.
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Offline IDEngineer

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Re: What's the use of OpAmps?
« Reply #40 on: May 01, 2019, 10:46:33 pm »
Class D amps are great for audio applications. The Apex products are actual opamps, just with power finals. If all you're doing is audio, Class D (TDA style) is a good choice and will save a ton of money. But since the OP asked:

Quote
can they drive loads? Are they suitable for "the power stage" of a project? It doesn't seem so. I would be interested, if someone could point me to a part what I would then call an "OpAmp Driver", that can handle bigger loads.

...something like an actual linear opamp with power capability seems like a better fit.

Basically, figure out your application and choose the proper component.
« Last Edit: May 01, 2019, 10:50:00 pm by IDEngineer »
 

Offline rstofer

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Re: What's the use of OpAmps?
« Reply #41 on: May 01, 2019, 11:39:55 pm »
Somebody must be buying/using op amps because the annual sales of analog ICs is expected to hit $69 billion by 2022.  Considering the 'jelly bean' price of these devices in volume, there's a lot of analog going on somewhere.

I just left op amps lumped in 'analog ICs' because I didn't see a stand-alone figure.

https://www.globenewswire.com/news-release/2018/04/03/1458822/0/en/Global-Analog-Integrated-Circuits-ICs-Market-Will-Reach-USD-68-97-Billion-by-2022-Zion-Market-Research.html
 

Offline james_s

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Re: What's the use of OpAmps?
« Reply #42 on: May 01, 2019, 11:47:50 pm »
Perhaps a good way to learn the use of op-amps is to try building some circuits commonly using them without one and see what problems you run into. Try low pass, mid pass and high pass filters, try making a sine wave oscillator, an integrator, or circuits to do simple analog addition or multiplication. Convert a signal between single ended and differential and vice versa, then once you've done all this the hard and painful way, try doing the same stuff with some jellybean op amps and the advantages should become obvious.
 

Offline dmills

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Re: What's the use of OpAmps?
« Reply #43 on: May 02, 2019, 12:11:07 am »
Lots of the Apex stuff ends up in sonar applications, you don't really care about 40 bucks for a transmit gain block when the transducer array is costing £10k or so.

I would bet their stuff ended up in some old design that Thales wound up the design authority for by purchasing the original manufacturer, at which point ANY change becomes a sufficient paperwork dance to make it not worth it.

Regards, Dan.
 

Offline Old Printer

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Re: What's the use of OpAmps?
« Reply #44 on: May 02, 2019, 01:19:04 am »
Apart from the hijackers: thank you very much guys!  :)  ;D

The linked PDF book is a very good read, I like  that! Must read more.  8)

I understand for now, that such an OpAmp is a very flexible signal amplifier and in terms of "manufacturing tolerance" and reliability and signal quality to prefer, instead of simple BJTs.

But what I don't understand is: can they drive loads? Are they suitable for "the power stage" of a project? It doesn't seem so. I would be interested, if someone could point me to a part what I would then call an "OpAmp Driver", that can handle bigger loads.

(Maybe that's why I never saw a use for them in any of my projects so far, because I understand 'amplification' as being able to 'amplify voltage AND current'. [A MOSFET with 1.4mΩ Rdson is not even slightly impressed, when there's a current of 5A or so ... would an OpAmp be?])


However, you guys helped me a lot! Thanks!  :-+

I was glad to see you ask this question as I don't really see the day to day use for them either, but I think that is because my projects have not progressed that far. I was hoping for some sort of revelation here, and I agree, you did get a lot of op amp information that did not answer your question. I think it is a question that will get clearer once we get more involved in the proper type of circuits. I did download a copy of the Op Amp book by Walt Jung, but at 800 pages it is going to take a while to digest :)  Tempted to buy a used copy as I find a printed book easier to work with, guess that's the printer in me :)
 

Offline David Hess

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Re: What's the use of OpAmps?
« Reply #45 on: May 02, 2019, 03:16:54 am »
But what I don't understand is: can they drive loads? Are they suitable for "the power stage" of a project? It doesn't seem so. I would be interested, if someone could point me to a part what I would then call an "OpAmp Driver", that can handle bigger loads.

Operational amplifiers are designed two drive their expected load, which is often the input network to another operational amplifier, *and* their feedback network.  So general purpose parts are usually specified for loads of 2 to 10 kilohms.  Low voltage noise parts can drive much heavier loads down to hundreds of ohms because they must use low impedance network to preserve their low voltage noise.  Audio parts might be expected to work with 32 ohm headphone loads.

Video parts can usually drive 150 ohms from a double terminated 75 ohm transmission line plus their own feedback network.  RF parts of course will do the same for 100 ohms from double terminated 50 ohm transmission line.  Some Video and RF parts can drive multiple double terminated transmission lines in parallel.

High output current comes at a price.  The larger transistors take up more space making the die more expensive.  The large transistors also will not fit in the smallest packages.  Higher output current requires higher power dissipation.  Higher power dissipation creates temperature gradients across the semiconductor die which compromises precision.  Thermal feedback from the output transistors to the input transistors actually limits open loop gain among other things so high precision (and low distortion) designs benefit from using an external buffer if they must drive heavy loads like the low impedance feedback network required for low noise.

Back in the 70s, just about everybody who made a 741 had a power version of the 741 like the uA791.

Quote
(Maybe that's why I never saw a use for them in any of my projects so far, because I understand 'amplification' as being able to 'amplify voltage AND current'. [A MOSFET with 1.4mΩ Rdson is not even slightly impressed, when there's a current of 5A or so ... would an OpAmp be?])

A very common audio power amplifier topology is just an operational amplifier scaled up to provide hundreds of watts; it has the same 3 or sometimes 4 stages doing the same things.
 

Online tggzzz

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Re: What's the use of OpAmps?
« Reply #46 on: May 02, 2019, 09:05:04 am »
It, of course, entirely possible to have an opamp that doesn't fit on a single die. The first opamps were all like that.

Compound opamps avid the weaknesses of integrated opamps, e.g. higher voltage or power etc.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline dmills

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Re: What's the use of OpAmps?
« Reply #47 on: May 02, 2019, 02:38:08 pm »
You can also get opamp boosters that are typically more or less unity gain POWER stages that are fast enough to place within the feedback loop after the opamp proper and could provide significant output current. The trick is to use a booster that is MUCH faster then the opamp that it is boosting so it does not eat too far into available phase margin.

See for example the BUF634 from TI.

BTW, that 1.4mR mosfet will absolutely notice 5A is you run it in the linear region with say 5V across it (25W will be dissipated), those things are generally designed as switches not linear components. You can get mosfets for linear use, but they tend to be packaged to dissipate the inevitable heat  and have much less emphasis on low RDS(on) as a figure of merit.

Regards, Dan.
 

Offline MrAl

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Re: What's the use of OpAmps?
« Reply #48 on: May 03, 2019, 12:24:47 pm »
Hello there,

The op amp is most basically a negative feedback amplifier with high gain.  This combination means we can get very low error at the output.
The basic operation in most pure analog op amp circuits is the output of the op amp tries to force the two inputs to be the same value.  As it does this, the side effect is that it provides some function like amplification of the input signal.

An interesting side effect is that the output is the inverse of whatever we put in the negative feedback path.  For example if we put a high pass filter in the feedback path then the output becomes a low pass filter for the input signal.  If we put a squaring circuit in the negative feedback path then we get the square root of the input at the output.

Probably the reason it is called an Operational Amplifier is because it is basically an amplifier that enjoys very widespread applications.  From amplifiers to filters to math operations to even logic.  The limiting factor comes from the specifications of the specific op amp being considered.

The LM358 is a very popular and low cost op amp very suitable for experimentation.  It has limitations but the cost is very low.
By contrast the LM741 that used to be popular is much harder to use.
 

Offline Jan Audio

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Re: What's the use of OpAmps?
« Reply #49 on: May 03, 2019, 03:37:55 pm »
If you take rail to rail input and output opamp you can have voltages at input and/or output at the supply voltages, else you have a rail offset with normal opamps wich limits your dynamic range.
These TLV272IP are nice opamps ( someone gave me the hint in this forum and i tryd ), they have input to negative rail, cost around 1 euro, if you use only 5 volt those MCP are cheaper.

For the rest i go with transistors to make things smaller if possible and it wont sound bad, these opamps cost more power.
« Last Edit: May 03, 2019, 03:41:54 pm by Jan Audio »
 


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