Author Topic: What's the use of OpAmps?  (Read 7000 times)

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Offline DarkwingTopic starter

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What's the use of OpAmps?
« on: April 27, 2019, 09:51:40 pm »
Hey mates!

Well, I don't get it ... I've now read a lot of times about OpAmps, but I don't see the use of them, when I can do amplifications of all sorts with MOSFETs, BJTs and Darlingtons or even Optocouplers (which have an advantage of their own).

What is this mystic operation, that is best to be amplified via an Operational Amplifier? Can someone please point out? Do they have an advantage in audio in terms of noise or the like?

Side quest 1: What's this "rail-to-rail" thing?

Side quest 2: Can you name your favorite OpAmp? Should there be such a thimg, what could be the best go-to jelly bean I should have in my box?

Thanks y'all!  :)


 

Offline Dubbie

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What's the use of OpAmps?
« Reply #1 on: April 27, 2019, 10:13:21 pm »
Sure you can make a basic amplifier out of a bunch of BJTs, but by the time you have designed it to be as quiet, stable, precise, efficient, small as a 20cent op amp, you would have effectively designed your own inferior op amp anyway. Opamps are for people who have more productive and interesting things to spend their design efforts on.

Of course there are applications where a custom amplifier design is necessary and an op amp won’t do, but for everything else there is an opamp.
 

Offline dmills

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Re: What's the use of OpAmps?
« Reply #2 on: April 27, 2019, 10:21:07 pm »
Indeed, the selling point of an opamp is that it is (more or less, ok actually less then more) an ideal gain block, within certain limits you can assume open loop gain is large, input impedances are high and bias currents are small (but never zero).

This means that rather then winding up with circuits where beta matters I can assume (again within limits) that the closed loop gain is set almost solely by the feedback network, epic maths time saver.

Also often my other option is to build an inferior difference amp out of discrete components (that will not match near as well as the ones in the opamp), it will be bigger, probably noisier (unless I am good) and worse behaved, worse, it will take me a day two design it.

Regards, Dan.
 

Offline soldar

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Re: What's the use of OpAmps?
« Reply #3 on: April 27, 2019, 11:16:09 pm »
The ideal op-amp has infinite gain and infinite input resistance. Then the gain of that stage is controlled by the feedback circuit. You can design an amp of any gain you want just by using a couple resistors for the feedback.

Try to do that with a transistor or two.
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Offline IDEngineer

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Re: What's the use of OpAmps?
« Reply #4 on: April 28, 2019, 12:10:22 am »
Another advantage of opamps over discrete amplifiers is matching of their components. Process variations mean you generally can't guarantee absolute values, but relative values on the same die can be amazingly close without trimming. And for even tighter matching things like laser trimming are a matter of automation. If matching is important for your homebrew discrete amplifier (think: differential input amplifier), you'll be mucking around with matched components or trimpots and the like just to achieve what a single monolithic opamp delivers for remarkably low cost.

Monolithic opamps are one of the modern miracles of electronics. The same three terminal device can yield hundreds of different results based solely on the feedback loop you put around it. Think of them as the analog equivalent of the microprocessor... both incredibly flexible building blocks, one in the analog domain, the other in the digital.

EDIT: As for "rail to rail", that refers to an opamp whose input and/or output voltage swings more closely approach the supply rails. Early opamps required some margin from the rails, but rail to rail has been available for literally decades and gives you more dynamic range within the supply rails.
« Last Edit: April 28, 2019, 12:12:14 am by IDEngineer »
 

Offline David Hess

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Re: What's the use of OpAmps?
« Reply #5 on: April 28, 2019, 12:33:58 am »
Well, I don't get it ... I've now read a lot of times about OpAmps, but I don't see the use of them, when I can do amplifications of all sorts with MOSFETs, BJTs and Darlingtons or even Optocouplers (which have an advantage of their own).

What is this mystic operation, that is best to be amplified via an Operational Amplifier? Can someone please point out? Do they have an advantage in audio in terms of noise or the like?

1. The matching between the two sides of the symmetrical differential input stage reduces DC offset and offset drift by orders of magnitude allowing great precision.  A bipolar transistor has an offset of 0.6 volts and drift of about -2 millivolts per degree C.  But use that same transistor in a differential stage and it can be 0.6 millivolts and +/-2 microvolts per degree C or a thousand times better.  Some parts are 10 times better even than that.

2. The differential input allows high common mode rejection.  A singled ended amplifier relies on one of the supply leads for common.

Quote
Side quest 1: What's this "rail-to-rail" thing?

This refers to the allowable input and output voltage swing and specifically how close it can approach the supply voltages.

Quote
Side quest 2: Can you name your favorite OpAmp? Should there be such a thimg, what could be the best go-to jelly bean I should have in my box?

My favorite part is the LM301A which is one of the earliest operational amplifiers but still produced.  Its input voltage range includes the positive supply and its compensation pin can be used as an input clamp or transconductance output.
 
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Offline vk6zgo

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Re: What's the use of OpAmps?
« Reply #6 on: April 28, 2019, 12:41:14 am »
Op Amps are very useful devices, & solve many problems in Electronics, but they do have "things you have to watch".
They are not at all fond of reactive loads, for instance, & many "young (& old) players" come to grief with this, although it is usually fairly easy to tame.

People do fall in love with them, & you will find "noobs" posting RF amps using "jellybean" Op Amps, with an LC circuit at the input, output or both, in puzzlement as to why the thing oscillates!


 

Online rstofer

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Re: What's the use of OpAmps?
« Reply #7 on: April 28, 2019, 05:16:15 am »
Maybe they're useful when you put a capacitor in the feedback path and create an integrator.  Analog computing...
Wait!  That's why they were invented in the first place!  Operational Amplifier - performs 'operations' like differentiation and integration or addition and subtraction.

See attached photo of Comdyna GP-6 doing "Mass-Spring-Damper" problem and .pdf of MATLAB Simulink schematic.

The wiring on the GP-6 is overkill because 2 different problems are patched.  The other simulates a swinging door like those between the kitchen and the dining area in a restaurant.  Same equations, different constants...

One way or another, hardware or software, these simulations bring differential equations to life!

 
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Offline 0culus

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Re: What's the use of OpAmps?
« Reply #8 on: April 28, 2019, 05:38:05 am »
A neat historical note; before solid state op amps existed, there were vacuum tube op amps:

 
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Offline IDEngineer

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Re: What's the use of OpAmps?
« Reply #9 on: April 28, 2019, 03:06:32 pm »
And before tubes, there were (essentially) mechanical opamps. Some early military aircraft had scary-elaborate mechanisms that basically performed integration/differentiation/etc. operations that we take for granted today in our monolithic devices. I tried to find a photo to link but no luck.
 

Offline David Hess

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Re: What's the use of OpAmps?
« Reply #10 on: April 28, 2019, 03:17:43 pm »
Maybe they're useful when you put a capacitor in the feedback path and create an integrator.  Analog computing...
Wait!  That's why they were invented in the first place!  Operational Amplifier - performs 'operations' like differentiation and integration or addition and subtraction.

Single transistors can do all of these things as well but with much less precision.  The single transistor "Miller integrator" is included inside most operational (and audio) amplifiers where it provides the frequency compensation and usually most of the voltage gain.

A single transistor shunt feedback adder or inverter is also completely feasible but compared to the version using an operational amplifier, it has terrible offset and drift.

 

Online rstofer

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Re: What's the use of OpAmps?
« Reply #11 on: April 28, 2019, 05:25:01 pm »
James Thomson first discussed solving differential equations with a mechanical ball-disk integrator back in 1876 but it was used earlier by Lord Kelvin in a tide prediction computer in 1872.  Bright brothers!

https://en.wikipedia.org/wiki/Differential_analyser

Lord Kelvin came up with the integration method of modeling differential equations and the magic is based on the '=' sign.

Take an equation like Ay'' + By' + Cy + D = 0 - just a simple second order DE with constant coefficients.  Lord Kelvin's contribution was to solve for y'' as y'' = (-By' - Cy - D) / A = (-1/A)*(By'+Cy+D).  Pretend you actually have y'' and integrate once to get y' and again to get y then mix in all the coefficients creating the right hand side of the expression.  Now, if we just HAD y'' we'd be all set.  But we DO have it because of the '=' sign.  All that mess we just created on the right hand side IS y'' so just feed to output back to the input, apply initial conditions and let the thing run.

Somehow I find this exciting!  I really wish I had had access to an analog computer back in college.  Not just for the Differential Equations course but also for Control Systems.

See that MATLAB Simulink schematic I posted earlier to see how this all plays out.

Oh, and accuracy is everything.  We want to be able to read values off a strip chart recorder or oscilloscope.  No drift, no effects from bias current (think 1 uA bias current with a 1 Mohm timing resistor - HUGE error!).  We can overcome some of that by scaling time such that the timing resistor can be 1/10 or 1/100 as large.  Of course, the timing capacitor has to be pretty high quality as well.  The op amps are the least of the problems.

Sometimes we only want the shape of the result and the absolute values don't matter.
« Last Edit: April 28, 2019, 05:33:45 pm by rstofer »
 

Online tggzzz

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Re: What's the use of OpAmps?
« Reply #12 on: April 28, 2019, 05:38:22 pm »
I really wish I had had access to an analog computer back in college.  Not just for the Differential Equations course but also for Control Systems.

Oh, and accuracy is everything.  We want to be able to read values off a strip chart recorder or oscilloscope.  No drift, no effects from bias current (think 1 uA bias current with a 1 Mohm timing resistor - HUGE error!).  We can overcome some of that by scaling time such that the timing resistor can be 1/10 or 1/100 as large.  Of course, the timing capacitor has to be pretty high quality as well.  The op amps are the least of the problems.

The analogue computer I saw in ~1973 at a local university when I was at school, had 100V supplies and a flying lead patchboard. Eventually people learned that one end of the patch cables bit.

My school math teacher had an interesting discussion with him about whether or not digital computers would supplant analogue and hybrid computers.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline IDEngineer

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Re: What's the use of OpAmps?
« Reply #13 on: April 28, 2019, 06:19:44 pm »
My school math teacher had an interesting discussion with him about whether or not digital computers would supplant analogue and hybrid computers.
There was a time when folks wondered if this newfangled "electronics" thing would supplant hydraulic circuitry. No, that is not a typo.
 

Offline Marco

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Re: What's the use of OpAmps?
« Reply #14 on: April 28, 2019, 06:32:27 pm »
Well, I don't get it ... I've now read a lot of times about OpAmps, but I don't see the use of them

High amplification allowing high linearity with feedback, rail to rail operation, auto-zero operation, very low offset with very low drift, very low bias and leakage current.

Most of that require either 100s of transistors to equal an opamp, or might even require very small area monolithic pairs which simply don't exist as discretes.

Quote
Side quest 1: What's this "rail-to-rail" thing?

They generally automagically transition between P/N input pairs near the rails so they maintain amplification from rail-to-rail, again requiring tons of transistors.
 

Offline David Hess

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Re: What's the use of OpAmps?
« Reply #15 on: April 28, 2019, 11:17:18 pm »
My school math teacher had an interesting discussion with him about whether or not digital computers would supplant analogue and hybrid computers.

I remember when digital multipliers caught up with analog multipliers in precision and speed.  Everybody who understood Moore's Law knew that it was only a matter of time before digital computing completely outperformed analog computing.
 

Online tggzzz

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Re: What's the use of OpAmps?
« Reply #16 on: April 28, 2019, 11:33:00 pm »
My school math teacher had an interesting discussion with him about whether or not digital computers would supplant analogue and hybrid computers.
There was a time when folks wondered if this newfangled "electronics" thing would supplant hydraulic circuitry. No, that is not a typo.

I once (1983) had a consultancy contract into exactly that.

My conclusion was that replacing the hydraulic logic was too difficult and with insufficient benefit. I included the possibility of remote measurement and control, well before the internet per se existed.

The application domain: off shore unmanned oil platforms where there was currently zero electricity, for obvious reasons.

There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Online tggzzz

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Re: What's the use of OpAmps?
« Reply #17 on: April 28, 2019, 11:37:12 pm »
My school math teacher had an interesting discussion with him about whether or not digital computers would supplant analogue and hybrid computers.

I remember when digital multipliers caught up with analog multipliers in precision and speed.  Everybody who understood Moore's Law knew that it was only a matter of time before digital computing completely outperformed analog computing.

Indeed. That was my maths teacher's opinion, and the person at the university didn't have their heart in a counter-argument.

As a knowingly incompetent schoolkid, I found that interplay fascinating and revealing.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
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Offline IDEngineer

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Re: What's the use of OpAmps?
« Reply #18 on: April 29, 2019, 12:03:45 am »
I remember when digital multipliers caught up with analog multipliers in precision and speed.  Everybody who understood Moore's Law knew that it was only a matter of time before digital computing completely outperformed analog computing.
There's always the quantization error of digital as compared to analog. But that becomes an argument between "number of bits" versus "signal to noise ratio".
 

Offline vk6zgo

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Re: What's the use of OpAmps?
« Reply #19 on: April 29, 2019, 02:29:27 am »
A neat historical note; before solid state op amps existed, there were vacuum tube op amps:



Interestingly, "scale of 2" binary dividers ( plain old bi-stables) were made in an almost identical format.
Apparently they used them in tube type computers.

I never saw any "in the flesh", all the TV SPGs used individual 12AT7s or similar.

Some early SPGs used special "black magic"  divide by 5 or 10 stages, but they had to be "babied" to get them to work properly.
 

Online tggzzz

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Re: What's the use of OpAmps?
« Reply #20 on: April 29, 2019, 08:18:34 am »
Interestingly, "scale of 2" binary dividers ( plain old bi-stables) were made in an almost identical format.
Apparently they used them in tube type computers.

I never saw any "in the flesh", all the TV SPGs used individual 12AT7s or similar.

Some early SPGs used special "black magic"  divide by 5 or 10 stages, but they had to be "babied" to get them to work properly.

I have a Tek 184 time mark generator. It uses nuvistor valves for the 10MHz crystal oscillator and multipliers up to 500MHz. It divides the 100ns crystal output down to 5s in divide by two and divide by 5 stages.

Each division is done with three transistors and some discrete components - including some tweakable components to "baby" the stage. The basic principle of operation is that each stage is an analogue divider: the input injects "glugs" of charge onto a capacitor, and when sufficient glugs have been received a pulse is output and the capacitor discharged.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
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Offline djnz

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Re: What's the use of OpAmps?
« Reply #21 on: April 29, 2019, 08:27:53 am »
Instead of having 20 transistors all over your PCB, you now have a neat little package that has those in silicon, and they are all connected nicely in the tiny little package.

What's not to like?
 
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Offline Wilksey

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Re: What's the use of OpAmps?
« Reply #22 on: April 29, 2019, 09:40:10 am »
If you know what one is and what it can do then you wouldn't be asking the question, but making the presumption that you do not know what it is or what it does then it is a very valid question.

It's easy to say stuff like "why wouldn't you use one" but if you didn't know how they worked then you probably wouldn't?  I know it took me a while to determine what the point of them are, but when you understand it "clicks" into place.

I think most people have answered your original question already, but here is a webpage with a small video that might help:
https://www.allaboutcircuits.com/video-lectures/op-amp-applications/
It tells you the applications for an op-amp.

There are also plenty of websites and videos that go into them.
 

Offline EEVblog

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Re: What's the use of OpAmps?
« Reply #23 on: April 29, 2019, 12:18:47 pm »
Well, I don't get it ... I've now read a lot of times about OpAmps, but I don't see the use of them

It's in the name, "operational", the opamp can do mathematical operations among all your basic amplifier stuff. They also make it easier to implement active filters and other blocks.
 

Online tggzzz

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Re: What's the use of OpAmps?
« Reply #24 on: April 29, 2019, 12:59:58 pm »
To give the OP a direct answer to his question, I suggest he looks at the opamp applications in
https://www.analog.com/en/education/education-library/op-amp-applications-handbook.html

That should keep him going and answer his questions.

Op Amp Applications Handbook, Edited by Walt Jung, Published by Newnes/Elsevier, 2005, ISBN-0-7506-7844-5 (Also published as Op Amp Applications, Analog Devices, 2002, ISBN-0-916550-26-5).

This may well be the ultimate op amp book. It is brimming with application circuits, handy design tips, historical perspectives, and in-depth looks at the latest techniques to simplify designs and improve their performance. But this is more than just the last word on applications. A brief but fascinating History section outlines the early development of the feedback amplifier, starting with H. S. Black’s invention of seventy years ago—and provides priceless insights into the application needs, technological developments, and creative personalities that drove the many generations of op-amp designs.

The Op Amp Applications book is available for download:
    Section H: Op Amp History (pdf)
    Section 1: Op Amp Basics (pdf)
    Section 2: Specialty Amplifiers (pdf)
    Section 3: Using Op Amps with Data Converters (pdf)
    Section 4: Sensor Signal Conditioning (pdf)
    Sections 5-1 to 5-4: Analog Filters (pdf)
    Sections 5-5 to 5-8: Analog Filters (pdf)
    Section 6: Signal Amplifiers (pdf)
    Section 7: Hardware and Housekeeping Techniques (pdf)
    Index (pdf)
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 


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