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# opamp structure

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#### qwertyqwq

##### Member
hi
i was looking at datasheet of old opamp (lm393) and saw circuit scheme of opamp inside. Then i wonder it how things working inside. But i didnt clearly understand structure of opamp. Searched on internet and didnt found what i was looking for , or i couldnt searched correctly.
I m looking for text or a video to explaine how its works. For example on comparison operation , how its work. Which transistor getting switch and which is not etc.
Do you know any pages or videos to explain step by step ?

EDIT: ıt doesnt need to be lm393's inside. Every opamp will be good for me.

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Err, vell yes - I can say I had excact the same problem as you several years ago when I started learning electronics. Think I plundered about this for a year because I had to ask myself the question that would lead me into the right direction.

I'd gladly explain this on a whiteboard if I could, but on a forum. . <?>

Ok try to think like this - assume you have two NPN BJT's inputs and emitter connected together. Just for the figure, say we operate within ±5V.
• On transistor A (don't care wich one is neg/plus), the input voltage (base on transistor) set to +1V.
• If there is nothing connected to transistor B (oposite input), then what must the voltage on the emitters have to be (approx) ?
• Then - you slowly rise the voltage on input B (base), what will happen when the voltage exceed +1V and above?

The basic of the opamp is dead simple .. All the rest is buffers and constant current sinks and sources..

The best way to learn is download the internals of the old 741...

There are a few members here that could identify the constant current sources, the constant current sink circuits, the over current circuit and the buffer... But the basic differential circuit is on the input..

There are quite a few transistors used as current sources & current mirrors in the 741, which may make it difficult to follow.

This is one that can be built with conventional components and is a bit easier to understand:

One of the important things when attempting to understand analog ICs, is that the integrated nature and the intimate relationships of the IC's integrated components, allow for circuit tricks which otherwise are very difficult or almost impossible to obtain with discrete components.

One simple example: the differential amp Q1 and Q2. Unless precisely selected and characterized devices are employed, plus some additional trimmings, the resulting voltage offset would be really bad.......especially across the full temperature range.

Nevertheless, prior to affordable IC technology, companies would indeed build discrete semiconductor opamps...... heck, they even built vacuum tube opamps, the most famous was the Philbrick Researches K2-W, as it was one of the first "low cost" general purpose opamps.

A fascinating history of opamps:

An LM393 is not an opamp, instead it is a dual comparator. Look at its datasheet. Its circuit is very simple.
To see how it works, learn about transistors, opamps and comparators.

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• LM393.png
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Below is the schematic for the LM324 opamp, which is also fairly simple.

The main difference between the opamp and the comparator is the opamp compensation capacitor Cc.
This forms the frequency response such that the opamp is stable for typical closed-loop negative feedback circuits.

To simplify the schematic, the transistor current-mirrors are shown as current sources (which is their function).

Here's the basic operation:
Q1 through Q4 form the differential input stage (it uses a Darlington configuration to minimize input bias currents).
Q8 and Q9 form a current-mirror that converts the differential stage voltage to single-ended, which is then be amplified and level-shifted by the rest of the circuitry to generate the Output signal.

Most op amps follow this basic configuration.

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Did you mean to say "I like this lousy one" ?

Looks really cool though - makes you want to build one

You can get them here:
They have 555 too.
$35 quid each though.... Did you mean to say "I like this lousy one" ? Lets not forget that "forerunners" deserve credit.. Albeit it's not up to today's standards, it was a much used device. When used on a split voltage rail, it performs admirably... You can get them here: They have 555 too.$35 quid each though....

No, I mustn't

No! You mustn't.... You could just print a PCB and make your own!!

That's what I was originally thinking of - but I mustn't do that either

It's really a pointless and useless thing to build - but it looks fun!.

It's really a pointless and useless thing to build - but it looks fun!.
Nothing is pointless or useless if you enjoy your time making or doing it.

Nothing is pointless or useless if you enjoy your time making or doing it.

Don't encourage me - I'm resisting (at the moment!).

hi
i was looking at datasheet of old opamp (lm393) and saw circuit scheme of opamp inside. Then i wonder it how things working inside. But i didnt clearly understand structure of opamp. Searched on internet and didnt found what i was looking for , or i couldnt searched correctly.
I m looking for text or a video to explaine how its works. For example on comparison operation , how its work. Which transistor getting switch and which is not etc.
Do you know any pages or videos to explain step by step ?

EDIT: ıt doesnt need to be lm393's inside. Every opamp will be good for me.

Today I am starting to solder the amplifier designed by member Dick Cappels, described here . I realized that provided you enable access to one of the inputs it becomes the simplest opamp I have ever seen. It is summarily described but easy to understand.

The best for me is The IC op amp Cookbook by Walter Jung, not sure if in the public domain already.

Then, more oriented to applications, by Ron Mancini, followed by Bruce Carter, Opamps for everyone (TI). Be aware that there are two versions, downloadable in .pdf.

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