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741 opamp out of discrete components?

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neazoi

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Hello, I was wondering if it is feasible to build a 741 opamp http://www.national.com/ds/LM/LM741.pdf out of discrete components. The internal schematic is clear and no double emitter transistors used.

What I need is a suggestion for the transistors that could be used.
(this will be used in a sweep circuit)
 

keith1200rs

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Why? Just use a 741 (or one of many, better opamps).

It will be tricky to build a discrete version of an IC because IC design usually relies on matching which you won't get with discrete devices. For example, Q8/Q9, Q1/Q2, Q3/Q4, Q5/Q6 etc all rely on decent device matching, as does the following circuitry.

Keith.
 

FvM

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I think, the shown schematic is simplified, I would expect that the circuit shown by Fairchildsemi is nearer to the real device.

There have been previous discussions related to the topic, I don't need to repeat what's already said.
https://www.edaboard.com/thread140242.html

Referring to your question, you won't get discrete transistors with a similar low area respectively current level, also no lateral PNP transistor with that low B and fT implemented in a traditional analog IC process. So there will be considerable differences. You should use small standard transistors, e.g. BC547/557.

P.S.:
IC design usually relies on matching which you won't get with discrete devices.
I don't think it's a big problem with uA741. The current mirror matching isn't particularly critical here. You'll see of course the mismatch of the input stage, may be few 10 mV offset voltage and respective temperature drift.
 

neazoi

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I do not really require the performance of this opamp. It is to be used in the attached schematic which is an old sweep generator for a very old spectrum analyzer.
 

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FvM

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The schematic uses three monolithic OPs, why don't you?
 

dick_freebird

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Many of the old op amps used lateral PNP input devices
which have a very high emitter breakdown. This makes
them good for general purpose use, which might include
high differential voltage. High speed, or high DC performance
(Iib, Iio, Vio) op amps on the other hand will not use those
lousy (but tough and cheap) devices. To the subject, most
PNPs will not let you put 40V Vbe on them. Whether you
need to emulate the 741 form, or its full functional envelope
(or both / neither) is something for you to think about.

Being as the price difference between a jellybean op amp
and a transistor is less than the postage, you might want
to pick a challenge with greater reward.
 
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