Discrete 741 build final project

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Wolframore

Joined Jan 21, 2019
1,530
Just ordered all the parts for a discrete 741 build for my final project. Using 0805 and SOT23 transistors 3906 and 3904. To keep it simple I will not be using bigger transistors for output stage. Hopefully the transistors will match pretty closely for the current mirrors (Since they are on reels my changes are good). So then I realize it won’t be any good without a demo circuit to show the operations. Thinking simple and perhaps an integrator to change waveforms which will parallel with an actual 741 dip doing the same thing with input and output showing on a oscilloscope. I’m having fun with the project if you guys have any suggestions or hints please feel free to share. Next semester is mostly opamps and this will get me started and ready. I will document and update the post with my build. Still playing with design ideas: build a large representation on the silicon die using discrete components... or build a small board with 8DIP pins so it can be drop in replaced with an actual IC.... will only do one version.

F641AB46-1CF1-408F-B741-4DF67C84FFAD.jpeg
 

SLK001

Joined Nov 29, 2011
1,543
As an undergrad, we had to build one of these - as a graduate student, we had to design one. You'd think that nowadays, the build would be of a more recent design.
 

dl324

Joined Mar 30, 2015
9,577
Hopefully the transistors will match pretty closely for the current mirrors (Since they are on reels my changes are good). So then I realize it won’t be any good without a demo circuit to show the operations.
You'd have better luck with something like this:
1571935210148.png

The transistors aren't on the same die, but will experience more similar temperature changes.

There's also a dual NPN (SMBT3904) and one that has complementary transistors (SMBT3946).
 

SLK001

Joined Nov 29, 2011
1,543
Hopefully the transistors will match pretty closely for the current mirrors (Since they are on reels my changes are good).
They won't. Even on the chip there is enough variation that they don't match. Don't worry, there are enough things that can be done externally to compensate for the differences. And by "perfect match", I'm referring to an output of ZERO potential with ZERO potential on both inputs when properly biased.
 

Thread Starter

Wolframore

Joined Jan 21, 2019
1,530
That's actually a great idea to use a two transistor array... might have to do some research into the correct ones that I can get. It's difficult to get them to match but being on the same piece of silicon helps a lot!
 

Thread Starter

Wolframore

Joined Jan 21, 2019
1,530
As an undergrad, we had to build one of these - as a graduate student, we had to design one. You'd think that nowadays, the build would be of a more recent design.
It was explained to me that we study the 741 because it's so common and it's easier to see the faults. The newer opamps are almost perfect. I studied electronics in the early 90's. I decided to take some refresher courses at our local community college. I have to say that it's been enjoyable and makes more sense the second time around. I'm looking forward to taking more classes. This project is for the first course on solid state circuits, so the project is to build a circuit using a least 3 solid state components, can use IC's but only if it's not a main component. I wanted to build a differential probe but without opamps would take a lot of work. So the 741 is a good challenge.
 
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ci139

Joined Jul 11, 2016
1,088
any suggestions or hints
Since Your Design in parts nearly matches that of µA776 then you might just about pass with 2N3906 , -04 -s otherwise they are quite . . . unincorporating . . . as is anything else -- off the shelf stuff (even if you can scale 'em it's difficult to "guess" the design . . . and i don't like to start computing anything . . . )
you may need https://www.google.com/search?tbs=ic:gray&tbm=isch&sa=1&q=op+amp+test+rig&oq=op+amp+test+rig
+ a design guess toolSpice - OpAmp Dev rig.png to check the differences in Pu Pd "force" use basic SqWv osc. referenced to SG = TOT.supply median ...
 

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Thread Starter

Wolframore

Joined Jan 21, 2019
1,530
yes I’m sticking with the smd versions of 3904/3906. I will dig into the calculations a bit since it’s for a school project. I haven’t done so much manual calculations in a while but I’m find value in this work. I’m already a little bored since we are just copying a Preexisting opamp design. I’m tempted to start working at improvements in the circuit. The only thing keeping me on track is that we will be working with the 741 next semester and it would be fun to bring this discrete version in for the labs.
 

ci139

Joined Jul 11, 2016
1,088
I’m already a little bored since we are just copying a Preexisting opamp design. I’m tempted to start working at improvements in the circuit.
actually that's it, just grab some random op amp of your interest and try to make it work with your xxxx390x... transistor models --or-- don't hesitate to play around with the preset 741 design ← all this grows your experience step by step - without which you can't be successful from the scratch by the unchanged circuit layout
 

OBW0549

Joined Mar 2, 2015
3,142
I’m having fun with the project if you guys have any suggestions or hints please feel free to share.
One thing you might want to consider: depending on how you construct this 741 mock-up, you could end up with considerable inter-nodal parasitic capacitances that don't exist (or are orders of magnitude less) in the actual IC. It's possible these could cause instability such as oscillation or overshoot/ringing. If your circuit acts "funny" that would be something to check out.

Good luck with the project and let us know how it turns out!
 

Thread Starter

Wolframore

Joined Jan 21, 2019
1,530
depending on how you construct this 741 mock-up, you could end up with considerable inter-nodal parasitic
I plan to keep you guys updated. It will take some time to design, test and gather all my parts... technically we start in 3 weeks but I might be done by then and then I can concentrate on a demonstration circuit to show the board working in parallel with an actual 741.

actually that's it, just grab some random op amp of your interest and try to make it work with your xxxx390x... transistor models --or-- don't hesitate to play around with the preset 741 design ← all this grows your experience step by step - without which you can't be successful from the scratch by the unchanged circuit layout
After I build the 741, I will go further but I need to build as is for use for second semester portion of this class. I'm studying the 741 in dept while pulling the stages apart, it's simple but effective design. I can get them for pennies, it's not perfect but I can see many uses for them.
 

Thread Starter

Wolframore

Joined Jan 21, 2019
1,530
Starting spice model... based on basic 20 transistor model... there are some variations out there... not sure why all the diagrams skip Q18 and Q21... to reduce confusion I've kept the numbering consistent.

1572122271858.png
 

bertus

Joined Apr 5, 2008
20,227
Hello,

Your schematic looks like the one from the National datasheet:

LM741_internal_schematic_national.png
In this schematic the R11 is 50 Ohms, in your schematic R10 is on the same position and has the value 51k.

The Fairchild schematic has some more parts at the output circuit:

LM741_internal_schematic_fairchild.png
Unfortunately there are no resistor values given.
 
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