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Project Notes

#150 ML741

The “Medium-Large 741” - an LM741-style OpAmp built with discrete components

Notes

This ia a “Medium-Large” version of the venerable 741, inspired by the XL741 kit from the Evil Mad Scientist. Just not so big - Lego size instead of Duplo!

The intention is to get inside a real op-amp circuit and see how it works.

First test was with the circuit on a breadboard, running a basic comparator test. That all worked fine. With the lower and upper rails at 0V and 8.92V respectively and no output load:

  • output swings to 1.21V (low) when non-inverting input (IN+) is below inverting input (IN-)
  • output swings to 8.52V (high) when non-inverting input (IN+) is above inverting input (IN-)

Next I put the circuit on a protoboard - see the layout details below. I’ve wired all the connection points to an 8-pin DIP socket that can be plugged into a breadboard, and provided pin headers for wiring directly to the board. The 8-pin DIP socket is wired in the same way as the LM741:

Pin Connection
1 Offset Null (non-inverting side)
2 Inverting input
3 Non-inverting input
4 V-
5 Offset Null (inverting side)
6 Output
7 V+
8 N/C

741DIP8

Some related projects to test the ML741 in various opamp topologies:

How Does it Work?

The Evil Mad Scientist has an excellent and detailed description on how the 741 circuit works in their “Principles of Operation” document. I won’t try to improve on it!

So just to pluck out the salient points and add some more references:

Differential Amplifier Stage

Bias Generator

Gain Stage

Output Stage

  • Q14/Q20two transistors configured as emitter followers to either source or sink current
  • Q15/R9 limits the current that can be sourced

Construction

Breadboard

The Schematic

Breadboard Construction

The Build

Protoboard Construction

Board Build

Board layout

Board Front

Board Rear

Credits and References

About
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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.