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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 LEAP#150 OpAmp
Project Source on GitHub Return to the LEAP Catalog

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

LEAP is my personal collection of electronics projects, 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 (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.