#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

A fairly conventional “long-tailed pair” arrangement
See w2aew’s excellent #193: Back to Basics: the differential amplifier, aka long-tailed pair, diff-pair for chapeter and verse
offset connections rarely used in practice, but wiring a 10kΩ pot across them allows fine-tuning the balance

Bias Generator

provides reference current for differential amplifier using a Widlar current source arrangement
current mirror feeds the gain stage
another excellent tutorial from w2aew: #190: Back to Basics: Transistor Current Sources and Mirrors

Gain Stage

multiplies the signal from the differential amplifier using Darlington configuration
Miller capacitor stabilizes the behavior of the op-amp at the cost of some gain
Vbe generator to avoid cross-over distortion in output from gain stage
Yet another topical video from w2aew: #198: Basics of a Vbe Multiplier: what it is, how it works & where it is used