The “Medium-Large 741” - an LM741-style OpAmp built with discrete components
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:
|1||Offset Null (non-inverting side)|
|5||Offset Null (inverting side)|
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
- 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
- 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
- Q14/Q20two transistors configured as emitter followers to either source or sink current
- Q15/R9 limits the current that can be sourced
Credits and References
- XL741 Discrete 741 Op-Amp kit by the Evil Mad Scientist
- LM741 Datasheet
- #75: Basics of Opamp circuits - a tutorial on how to understand most opamp circuits - w2aew