I kind of fell into electronics later than many and that’s led me to mainly be a digital type. Show me some 4000-series CMOS chips and I’m on reasonably familiar ground. It often embarrasses me how little I know of the dark art of the Op-Amp! Despite this, even I have heard of the classic 741 op-amp originally designed by Fairfield Semiconductors. If you’re interested in knowing what’s going on under the hood of this classic chip the OA741D Op-Amp Bare PCB kit is what you need!
This PCB allows you to build a through-hole discrete component version of the 741, and the components needed are commonly available. It’s built around 2N3904 and 2N3906 complementary NPN/PNP transistors with the rest of the components being pretty standard; capacitors, resistors and a couple of diodes. Once built you have a fully functional 741 with nice header pins for you to test or wire into your circuits.
Being able to probe the normally inaccessible internals of the 741 is also a great way to learn about what goes on inside an op-amp, and how each section of the circuit works. You can learn about differential inputs, current mirrors, and transistor biasing! Ken Shirriff has an amazing blog post about the 741 and if you read through that while playing around with this discrete version, you’re sure to learn more about integrated circuits than you thought possible!
Mostly DIY RF has not only done a great job on the PCB, but they have shared the source on their GitHub page; this includes a nicely annotated PDF schematic that acts as a build guide. Whilst you could buy some modern silicon 741’s for your next op-amp project there’s nothing, other than size, stopping you from using this excellent PCB in a project or on your lab bench!
