After a bit of browsing on eBay I found several “new” versions of my favourite classic op amp, the 741. We’ve already seen about twelve different layouts so far, but today we’ll see that there are still more out there.
Starting with the oldest, we go back more than four decades to 1974. Texas Instruments at that point sold the SN72741, adapting the original Fairchild part number to their own nomenclature: SN stands for “Semiconductor Network”, meaning “integrated circuit”, while the numbers indicate the product series and temperature range (0 to 70 °C in this case; there was also an SN52741 with a wider temperature range).
After dissecting a whole batch of 555 timer ICs I thought it would be an interesting project to make a working copy of the 555’s internal circuit out of discrete components, in the same physical space as the original IC. I’ve done this before with the 741 opamp, and the steps are similar. First, I drew the schematic in KiCAD:
I used the original circuit as designed by Hans Camenzind, as a tribute to his design but also because it uses the smallest number of transistors among all different designs I found while dissecting the various 555 ICs. Discrete transistors are larger than discrete resistors, so the original design saves space compared to newer versions that include several more transistors. Note that it’s the opposite situation when you’re designing an actual IC: integrated resistors are usually the largest parts in your layout.