The most basic building block of active semiconductor circuits, and the particular one we’ve chosen is a ubiquitous NPN signal transistor, the 2N3904. It’s been around for a very long time ...

It was not uncommon to see circuits with only a few transistors ... On the left: J310 RF amp, centre: J310 regenerative receiver, right: 2N3904 audio amp. It is this last application that points ...