BC547 and 2N3904 transistors
They are Bipolar Junction Transistor. Both are NPN Transistors. Their difference is manufacturer and hFE and datasheet parameters, seriously. BC547 is European transistor while 2N3904 is American transistor. Both are common NPN bipolar junction transistors used for general-purpose low-power amplifying or switching applications. https://en.wikipedia.org/wiki/2N3904 The BC548 was released with, and often is found together in →