The BC547 is considered a jellybean part: an NPN transistor able to drive up to 100 mA of collector current, it is commonly used in analog and digital circuits. The BC847 is similar but provided in a pick-and-place-friendly surface mount package
NPN transistors are commonly used in transistor switches and a variety of signal-conditioning circuits. If a circuit uses a BC547, it can be substituted with a BC847 to ease assembly.
BC847A vs BC847B vs BC847C
The BC847 is most commonly available in three grades. The three transistor types have the same pinout but differ in the typical values of the amplification commonly known as Hfe (Hybrid parameter forward current gain, common emitter). Nexperia’s data sheet, found at https://assets.nexperia.com/documents/data-sheet/BC847X_SER.pdf, breaks up the different classes as shown in the figure.
Keep in mind that Hfe stability is low: Horowitz and Hill clearly state in “The Art of Electronics” that any transistor circuit which works solely with a very tight range of Hfe is, per definition, a bad circuit
If a very high Hfe is required, a Darlington transistor can be useful. The BC847 can easily be used as the amplifier transistor of the pair, making sure that weak signals develop enough drive capability to power the main power transistor.
A quick check for inventory reveals that the transistor is easily available from all semiconductor distributors. On average, the BC847 price hovers around 7 US cents in quantities of 100, with prices dropping significantly as order volume increase.
The main limitation is the low maximum collector current, which sits at only 100 mA. In a similar form factor, the FMMT449TA by Diodes Incorporated provides a significantly higher maximum collector current of one Ampere. This, of course, is not unlimited – the thermal capacity of the semiconductor housing must be kept in mind, as heat dissipation becomes a limiting factor in some applications (such as linear regulators).
If an NPN transistor is needed, specifying the BC847 is a good idea. If currents are small, not much can go wrong by selecting this component. The part, furthermore, is ideally suited for upgrading through hole-based assemblies to surface mount with minimal redesign.