There are many general use transistors available. The BC557 (RM0385) is a PNP transistor and a heavy duty drop-in replacement for a 2N2222 or 2N3906.
The three terminals of a bipolar transistor are called the emitter, base, and collector. A small current into the base controls a large current flow from the collector to the emitter. The current at the base is typically one-hundredth of the collector-emitter current. Moreover, the large current flow is almost independent of the voltage across the transistor from collector to emitter. This makes it possible to obtain a large amplification of voltage by taking the output voltage from a resistor in series with the collector.
From the simplest point of view a bipolar transistor is a current amplifier. The current flowing from collector to emitter is equal to the base current multiplied by a factor. An NPN transistor operates with the collector voltage at least a few tenths of a volt above the emitter voltage, and with a current flowing into the base. The base-emitter junction then acts like a forward-biased diode with an 0.6 Vdrop: VB≈VE+ 0.6V. Under these conditions, the collector current is proportional to the base current: IC= hFEIB. The constant of proportionality is called hFE because it is one of the "h-parameters," a set of numbers that give a complete description of the small-signal properties of a transistor (see Bugg Section 17.4). It is important to keep in mind that hFE is not really a constant.
What is a Transistor?
A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal.