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BC557 is a widely used PNP bipolar junction transistor manufactured in small TO-92 package. It is a general purpose PNP transistor that can be used as a switch or amplifier in electronic circuits. Its HFE ratings are 125 to 800 which makes it an ideal transistor to use as a small signal amplifier in electronic circuits for example for audio signal amplification or other, the HFE ratings of BC557 can be determined by the last letter after its number for example BC557A HFE ratings are 110-220, BC557B HFE ratings are 200-450 and BC557C HFE ratings are 420-800. The max collector dissipation is 500 milliwatt which is also an another good point to use it in amplification stages. Moreover it will also perform well when used it as a switch for load under 100mA. The transistor has also good collector to emitter (VCE) value that is -45V, therefore you can easily use it in circuits using 40 to 45 volts DC.

2N2222 is NPN BJT?normally used in low power amplifiers and also for switching application.

2n2222 is used as a?small signal transistor?and used as a general purpose transistor and is very popular. ?This is in TO-92 package form.

BC548 is another general purpose widely used transistor that can be easily get from reputable electronic components store, this transistor also have bunch of good features on the basis of which one can use it in their electronic circuit, it can handle maximum current of 500mA which is enough to drive many other components such as ICS, other transistors, portion of a circuits, relays, LEDs etc. The max collector dissipation of the device is 625 miliWatt which is another good feature to use it as a small amplifier.