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BC547 is a general purpose BJT NPN transistor mostly used in electronics hobbyists and educational electronics projects. Besides these uses it can also be used in commercial circuits. It comes in TO-92 packaging and the maximum output current this transistor can handle is 100mA. The transistor is having very good DC current gain and low noise capabilities due to which it is ideal to use in signal amplification stages. The typical saturation voltage is only 90 millivolts which is also a good sign to use it as a switch.

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.