The BC547 is an SMD SOT23-3 package NPN Transistor with VCE of 45V and a collector current of 100mA. It can be used as a small signal switching transistor. It also has a low base voltage of 6V.
BC547 is a NPN transistor hence the collector and emitter will be left open (Reverse biased) when the base pin is held at ground and will be closed (Forward biased) when a signal is provided to base pin.
2SC828 is an NPN, Silicon-based AF Amplifier Transistor that is mainly used in AF based audio equipment for amplification and switching. It has a Collector-Emitter Voltage of 45 Vdc with a Collector-Base voltage of 45 Vdc.
The C1815 is a 50V 150mA general purpose NPN transistor with a maximum gain value of 700. This makes the transistor suitable for amplifier applications.
The 2SC1061 is a NPN Power Transistor in To-220 package. It has a DC current gain from 35 to 320 and transition frequency of 5MHz. It is commonly used in high power amplifier application along with its complementary PNP part 2SA671.