The 2N5551 is an NPN amplifier Transistor with an amplification factor (hfe) of 80 when the collector current is 10mA. It also has decent switching characteristics (Transition frequency is 100MHz) hence can amplify low-level signals.

Due to this feature, the transistor is commonly used for amplification of audio or other low power signals. So if you are looking for an NPN transistor for you amplifier circuit then this transistor might be the right choice.

Features:

• Amplifier NPN Transistor
• High DC Current Gain (hFE), typically 80 when IC=10mA
• Continuous Collector current (IC) is 600mA
• Collector-Emitter voltage (VCE) is 160 V
• Collector-Base voltage (VCB) is 180V
• Emitter Base Voltage (VBE) is 6V
• Transition Frequency is 100MHz
• Available in To-92 Package

Datasheet:

http://www.onsemi.com/pub/Collateral/2N5550-D.PDF