A42 is an NPN transistor hence the collector and emitter will be left open (Reverse biased) when the base pin is held at the ground and will be closed (Forward biased) when a signal is provided to the base pin. It has a gain value of 30; this value determines the amplification capacity of the transistor normally MPSA42 will not be used for amplification due to its low gain value. The maximum amount of current that could flow through the Collector pin is 500mA, hence we cannot drive loads that consume more than 500mA using this transistor. To bias a transistor we have to supply current to the base pin, this current (IB) should be limited to 5mA.

When this transistor is fully biassed then it can allow a maximum of 500mA to flow across the collector and emitter. 

Specifications:

• Bi-Polar, High Voltage NPN Transistor
• DC Current Gain (hFE) is 40
• Continuous Collector current (IC) is 500mA
• Base- Emitter Voltage (VBE) is 6V
• Collector-Emitter Voltage (VCE) is 300V
• Collector-Base Voltage (VCB) is 300V
• Available in To-92 Package

Applications:

• Switching high voltage loads
• Used to control loads with high peak voltage
• Speed control of low current motors
• Drive high voltage stepper motors
• Drive high power LED