How to choose a transistor for amplification

Question

I've been trying to find information on choosing an NPN transistor but everything I've found seems to apply to saturated transistors only. I'm basically trying to amplify the current given from a 3mm audio headphone jack to a useful level. Knowing the voltage and current required for the load at the collector, plus the voltage and current supplied to the base how can I determine from a spec sheet if a transistor is suitable?

Also, am I correct in thinking that the voltage supplied by the audio source will need to be at least 0.6V?

EDIT: For the sake of an example let's say the load is a bunch of LED's totaling 9V and 800mA. Obviously with an appropriate power source. I'm still researching the current and voltage of the audio signal but for this example let's say it's 0.3V and 5mA.

Answer 1

Simple amplification of audio is the most trivial demand for a BJT and virtually any transistor (including germanium) will do the job you want. Maybe look for BJTs that are referenced as good audio amplifiers but, given that the marketing boys would see some folk being put off buying a particular transistor because it is pigeon-holed by the label.

The level of audio does not need to be greater than 0.6 volts because the right BJT amplifier will set its bias point to accommodate all the + and - swings of the audio signal it wishes to amplify.

Answer 2

Normally, to amplify an audio signal, you should know what is the excursion signal you want to achieve over the load. A good method is to use the load line.
This will let you know the maximum collector current to develop and the maximum voltage that must support the collector-emitter transistor. These are the basic parameters to select the transistor.