The Amplifier in a nutshell

Yes it could be done, with a single transistor! An amplifier, is a device, either as a circuit in a unit, or a whole unit, that increases a signal's voltage, current, or both. The simplest amplifier is a 1-transistor circuit. Therefore, nearly all electronics perform amplification in different ways.

Two examples;
- A microphone generates a signal with a small voltage. The voltage is amplified and the signal can be processed through filters or a volume control. If the current is amplified too, it can power a speaker.
- A thermistor (temperature dependent resistor) is applied with a constant current, where the varying resistance produces a varying voltage drop across it. The voltage can control a circuit that amplifies current to switch an electromechanical relay.

There are links in the end of some amplifier type texts below. A very informative site about amplifiers, audio electronics and more is Elliott Sound Products - sound.westhost.com.