When the base of NPN transistor is grounded (0 volts), no current flows from the emitter to the collector (the transistor is "off"). If the base is forward-biased by at least 0.6 volt, a current will flow from the emitter to the collector (the transistor is "on"). When operated in only these two modes, the transistor function as a switch. If the base is forward-biased, the emitter-collector current will folow variations in amuch smaller base current. The transistor then function as an amplifier. This discussion applies to a transistor in which the emitter is the ground connection for both the input and output and is called the common-emitter circuit. Some simplified common-emitter circuit are shown below. So you can see how they are used in real circuit.
A Bipolar Transistor Switch
Only two inputs are possible: ground (0 volt) and the positive battery (+V). Therefore the transistor is off or on. A typical base resistance is 5,000 to 10,000 ohms. (if the resistor is replaced by a wire, the lamp can be switched on or off from a considerable distance.)
A Bipolar Transistor DC Amplifier
The variable resistor forward biases the transistor and controls the input (base-emitter) current. The meter indicates the output (collector-emitter) current. The series resistor protects the meter from excessive current. In a working circuit, the variable resistor may be in series with a second component having a resistance that varies with temperature, light, moisture, etc. When the input signal changes rapidly, an AC amplifier such as the one below is used.
A Bipolar Transistor AC Amplifier
This is the simplest of several basic AC amplifiers. The input capacitor blocks any DC in the input signal.
The bias resistor is selected to give an output voltage of about half the battery voltage. The amplified signal "rides" on this steady output voltage and varies above and below it. (without the basis resistor, only the positive half of the input signal above 0.6 volt will be amplified. This will cause severe distortion.)