Bipolar Junction Transistors (BJT) has two types: PNP and NPN. Bipolar transistors are so named because the controlled current must go through two types of semiconductor material: P and N. The only functional difference between a PNP transistor and an NPN transistor is the proper biasing (polarity) of the junctions when operating. For any given state of operation, the current directions and voltage polarities for each type of transistor are exactly opposite each other. Transistors function as current regulators by allowing a small current to control a larger current.
bipolar transistors are current-controlled devices: they regulate collector current based on the existence of base-to-emitter current, not base-to-emitter voltage.
When a transistor has zero current through it, it is said to be in a state of cutoff. When a transistor has maximum current through it, it is said to be in a state of saturation. If this limit for the controlled current is greater than zero but less than the maximum allowed by the power supply and load circuit, the transistor will control the collector current in a mode somewhere between cutoff and saturation. This mode of operation is called the active mode.
Transistor's characteristic curves. I_base(uA), I_collector(mA). The ration of collector current to base current is Beat ration: β = I_collector/I_base. (I_collector = β I_base). β ratios are different for every transistor, and they tend to change for different operating conditions
Common-emitter amplifier: both the signal source and the load share the emitter as a common connection point. Transistors are essentially DC devices: they cannot directly handle voltages or currents that reverse direction. In order to make them work for amplifying AC signals, the input signal must be offset with a DC voltage to keep the transistor in its active mode throughout the entire cycle of the wave. This is called biasing. If the output voltage is measured between emitter and collector on a common-emitter amplifier, it will be 180o out of phase with the input voltage waveform. For this reason, the common-emitter amplifier is called an inverting amplifier circuit.
Common-collector amplifier: the common-collector produces an output voltage in direct rather than inverse proportion to the rising input voltage. The current gain of a common-collector amplifier is equal to β plus 1. The voltage gain is approximately equal to 1. A Darlington pair is a pair of transistors "piggybacked" on one another so that the emitter of one feeds current to the base of the other in common-collector form. The result is an overall current gain equal to the product (multiplication) of their individual common-collector current gains (β plus 1).
Common-base amplifier: The current gain of a common-base amplifier is always less than 1. The ratio of a transistor's collector current to emitter current is called α. The α value is less than 1.