Bipolar Junction Transistor (BJT)

A junction transistor which is a semiconductor device having two junctions and three terminals, in which the current is carried by both electrons and holes (hence the name Bipolar), is called a Bipolar Junction Transistor.

A transistor in which a p-type semiconductor (base) layer separates two layers of the n-type semiconductor (emitter and collector), and in whose circuit symbol the arrow is drawn from base pointing towards emitter, is called an n-p-n Transistor.

A transistor in which an n-type semiconductor (base) layer separates two layers of the p-type semiconductor (emitter and collector), and in whose circuit symbol the arrow is drawn from emitter pointing towards base, is called a p-n-p Transistor.

A transistor configuration in which the emitter of the transistor is common to both the input and the output is called the Common Emitter (CE) Configuration.

A transistor configuration in which the base of the transistor is common to both the input and the output is called the Common Base (CB) Configuration.

A transistor configuration in which the collector of the transistor is common to both the input and the output is called the Common Collector (CC) Configuration.

\[\alpha_{ac} = \dfrac{\Delta I_C}{\Delta I_E}\]

α_dc ​= \[\frac {I_C}{I_E}\]​ (value lies between 0.95 to 0.99)

\[\beta_{ac} = \dfrac{\Delta I_C}{\Delta I_B}\]

β_dc ​= \[\frac {I_C}{​I_B}\]​​ (always > 1)

1. For a transistor operating as an amplifier in CE configuration, the E-B junction is forward biased while the C-B junction is reverse biased.
2. The AC current gain (βac) is the ratio of small change in collector current (ΔI_C) to small change in base current (ΔI_B) at constant V_CE. The DC current gain (βdc) is the ratio of I_C to I_B, which is always greater than 1.
3. The Voltage Gain (A_v) is the ratio of change in output voltage (ΔV_o) to change in input voltage (ΔV_i). The Power Gain is the ratio of change in output power (ΔP_o) to change in input power (ΔP_i).
4. In output characteristics, I_C is independent of V_CE as long as the collector-emitter junction is reverse biased. I_C is large for large values of I_B when V_CE is constant.