Bipolar Junction Transistor (BJT's)

Bipolar Junction Transistors (BJTs) are semiconductor three-terminal devices utilized in a variety of electronic applications, including signal modulation, switching, and amplification. NPN (Negative-Positive-Negative) and PNP (Positive-Negative-Positive) are the two primary categories of BJTs. The placement of their semiconductor layers and the way current flows differ between these two types. An overview of NPN and PNP transistors is given below:

1. NPN Transistor:

• Structure: A layer of P-type semiconductor material (positive) is sandwiched between two layers of N-type semiconductor material (negative) in an NPN transistor. The N-type layers serve as the emitter and collector, with the P-type layer serving as the base.
• Current Flow: In an NPN transistor, the base functions as a control terminal to regulate the current flow, which travels from the emitter to the collector.
• Amplification: NPN transistors are frequently employed in amplifying applications. A signal is amplified when a tiny current enters the base and controls a bigger current that is flowing from the emitter to the collector.

2. PNP Transistor:

• Structure: A layer of N-type semiconductor material (negative) is sandwiched between two layers of P-type semiconductor material (positive) in a PNP transistor. The base is an N-type layer, whereas the emitter and collector are P-type layers.
• Current Flow: In a PNP transistor, the base regulates the current flow from the collector to the emitter.
• Amplification: Similar to NPN transistors, PNP transistors can also be used for amplification. A tiny current at the base regulates the flow of current from the collector to the emitter.

Transistor Amplifier Configurations: 

Transistors are frequently employed as amplifiers to strengthen a weak input signal. The common emitter (CE), common base (CB), and common collector (CC) configurations are the three fundamental types of transistor amplifiers.

1. Common Emitter (CE) Configuration:

• The emitter terminal is shared by the input and output circuits in a CE setup.
• High voltage gain and moderate current gain are provided by CE amplifiers.
• The base-emitter junction receives the input, while the collector-emitter junction receives the output.
• Phase inversion exists between the input and output of CE amplifiers.

2. Common Base (CB) Configuration:

• The base terminal is shared by the input and output circuits in a CB design.
• High current gain and unity voltage gain (i.e., no voltage amplification) are provided by CB amplifiers.
• The emitter-base junction receives the input, while the collector-base junction receives the output.
• Low input impedance and high output impedance characterize CB amplifiers.

3. Common Collector (CC) Configuration (Emitter Follower):

• The collector terminal is shared by the input and output circuits in a CC design.
• CC amplifiers offer high voltage gain and unity current gain.
• The base-emitter junction receives the input, while the collector-emitter junction receives the output.
• High input impedance and low output impedance characterize CC amplifiers.

Engineers can select the optimal amplifier layout for a given application by weighing trade-offs in terms of voltage gain, current gain, input/output impedance, and phase relationships.