Class 12 > Unit # 22:Solid State of Electronics > Transistor Operations

Transistors, PNP vs NPN, and Common Emitter Configuration - Talha's Physics Academy

Talha's Physics Academy

Transistors, PNP vs NPN Transistors, and Common Emitter Configuration

Video Lecture

Watch the complete video lecture below to understand transistors, the differences between PNP and NPN types, and the common emitter configuration.

Transistor Definition and Comparison (PNP vs. NPN)

"When one type of semiconductor is sandwiched between two layers of the other type, this new device is called a transistor. A transistor is a semiconductor device that amplifies or switches electronic signals by controlling the flow of current between three layers: base, emitter, and collector."

It acts as a gatekeeper, allowing or blocking the flow of current between the collector and emitter based on the voltage applied at the base. There are two primary types of transistors:

  • (i) NPN Transistor
  • (ii) PNP Transistor

Difference Between NPN and PNP Transistors

Feature NPN Transistor PNP Transistor
Structure One P-type semiconductor is sandwiched between two N-type semiconductors. Two P-type material layers with an N-type sandwiched between them.
Current Flow Direction The current flows from the collector terminal to the emitter terminal. The current flows from the emitter terminal to the collector terminal.
Voltage Generation Current flow from the collector is generated by keeping a positive ($+\text{ve}$) voltage there. Current flow from emitter to collector is generated at the emitter terminal by keeping a positive ($+\text{ve}$) voltage there.
Switching Behavior Switches ON with an increase in current at the base terminal. Switches ON when there is no current flow at the base terminal (turns OFF when current is present at the base).

Common Emitter Configuration of a Transistor

In a common emitter configuration, the base is the input terminal, the collector is the output terminal, and the emitter is the common terminal shared for both input and output. That means the base terminal and common emitter terminal form the input terminals, whereas the collector terminal and common emitter terminal form the output terminals.

Transistor Operation

The supply voltage between the base and emitter is denoted by $V_{BE}$, while the supply voltage between the collector and emitter is denoted by $V_{CE}$. The input current (base current) is denoted by $I_B$, and the output current (collector current) is denoted by $I_C$.

  • The base-emitter junction is forward-biased so that a base current $I_B$ is generated. Once this junction is conducting, $V_{BE} \approx 0.6\text{ V}$.
  • The base-collector junction is reverse-biased. Transistor action then translates into the relationship:

    $$I_C = \beta I_B$$

    Where $\beta$ is the current gain, which is typically around $100$.
  • For completeness, the emitter current is given by:

    $$I_E = I_C + I_B$$

Characteristics and Regions of Operation

The characteristics graph shows the collector current $I_C$ as a function of the collector-emitter voltage $V_{CE}$ for different values of base current $I_B$:

  1. Cutoff Region: When input conditions are not met, the transistor is off and $I_C = 0$.
  2. Saturation Region: Initially, $I_C$ rises sharply with a small increase in voltage until it reaches a certain point, indicating full current flow.
  3. Active Region: The flat region that follows where the transistor operates normally, and further increases in collector-emitter voltage do not drastically alter the steady active amplification behavior.
Fig: Common emitter transistor circuit configuration and its output I-V characteristics graph.

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