Test: Common Emitter Configuration - Electronics and Communication Engineering (ECE) MCQ

• In common emitter configuration, the emitter is common to both the base and collector. The emitter-base junction is always forward-biased as the positive terminal of the battery is connected to the p-side of the transistor and the negative terminal of the battery is connected to the n-side of the transistor.
• The input impedance is typically around 1kΩ, although this can vary considerably according to the circuit values and conditions. The low input impedance results from the fact that the input is applied across the base and emitter where there is a forward-biased junction, Also the output impedance can be relatively high. Again this varies considerably on the electronic component values chosen and current levels permitted. The output impedance may be as high as 10kΩ or possibly more
• However, if the current drain allows higher current levels to be drawn, the output impedance can be reduced considerably. The level of resistance or impedance comes from the output taken from the collector where there is a reverse-biased junction.

Amplifier: An electronic device that increases the voltage, current, or power of a signal is called an amplifier.

• Amplifiers are used in wireless communications and broadcasting, and in audio equipment of all kinds.

Common emitter amplifier: It is a three basic single-stage bipolar junction transistor and is used as a voltage amplifier.

• The input is applied to the base, and the output is taken from the collector, the common terminal for both circuits is the emitter.

The correct option is 1.

Bipolar junction transistor

• The Bipolar Junction Transistor (BJT) is a three-terminal device i.e. Base, Emitter, and Collector.
• There are two main types of bipolar junction transistors the NPN and the PNP transistor.
• The emitter is a heavily doped region of the BJT transistor, providing the majority of carriers into the base region.
• The base region is a thin, lightly doped region sandwiched between the emitter and collector.
• The majority of carriers from the emitter pass through the base region and its flow can be externally controlled.

Working configuration of BJT

Common Base configuration has high output impedance but low input impedance.
Concept:
In common emitter configuration, the emitter terminal is grounded so the common emitter configuration is also known as grounded emitter configuration.

As the emitter current is zero in the cut-off region, the potential VE is called floating emitter potential.

• The Bipolar Junction Transistor (BJT) is a three-terminal device i.e. Base, Emitter, and Collector.
• There are two main types of bipolar junction transistors the NPN and the PNP transistor.
• The emitter is a heavily doped region of the BJT transistor, providing the majority of carriers into the base region.
• The base region is a thin, lightly doped region sandwiched between the emitter and collector.
• The majority of carriers from the emitter pass through the base region and its flow can be externally controlled.

Working configuration of BJT

Power gain of amplifier:
The power gain of an amplifier is defined as the product of voltage gain and current gain.

where, A_p = Power gain
A_v = Voltage gain
A_i = Current gain
Comparison between CE, CC, and CB configuration:

Since the voltage and current gain in the CE amplifier are high, therefore the power gain is maximum.

Given:
V_CC = 10V
I_C = 1 mA
R_C =  4 kΩ
Apply KVL in the collector to Emitter loop:
V_CC - I_CR_C = V_C
V_C = 10 - 1 mA × 4 kΩ
V_C = 10 - 4 = 6V
We know that the operating point is represented in terms of (V_C, I_C)
Hence the operating point of the given CE Amplifier is 6 V, 1 mA

Characteristics of Common Emitter Amplifier:
1) The voltage gain and current gain are high
2) The power gain is high
3) There is a phase relationship of 180 degrees in input and output
4) The input and output resistors are medium

Common emitter configuration:

• The emitter is connected between the collector and base.
• The emitter is common to both the input and the output circuit.
• Both current and voltage gain can be described as medium.
• This provides good overall performance and as such, it is often the most widely used configuration.

The quiescent Base voltage (V_b) is determined by the potential divider network formed by the two resistors R₁, R₂ (connected to the base of the transistor), and the power supply voltage V_cc as shown, with the current flowing through both the resistors.



The Thevenin equivalent resistance connected to the base of the circuit will be:

Assuming I_E ≈ I_C, the output voltage will be: