Test: Configuration of BJT - Electronics and Communication Engineering (ECE) MCQ

• 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.

The base current amplification factor in CE configuration is denoted by the symbol β which is the ratio of change in collector current (ΔI_C) to the change in base current (ΔI_B).

• The common collector transistor circuit configuration gains its name from the fact that the collector circuit is common to both input and output circuits, the base is associated with only the input, and then the emitter with the output only.
• The other name for the common collector is emitter follower. This name is derived from the fact that the emitter voltage "follows" that of the base circuit - the circuit has unit voltage gain.
• The emitter follower transistor amplifier has a very straightforward circuit. The base is connected to the previous stage, and often this may be directly connected as this can save on additional bias resistors which lower the input impedance and hence increase the loading to the previous stage.
• The power gain of a common emitter (CE) amplifier is higher than the power gain of a common collector (CC) amplifier or a common base (CB) amplifier

When the emitter-base (EB) junction of a transistor is reverse-biased, the collector current typically becomes very small or approaches zero. Therefore, the correct answer is 1
Concept:

• In a normal operating condition, when the emitter-base junction of a transistor is forward-biased, it allows current to flow from the emitter to the base. This forward biasing establishes the necessary conditions for the transistor to function as an amplifier or switch.
• However, when the emitter-base junction is reverse-biased (reverse voltage applied), the depletion region of the junction widens, making it difficult for the majority carriers to cross the junction. As a result, the flow of current from the collector to the emitter is significantly reduced.
• In this reverse-biased condition, only a small amount of leakage current may flow through the junction due to minority carriers (minority carrier leakage), but the collector current is typically negligible or considered close to zero.

Current gain(α) of a Common base amplifier is unity (ideally).
The Input resistance is very low ( 30-150 Ω ).
The output resistance is very high ( ≈ 500K ).
The Voltage gain is high.
The Phase between input and output is the same.

For a transistor, the base current, the emitter current, and the collector current are related as:
I_E = I_B + I_C
where I_C = αI_E
α = Current gain of the transistor
Calculation:
Given,
IC = 0.95 mA
IB = 0.05 mA
∴ I_E = I_B + I_C = 0.05 + 0.95 = 1 mA
From the above concept,
I_C = αI_E

Common base configuration:

Input Terminal Emitter – Base (E_B)
Output Terminal Collector – Base (C_B)
Since for Amplification Application in BJT
E_B Junction → Forward Biased (Low Impedance)
C_B Junction → Reversed Biased (High Impedance)
Various transistor configurations and their characteristics are shown in the table

Region of operation of BJT:
In a given circuit, the region of the transistor can be decided through the following procedure.

• Assume that the transistor is in the saturation region.
• Calculate Base current (I_B) and collector current (I_C) separately.
• Calculate the minimum Base current (IB)min required to operate the transistor in saturation.
• Compare Base current (I_B) and (I_B)min

I_B  ≥   (I_B)_min, Transistor is in the Saturation region.
I_B < (I_B)_min, Transistor is in the Active region
I_B  < 0, Transistor is in the Cut-off region.
Formula:
In saturation,
Base-Emitter voltage = V_BE = 0.8 V
Collector-Emitter voltage V_CE = 0.2 V
(I_B)_min = (I_C)_sat / β  ----(1)
(I_C)_sat  = Collector current in Saturation 
β = DC current gain

Assume transistor in Saturation region.
Apply KVL in the outer loop:
5 - I_C(Sat) x 5kΩ - V_CE(Sat) = 0

Now, Apply KVL in Emitter Loop; 

Hence, BJT is in saturation region.

Common base configuration:

Input Terminal Emitter – Base (E_B)
Output Terminal Collector – Base (C_B)
Since for Amplification Application in BJT
E_B Junction → Forward Biased (Low Impedance)
C_B Junction → Reversed Biased (High Impedance)