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

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

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.

I_E = I_C + I_B = αI_E + I_CBO
Where,
I_E is emitter current
I_C is collector current
I_B is base current
Amplification factor (α) = IC/IE
I_CBO is an open circuit current
Application:
We have,
I_E = 1 mA
I_CBO = 50 μA
α = 0.92
Hence,
IC = αI_E + I_CBO = 0.92 × 1 + 50 × 10^-3 = 0.97 A

The output resistance is boosted when a BJT is connected in a circuit in a common base configuration. 
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 Power gain is high.
The Phase between input and output is 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
Given,
I_C = 0.95 mA
I_B = 0.05 mA
∴ I_E = I_B + I_C = 0.05 + 0.95 = 1 mA
From the above concept,
I_C = αI_E

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

Given that,
α = .99
I_CBO = 1 μA
I_C = 1 mA

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)

Current amplification factor: It is defined as the ratio of the output current to the input current. In the common-base configuration, the output current is emitter current IC, whereas the input current is base current IE.
Thus, the ratio of change in collector current to the change in the emitter current is known as the current amplification factor. It is expressed by the α.

β is the current amplification factor in the common-emitter configuration.

​​​​​​​γ is the current amplification factor in the common collector configuration.

Current amplification factor: It is defined as the ratio of the output current to the input current. In the common-base configuration, the output current is emitter current IC, whereas the input current is base current IE.
Thus, the ratio of change in collector current to the change in the emitter current is known as the current amplification factor. It is expressed by the α.

Where, I_E = I_C + I_B
Given,
I_B = 0.25 mA and I_E = 1.4 mA
We know that,
I_E = I_C + I_B
⇒ I_C = I_E - I_B = 1.4 - 0.25 = 1.15 mA