Practice Questions: Bipolar Junction Transistor (BJT) - 2 - Electronics and Communication Engineering (ECE) MCQ

• A large collector base reverse bias is the reason behind early effect manifested by BJTs
• The depletion layer penetrates more into the base as the base is lightly doped increasing the concentration gradient in the base
• As reverse biasing of the collector to base junction increases, depletion region penetrates more into the base, this reduces the effective base width
• This reduction in base width causes less recombination of carriers in the base region
• This is known as an early effect

The modes of operation of an npn BJT are as follow:

Concept:

• A large collector base reverse bias is the reason behind the early effect manifested by BJTs.
• As reverse biasing of the collector to base junction increases, the depletion region penetrates more into the base, as the base is lightly doped.
• This reduces the effective base width and hence the concentration gradient in the base increases.
• This reduction in the effective base width causes less recombination of carriers in the base region which results in an increase in collector current. This is known as the Early effect.
• The decrease in base width causes ß to increase and hence collector current increases with collector voltage rather than staying constant.
• The slope introduced by the Early effect is almost linear with IC and the common-emitter characteristics extrapolate to an intersection with the voltage axis VA, called the Early voltage.

This is explained with the help of the following VCE (Reverse voltage) vs IC (Collector current) curve:

Transistor has three regions:

Emitter:

• Highly doped
• medium area

Collector:

• moderate doping
• largest area

Base:

• lightest doping
• lightest area

Bias stabilization is a process of making Q-point independent of changes in temperature and changes in the transistor parameter.

Current stability factor

Fixed bias circuit:

Base current I_B is fixed. Hence it is called a fixed biased circuit.
S = 1 + β
Since S is very large, IC will be unstable
Collector to base bias circuit:

It gives a more stable IC­ than a fixed bias circuit. The stability factor is dependent upon RC. If RC­ becomes smaller or zero, the stability factor becomes very large and IC does not remain stable.

Self-bias circuit:

The Self-bias circuit has the least stability factor. The stability factor is independent of R_C. it can b used to bias BJT for any configuration.

• The term impedance matching is simply defined as the process of making one impedance look like another
• Impedance matching is a process in which the impedance of an electrical load is made equal to the source impedance to maximize the power transfer or minimize signal reflection from the load
• The Power amplifiers generally use transformer coupling because the transformer permits impedance matching
• The disadvantage of impedance matching is that it gives distorted output

Concept:
I_CEO is the reverse leakage current in the common-emitter configuration of BJT when the base is open.
I_CBO is the reverse leakage current in the common-base configuration of BJT when the emitter is open.
Also, I_CEO > I_CBO
And they are related by the relation:
I_CEO = (1 + β) I_CBO
Total collector current is given by:
I_C = βI_B + I_CBO(β + 1)

Application:
The total collector current for the given configuration will be:
IC = βI_B + I_CBO(β + 1)   ---(1)
Given: I_B = 0 A, β = 50, and I_CBO = 2.5 μA.
Substituting these values in Equation (1), we get:
IC = (50)(0) + (2.5 μ) (50 + 1)
IC = 2.5 x 10^-6 x 51 A
IC = 0.1275 mA

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

Comparison: