4.

Bipolar Junction Transistors

BJT structure,

A transistor is a semiconductor device made of three

regions separated by two p-n junctions.

Both electrons (from n-type) and holes (from p-type) are

involved in the current flow through the transistor.
Therefore, it is called a Bipolar Junction Transistor (BJT).

A BJT (Bipolar Junction Transistor) has three layers of semiconductor material:

1. Emitter (E) – Heavily doped region that emits majority charge carriers

2. Base (B) – Thin and lightly doped layer that controls the flow of charge carriers between the emitter
and collector.

3. Collector (C) – Moderately doped and larger in size to collect charge carriers from the emitter.

Types of Transistors:

1. NPN Transistor: 2. PNP Transistor:

Emitter and Collector are made of n-type material Emitter and Collector are made of p-type material
(has free electrons). (has holes).
Base is made of p-type material (has holes). Base is made of n-type material (has free
electrons).

In transistor there are two depletion layers

Depletion Layers:

• The EB-junction has a narrow depletion

layer because the emitter is heavily doped, and the
base is lightly doped. The depletion layer goes deeper
into the base.

• The CB-junction has a wide depletion

layer because the collector is moderately doped. The
depletion layer also goes deeper into the base.
 A transistor can be biased in four different ways, depending on how its two junctions (Emitter-Base and
Collector-Base) are biased:

1. Both Junctions 2. Both Junctions 3. Emitter-Base (EB) 4. Emitter-Base (EB)

Forward Biased: Reverse Biased: Junction Reverse Junction Forward
Biased, Collector- Biased, Collector-
Base (CB) Junction Base (CB) Junction
Forward Biased: Reverse Biased:

Transistor operates in Transistor operates Transistor operates Transistor operates in

the saturation in the cut-off in inverted mode (not the active region (normal
region (fully ON, like a region (fully OFF, commonly used). mode, used for
closed switch). like an open amplification).
Allows maximum switch).
current to flow between Does not allow
its collector and emitter significant current
to flow between its
terminals.

The most commonly used is the active region for amplification.

A transistor is a three-terminal device (Emitter, Base, Collector), and it can be connected in three different
circuit configurations depending on which terminal is common to both the input and output.

These configurations are:

1. Common-Base (CB) 2. Common-Emitter (CE) 3. Common-Collector (CC)

Configuration: Configuration: Configuration:
• Base is common to • Emitter is common to both • Collector is common to both
both input and output. input and output. input and output.
• Input: Applied • Input: Applied • Input: Applied
between Emitter and between Base and Emitter. between Base and Collector.
Base. • Output: Taken • Output: Taken
• Output: Taken between Collector and between Emitter and
between Collector and Emitter. Collector.
Base.
An important point to note

➢ Base cannot be output: The base terminal of the transistor is not used as an output.

➢ Collector cannot be input: The collector terminal is not used as an input.

➢ Output side will be Reversed Biased: The output side (likely the collector-base junction) is reverse-
biased, which is typical for amplification purposes.

➢ IE= IC + IB This equation is valid for all configurations.

Modes of operation,

CB, CE I-V characteristics

Common-Base (CB) Configuration:

Input Signal: A small signal is applied to the emitter.

Output Signal: A larger signal is taken from the collector.

The base acts as a common reference point for both input

and output.

The transistor is forward biased at the emitter-base junction

and reverse biased at the collector-base junction.

• When a small forward voltage is applied to the emitter-base junction, electrons are injected from the
emitter into the base.

• Since the base is thin and lightly doped, most of these charge carriers pass through to the collector,
creating collector current (IC).

• The collector current is approximately equal to the emitter current

IE = IC+IB
Since IB (base current) is very small, IE≈IC.

Current Gain (α):

α=IC /IE ≈0.95−0.99

(Since most of the emitter current flows into the collector)

Voltage Gain:

It provides high voltage gain, meaning the output voltage is much larger than the input voltage.

(The voltage gain is achieved by the transistor controlling current flow and the collector resistor
converting that current into a larger voltage.)

Input Impedance:
 The input impedance is very low, so it does not need much voltage to allow current to flow.

Output Impedance:

The output impedance is very high, meaning it can drive high-resistance loads.

Input Characteristics: Output Characteristics:

The input characteristics show the relationship The output characteristics show the relationship
between the input voltage and the input current for between the output voltage and the output
different values of the output voltage current for different values of the input current

The input characteristics are similar to the forward-

biased diode curve since the emitter-base junction
is forward-biased.

As VCB increases, the width of the depletion region

at the collector-base junction increases. This
reduces the effective base width, base current
decreases which results in a slight increase in IE for
a given VBE.

Common-Emitter (CE) Configuration:

In the CE configuration:

The emitter is common to both the input and output circuits.

The input signal is applied between the base and emitter.

The output signal is taken between the collector and emitter.

 The transistor is forward biased at the Base-Emitter junction and reverse biased at the Base-
Collector junction.

Current Gain (β): the ratio of change in collector current with respect to base current is known as the base
amplification factor. It is represented by β (beta).

β =IC /IB

Input Characteristics: Output Characteristics:

The curve plotted between base current IB and the Output characteristics describe the relationship
base-emitter voltage VBE is called Input between the collector current (IC) and the collector-
characteristics curve emitter voltage (VCE) for different base currents (IB).

The base current (IB) depends on VCE because

as VCE increases, the reverse bias across the base- Output Resistance: The ratio of the variation in
collector junction grows. This changes the collector-emitter voltage to the collector-emitter
distribution of minority carriers in the base region, current is known at collector currents at a constant
reducing recombination. As a result, for a base current IB is called output resistance ro.
fixed VBE, IB decreases slightly as VCE increases.

Input Resistance: The ratio of change in base-

emitter voltage VBE to the change in base current
∆IB at constant collector-emitter voltage VCE is
known as input resistance, i.e.,
In a common base transistor, the current gain (α) is defined as the ratio of the collector current (IC) to the
emitter current (IE):
α= IC / IE (1)

In a common emitter transistor amplifier, the current gain (β) is defined as the ratio of the collector current
(IC) to the base current (IB):

β=IC / IB (2)

For a transistor, the emitter current (IE) is the sum of the collector current (IC) and the base current (IB):

IE=IC+IB

We can express the emitter current in terms of the collector current:

IE=IC + IB ⟹

IE=IC + ICβ ( IB=IC β)

IE=IC (1+1/β)

α = IC/IE

= IC / { IC (1+1/β)}

= 1 / (1+1/β)

α = β / (β+1)

Rearranging the above equation gives:

β = α / (1−α)
BJT as a switch,

• The input and base are grounded.

• Base emitter voltage VBE<0.7v

• Base Emitter junctions is reversed.

• Base- collector junction us reversed biased.

• Transistor is "fully-Off" (cut off region)

• No collector current flows(Ic=0)

• Vout = VcE = VCC= 1

• Transistor operates as open switch

• Saturation Characteristics:

• The base and input are connected to Vcc.

• Base-Emitter voltage VBE>0.7V.

• Base-Emitter junction is forward biased.

• Base-collector junction is forward biased.

• Transistor is "fully-On"

• Max Collector current flows (Ic = Vcc/ RL)

• VCE=0 (ideal saturation)

• Vout = VCE ="0".

• Transistor operates as a closed switch.

• Cutoff Region:

Vi is low, transistor is off, Vo is high (close to VCC).

Acts like an open switch.

• Active Region:

Vi is moderate, transistor is partially on, Vo decreases

linearly with Vi.

Used for amplification.

• Saturation Region:

Vi is high, transistor is fully on, Vo is very low (~0.2V).

Acts like a closed switch.

BJT Amplification

The Common Emitter (CE) transistor amplifier is one of the most widely used transistor configurations in
analog circuits. It is called a common emitter because the emitter terminal is shared between the input and
output circuits. This configuration provides significant voltage and current amplification, making it highly
useful in various applications.

When a small AC signal is applied to the base the base current, controls the collector current, resulting in a
much larger amplified output voltage across the load resistor.

Working Principle:

• A small AC input voltage (Vin) is applied to the base.

• This causes variations in the base current (IB).

• Due to transistor action, the collector current (IC=β IB) also varies.

• This variation in collector current leads to voltage changes across Rc, producing the amplified
output.

• The output voltage (Vo) is inverted (180° phase shift) compared to the input.

Current amplification:

Given by β= Io / Ii

IC = β IB

Collector-emitter voltage:

VCE = VCC−ICRC

Input Resistance (Ri)

 • Defined as Ri=Vi/Ii

• Expressed as Ri=ΔVBE/ΔIB

1. Output Resistance (Ro)

• Defined as Ro=Vo/Io

• Expressed as Ro=ΔVCE/ ΔIC

Voltage Gain (Av)

• Defined as Av=Vo/Vi

• Expressed as Av=ΔVCE/ΔVBE

• Using Ohm’s Law V=IR, it simplifies to Av=βRL/Ri

Problem:
A transistor in CE configuration has a base current 𝐼𝐵 = 20𝜇𝐴 and a collector current 𝐼𝐶 = 4𝑚𝐴. Find the
current gain (𝛽) of the transistor. If the base current is increased to 50𝜇𝐴, what will be the new collector
current?

Solution:
Current gain (𝛽) is given by:
𝐼𝐶
𝛽=
𝐼𝐵
Substituting the values:

4 × 10−3
𝛽= = 200
20 × 10−6
Now, if 𝐼𝐵 is increased to 50𝜇𝐴, the new collector current is:

𝐼𝐶′ = 𝛽 × 𝐼𝐵′

𝐼𝐶′ = 200 × 50 × 10−6 = 10𝑚𝐴

Answer:

• Current gain, 𝛽 = 200

• New collector current, 𝐼𝐶′ = 10𝑚𝐴

Problem:
A transistor is operating in CE configuration with 𝑉𝐶𝐶 = 12𝑉, 𝑅𝐶 = 2𝑘𝛺, 𝐼𝐶 = 5𝑚𝐴, and 𝑉𝐵𝐸 = 0.7𝑉. Find
the collector-emitter voltage 𝑉𝐶𝐸 .

Solution:
Using Kirchhoff’s Voltage Law (KVL) in the output loop:
 𝑉𝐶𝐸 = 𝑉𝐶𝐶 − 𝐼𝐶 𝑅𝐶
Substituting values:

𝑉𝐶𝐸 = 12𝑉 − (5 × 10−3 𝐴 × 2 × 103 𝛺)

𝑉𝐶𝐸 = 12𝑉 − 10𝑉 = 2𝑉
Answer: 𝑉𝐶𝐸 = 2𝑉

Problem:
A transistor with 𝛽 = 100 is used in a CE amplifier circuit. The base-emitter voltage is 𝑉𝐵𝐸 = 0.7𝑉, and the
supply voltage is 𝑉𝐶𝐶 = 12𝑉. The desired collector current is 𝐼𝐶 = 2𝑚𝐴. If a base resistor 𝑅𝐵 is used to
provide the required base current, find its value.

Solution:
Base current is:

𝐼𝐶 2 × 10−3
𝐼𝐵 = = = 20𝜇𝐴
𝛽 100
Applying KVL to the input loop:

𝑉𝐶𝐶 − 𝐼𝐵 𝑅𝐵 − 𝑉𝐵𝐸 = 0
Solving for 𝑅𝐵 :
𝑉𝐶𝐶 − 𝑉𝐵𝐸
𝑅𝐵 =
𝐼𝐵
12𝑉 − 0.7𝑉
𝑅𝐵 =
20 × 10−6 𝐴
11.3𝑉
𝑅𝐵 = = 565𝑘𝛺
20 × 10−6 𝐴
Answer: 𝑅𝐵 = 565𝑘𝛺