GATE Electrical – Analog Electronics

Page 15: Emitter Follower (Common Collector Amplifier)

The Emitter Follower amplifier is also called the Common Collector (CC) amplifier. In this configuration, the collector terminal is common to both input and output circuits.

                                            

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Circuit Configuration

In the emitter follower configuration:

• Input signal is applied between base and collector
• Output signal is taken from the emitter terminal
• The collector is connected to the supply voltage

Since the output voltage follows the input voltage, this amplifier is called an Emitter Follower.

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Working Principle

When an input signal is applied to the base, the emitter voltage changes accordingly.

The emitter voltage is approximately:

VE ≈ VB − VBE

Where:

• VB = Base voltage
• VBE ≈ 0.7 V (for silicon transistor)

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Voltage Gain

Voltage gain of emitter follower is approximately:

Av ≈ 1

Thus the output voltage nearly follows the input voltage.

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Input Resistance

Input resistance is very high:

Rin ≈ β RE

Typical value ranges from:

100 kΩ – 1 MΩ

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Output Resistance

Output resistance is very low:

Rout ≈ RE / β

Typical value ranges from:

10 Ω – 100 Ω

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Applications

• Impedance matching
• Buffer amplifier
• Voltage follower circuits
• Signal isolation

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Important GATE Points

• Voltage gain ≈ 1
• Very high input resistance
• Very low output resistance
• No phase inversion
• Used as buffer amplifier

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Next Page → Multistage Amplifiers

 

GATE Electrical – Analog Electronics

Page 14: Common Emitter (CE) Amplifier

The Common Emitter amplifier is the most widely used transistor amplifier configuration because it provides high voltage gain and moderate input and output resistance.

                                       

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CE Amplifier Circuit

In the CE amplifier configuration:

• Input signal is applied between base and emitter
• Output is taken between collector and emitter
• The emitter terminal is common for both input and output

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Working Principle

A small change in base current produces a large change in collector current. This results in amplification of the input signal.

Collector current relationship:

IC = β IB

Since the collector resistor converts current variation into voltage variation, a large output voltage appears across the load.

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Voltage Gain

Using the small signal model, voltage gain of CE amplifier is:

Av = -gm RC

Where:

• gm = transconductance
• RC = collector resistance

Negative sign indicates phase inversion.

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Input Resistance

Input resistance of CE amplifier is approximately:

Rin = rπ

Typical value ranges from:

1 kΩ – 10 kΩ

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Output Resistance

Output resistance of CE amplifier is approximately:

Rout ≈ RC

Typical value ranges from:

10 kΩ – 50 kΩ

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Phase Inversion

One important feature of the CE amplifier is that the output signal is 180° out of phase with the input signal.

This means when the input signal increases, the output voltage decreases.

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Important GATE Points

• CE amplifier provides high voltage gain.
• Output signal is inverted (180° phase shift).
• Moderate input resistance.
• Moderate output resistance.
• Most widely used transistor amplifier.

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Next Page → Emitter Follower (Common Collector Amplifier)

 

GATE Electrical – Analog Electronics

Page 13: Small Signal Model of BJT

In amplifier analysis, signals are usually small compared to the DC bias values. To analyze such circuits easily, the transistor is replaced with a simplified equivalent circuit called the Small Signal Model.

This model helps determine voltage gain, input resistance and output resistance of transistor amplifiers.

                                              

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Why Small Signal Model is Needed

• Simplifies transistor analysis
• Helps calculate amplifier gain
• Used in AC analysis of amplifier circuits

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Hybrid-π Model

The most widely used small signal model is the Hybrid-π Model.

It consists of the following elements:

• rπ → Base-emitter resistance
• gm vπ → Controlled current source
• ro → Output resistance

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Important Small Signal Parameters

1. Transconductance (gm)

Transconductance represents the relationship between collector current and base-emitter voltage.

gm = IC / VT

Where:

• IC = Collector current
• VT ≈ 25 mV at room temperature

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2. Input Resistance (rπ)

Input resistance between base and emitter is:

rπ = β / gm

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3. Output Resistance (ro)

Output resistance is caused by Early effect.

ro = VA / IC

Where:

• VA = Early voltage

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Voltage Gain of CE Amplifier

Using the small signal model, the voltage gain becomes:

Av = -gm RC

Negative sign indicates phase inversion.

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Important GATE Points

• Small signal model is used for AC analysis.
• Hybrid-π model is most commonly used.
• gm increases with collector current.
• Voltage gain of CE amplifier is negative.

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Next Page → CE Amplifier Analysis

 

GATE Electrical – Analog Electronics

Page 12: BJT Biasing Techniques

Biasing is the process of establishing the correct operating point (Q-point) of a transistor so that it can work properly as an amplifier.

Proper biasing ensures:

                                 

• Stable operation
• Linear amplification
• Minimum signal distortion

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1. Fixed Bias (Base Bias)

This is the simplest biasing method where a resistor is connected between the base and the supply voltage.

Base current is determined by:

IB = (VCC − VBE) / RB

Collector current becomes:

IC = β IB

Advantages

• Simple circuit
• Easy to design

Disadvantages

• Very poor stability
• Q-point changes with temperature

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2. Collector-to-Base Bias

In this method the base resistor is connected to the collector instead of the supply voltage.

This provides a small negative feedback which improves stability.

Advantages

• Better stability than fixed bias
• Provides negative feedback

Disadvantages

• Still not very stable
• Limited practical use

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3. Voltage Divider Bias

Voltage divider bias is the most widely used biasing method.

Two resistors form a voltage divider network to set the base voltage.

VB = VCC × R2 / (R1 + R2)

Emitter resistor improves thermal stability.

Advantages

• Excellent stability
• Q-point remains nearly constant
• Widely used in amplifier circuits

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Comparison of Biasing Methods

Bias Method	Stability	Complexity
Fixed Bias	Poor	Simple
Collector Bias	Moderate	Medium
Voltage Divider Bias	Excellent	Most Used

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Important GATE Points

• Voltage divider bias is the most stable method.
• Emitter resistor improves thermal stability.
• Biasing ensures correct Q-point.
• Without proper biasing, amplifier distortion occurs.

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Next Page → Small Signal Model of BJT

 

GATE Electrical – Analog Electronics

Page 11: Load Line Analysis of BJT

Load line analysis is a graphical method used to determine the operating point (Q-point) of a transistor amplifier circuit.

It shows the relationship between collector current (IC) and collector-emitter voltage (VCE) for a given load resistance.

                                              

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DC Load Line Equation

Applying Kirchhoff's Voltage Law (KVL) to the collector circuit:

VCC = IC RC + VCE

Rearranging the equation:

VCE = VCC − IC RC

This equation represents a straight line called the DC Load Line.

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Important Points on Load Line

1. Cutoff Point

At cutoff region:

IC = 0

Therefore:

VCE = VCC

This point lies on the voltage axis.

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2. Saturation Point

At saturation region:

VCE ≈ 0

Therefore:

IC = VCC / RC

This point lies on the current axis.

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Q-Point (Operating Point)

The intersection of the DC Load Line and the transistor characteristic curve is called the Q-point.

Q-point stands for:

Quiescent Point

It represents the steady-state operating condition of the transistor when no input signal is applied.

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Importance of Proper Q-Point

• Prevents signal distortion
• Ensures linear amplification
• Maintains stable operation

For proper amplifier operation, the Q-point is usually placed near the middle of the load line.

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Important GATE Points

• Load line is a straight line on IC–VCE graph.
• Intercepts represent cutoff and saturation points.
• Q-point determines amplifier performance.
• Proper biasing ensures stable Q-point.

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Next Page → BJT Biasing Techniques

 

GATE Electrical – Analog Electronics

Page 10: BJT Characteristics

The behavior of a Bipolar Junction Transistor (BJT) can be understood using its characteristic curves. These curves show the relationship between currents and voltages in the transistor.

For analysis, the Common Emitter (CE) configuration is most widely used.

                                               

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1. Input Characteristics

The input characteristic shows the relationship between:

Base Current (IB) vs Base–Emitter Voltage (VBE)

Important observations:

• Input characteristic resembles a PN junction diode.
• Base current increases rapidly after threshold voltage.
• Typical VBE value for silicon transistor ≈ 0.7 V.

Input resistance is defined as:

R_in = ΔVBE / ΔIB

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2. Output Characteristics

The output characteristic shows the relationship between:

Collector Current (IC) vs Collector–Emitter Voltage (VCE)

This curve is drawn for different values of base current (IB).

Three important regions appear in this graph:

• Cutoff Region → IB = 0
• Active Region → Amplifier operation
• Saturation Region → Transistor fully ON

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3. Transfer Characteristics

Transfer characteristics represent the relationship between:

Collector Current (IC) vs Base Current (IB)

This relationship gives the current gain of the transistor.

IC = β IB

Where:

• β = current gain
• Typical β value = 50 to 300

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Important BJT Parameters

• Input resistance (Rin)
• Output resistance (Rout)
• Current gain (β)
• Voltage gain

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Important GATE Points

• CE configuration is most commonly used.
• Input characteristic is similar to diode curve.
• Active region is used for amplification.
• Saturation region is used for switching.

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Next Page → Load Line Analysis of BJT

 

GATE Electrical – Analog Electronics

Page 9: BJT Configurations

A Bipolar Junction Transistor (BJT) can be connected in three different ways depending on which terminal is common between input and output circuits.

The three configurations are:

                                     

• Common Base (CB)
• Common Emitter (CE)
• Common Collector (CC)

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1. Common Base (CB) Configuration

In this configuration the base terminal is common between input and output.

• Input is applied between Emitter – Base
• Output is taken between Collector – Base

Important Characteristics

• Current Gain (α) is less than 1
• Very high voltage gain
• Low input impedance
• High output impedance

α = IC / IE

Typical value of α ≈ 0.95 – 0.99

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2. Common Emitter (CE) Configuration

In this configuration the emitter terminal is common between input and output.

• Input is applied between Base – Emitter
• Output is taken between Collector – Emitter

Important Characteristics

• High current gain
• High voltage gain
• Moderate input impedance
• Moderate output impedance

β = IC / IB

Typical value of β = 50 to 300

This is the most widely used configuration for amplifiers.

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3. Common Collector (CC) Configuration

In this configuration the collector terminal is common.

• Input between Base – Collector
• Output between Emitter – Collector

Important Characteristics

• Current gain is very high
• Voltage gain approximately 1
• Very high input impedance
• Very low output impedance

This configuration is also called:

Emitter Follower

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Comparison of BJT Configurations

Configuration	Current Gain	Voltage Gain	Input Impedance	Output Impedance
CB	Low	High	Low	High
CE	High	High	Medium	Medium
CC	Very High	≈ 1	Very High	Very Low

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Important GATE Points

• CE configuration is most widely used.
• CC configuration is used for impedance matching.
• CB configuration is used in high-frequency circuits.
• Relationship between gains:

β = α / (1 − α)

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Next Page → BJT Characteristics (Input & Output Curves)