common emmiter amplifier

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Common-Emitter Configuration Overview

The common-emitter (CE) amplifier is one of the most popular transistor amplifier configurations due to its good balance of voltage gain, input impedance, and output impedance. It provides inversion (i.e., a negative voltage gain), meaning that an increase in the input signal results in a decrease in the output signal (and vice versa).

Basic Circuit Diagram

Here’s a simplified diagram of a common-emitter amplifier using a Bipolar Junction Transistor (BJT):

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RCR_CRC​: Collector resistor

RER_ERE​: Emitter resistor

R1R_1R1​ and R2R_2R2​: Biasing resistors for setting the Q-point

VCCV_{CC}VCC​: Supply voltage

CCC: BJT (NPN transistor)

Key Components and Their Roles

Transistor (BJT): The active component that amplifies the input signal.

Biasing Resistors (R1R_1R1​ and R2R_2R2​): They form a voltage divider that establishes the appropriate biasing conditions for the transistor.

Collector Resistor (RCR_CRC​): Converts changes in collector current (ICI_CIC​) to changes in output voltage (VoutV_{out}Vout​).

Emitter Resistor (RER_ERE​): Provides stability by offering negative feedback. This helps maintain a steady Q-point against variations in transistor parameters.

Coupling Capacitors: (Not shown in the basic diagram but often included) allow AC signals to pass while blocking DC voltages.

Operating Principle

The common-emitter amplifier works by using the transistor to amplify a small AC input signal superimposed on a DC biasing voltage.

1. DC Biasing:

The purpose of DC biasing is to set the Q-point (operating point) of the transistor in the active region, where it can amplify signals effectively without distortion.

Q-point: The point on the transistor's output characteristics where it operates when no input signal is applied.

The DC biasing circuit ensures that the transistor remains in the active region for the entire range of the input signal.

2. AC Signal Amplification:

Once the transistor is properly biased, an AC signal can be applied to the base terminal:

Input Signal: A small AC voltage signal is applied between the base and emitter of the transistor.

Current Changes: As the input signal varies, it causes changes in the base current (IBI_BIB​), which in turn leads to larger changes in the collector current (ICI_CIC​) due to the transistor's current gain (β\betaβ).

This amplification effect is expressed as:

IC=βIBI_C = \beta I_BIC​=βIB​

3. Output Signal:

The output is taken from the collector terminal. Changes in ICI_CIC​ cause corresponding changes in the voltage across RCR_CRC​:

Vout​=VCC​−IC​RC​

Characteristics of the Common-Emitter Amplifier

Voltage Gain (Av​): The voltage gain is a key feature and can be approximated as:

Av=−RCreA_v = -\frac{R_C}{r_e}Av​=−re​RC​​

where rer_ere​ is the small-signal emitter resistance given by:

re≈26mVIEr_e \approx \frac{26mV}{I_E}re​≈IE​26mV​

IEI_EIE​ is the emitter current.

Phase Inversion: The output signal is inverted compared to the input signal. This means that a positive change in the input results in a negative change in the output.

Input and Output Impedance:

Input Impedance (ZinZ_{in}Zin​): Generally moderate, depending on the base biasing network.

Output Impedance (ZoutZ_{out}Zout​): Primarily determined by RCR_CRC​.