What is Common-Emitter Amplifier?

A common-emitter amplifier is a type of transistor amplifier circuit that is widely used in electronic circuits. Its name comes from the way the signal is input and output: the signal is input from the base and emitter of the transistor, and output from the collector and emitter. Because the emitter acts as a common ground in the circuit, this circuit is called a common emitter amplifier circuit. This circuit is known for its high voltage gain, large current amplification, and moderate input-output impedance, and although its operating point may not be stable enough, it still performs well in many practical applications.

Basic Composition of Common-Emitter Amplifier

A common-emitter amplifier circuit consists of three main components: an input circuit, an amplifier circuit, and an output circuit. The input circuit is responsible for introducing external signals into the amplifier circuit and usually includes a signal source, input resistor and bias circuit. The amplifier circuit is the core part, consisting of a transistor (e.g., triode) and its peripheral circuits, and is responsible for amplifying the input signal. The peripheral circuits include the power supply, base bias resistor, collector load resistor, etc., which provide the transistor with suitable operating conditions and amplification. The output circuit is responsible for outputting the amplified signal to an external load, which usually includes an output resistor and load.

Key Features of Common-Emitter Amplifier

Common emitter amplifier circuits have the following features:

Low input resistance: Since the load resistor is connected between the base and the emitter, the input resistance is very low and can accommodate signals from different sources.

High output resistance: The collector is directly connected to the power supply and the output resistance is high enough to provide a large load voltage without affecting the preamplifier circuit.

Large voltage gain: With a large base current compared to the output current, the voltage gain is large, up to tens of times or even higher values.

Unidirectional amplification: the common emitter amplifier circuit can only amplify the signal in one direction, can not enhance the signal in both directions equally.

How does a Common-Emitter Amplifier Work?

The operating principle of a common-emitter amplifier circuit is based on several key properties of a PNP or NPN transistor, including current control, amplification, and inversion. When an input signal is coupled into the base, it causes a change in the base current. In the case of a PNP type transistor, an increase in base current leads to an increase in emitter current, which produces a large voltage output across the load resistor.The opposite is true for an NPN type transistor, where a decrease in base current leads to a reduction in current and voltage, and an inverting signal at the output. This amplification of current and voltage allows the common-emitter amplifier circuit to effectively amplify the input signal.

Signal Input and Output Relationship of Common-Emitter Amplifier

In a common-emitter amplifier circuit, the input signal is injected into the transistor in the form of a base current, which is amplified and then presented in the form of an output voltage from the load resistor. Due to the control relationship between current and voltage inside the transistor, a change in the input signal causes a greater change in the collector current, which results in the amplification of the voltage. The fact that the output signal is opposite in phase to the input signal is an important characteristic of the common-emitter amplifier circuit. In addition, the higher output resistance of the common-emitter amplifier circuit provides a larger load voltage without affecting the preamplifier circuit.

Performance Specifications of Common-Emitter Amplifier

The performance indexes of a common-emitter amplifier circuit include magnification, input resistance, output resistance, dynamic range, and bandwidth. Amplification is an important indicator of the amplification ability of the amplifier circuit, which is equal to the ratio of the output to the input of the amplifier circuit. The input resistance characterizes the loading effect of the input circuit of the amplifier circuit on the signal source. The output resistance characterizes the internal resistance of the output loop of the amplifier circuit. Dynamic range is the peak-to-peak value of the output voltage when the nonlinear distortion of the output voltage reaches a specified value as the input signal voltage increases. The bandwidth, on the other hand, determines the frequency range of the signal that the amplifier circuit can amplify.

Application Scenarios for Common-Emitter Amplifier

Common emitter amplifier circuits are widely used in many fields because of their excellent performance. In audio equipment, such as radios, sound systems, and headphone amplifiers, common-emitter amplifier circuits amplify audio signals to achieve sufficient power to drive output devices such as speakers. In communication systems, common-emitter amplifier circuits are used to amplify RF signals to ensure the stability and effectiveness of the signal during transmission. In addition, common-emitter amplifier circuits are also widely used in measuring instruments, automatic control circuits, as well as high-frequency resonant amplifiers, wide-band amplifier circuits and other occasions.

Common-Emitter amplifier VS other amplifier

Compared with other amplifier circuits, common-emitter amplifier circuit has its unique advantages and disadvantages. Compared with common-base amplifier circuits, common-emitter amplifier circuits have higher voltage gain and moderate input-output impedance, but the frequency response may not be as good as that of common-base amplifier circuits. Compared to common-collector amplifier circuits, common-emitter amplifier circuits have a higher output resistance and are capable of delivering larger load voltages, but have a relatively low input impedance. Therefore, when selecting an amplifier circuit, tradeoffs and choices need to be made based on specific application requirements and performance specifications.

A common-emitter amplifier circuit refers to a transistor amplifier circuit with a load resistor connected between the base and emitter, and the collector connected to the power supply. This circuit is characterized by low input resistance, high output resistance and large voltage gain, and is commonly used for signal amplification.

Historical background and technological development

Common emitter amplifier circuit has a wide range of applications in the field of electronic technology, is one of the foundations of electronic technology. With the development of electronic technology, common-emitter amplifier circuits are constantly optimized and innovated to adapt to more complex application scenarios and needs.

The common-emitter amplifier circuit is an amplifier circuit in which the signal is input from the base and emitter of the transistor and output from the collector and emitter.

Common emitter amplifier circuit has which several characteristics, because the emitter is named for the common ground terminal, has a large voltage and current amplification, the input and output resistance is moderate, the operating point is not stable.