Comprehensive Handbook for the Common Emitter Amplifier Circuit

The common emitter amplifier is one of the first circuits that any student of electronics learns. It is used extensively in myriad fields like audio amplification, signal processing, etc. This article covers every essential detail about the common emitter configuration — its working principles, circuit elements, as well as benefits — to provide you with a well-informed perspective. It doesn’t matter if you are a novice exploring amplifier circuits or an expert wanting to dive deeper; this piece will offer ample information regarding the operation and construction of this circuit. You will understand the operational logic of a common emitter amplifier, its important performance indicators, and ways to operationally incorporate it in various situations of practical life.

What is a Common Emitter Amplifier?

A common emitter amplifier is a type of transistor amplifier configuration where the emitter terminal is common or shared between the two signals: input and output. It is used widely in electronic circuits because of its significant voltage gain, medium current gain, and inverted output. This configuration works with a small signal at the base to control a bigger current between the collector and emitter, so it will amplify the input. Some of the important parameters are the input and output impedances, the gain bandwidth product, and the performance of distortions. It can be used in many applications in analog circuits.

Understanding the Common Emitter Configuration

The performance and application of the common emitter configuration are influenced by several critical parameters. Below is a detailed breakdown of these specifications:

Voltage Gain (Av):

Describes the ratio of the output voltage and the input voltage in terms of their change.

This configuration is characterized by a high voltage gain, thus making it suitable for voltage amplification.

Current Gain (β or hFE):

Represents the output current, which is the collector current, over the input current, which is the base current.

With this feature, larger currents can be controlled effectively with small input signals.

Input Impedance (Zin):

This is described as the resistance presented by the input signal.

Typically, set to average, enables good interaction with several circuit stages before it.

Output Impedance (Zout):

This can be defined as the resistance offered to the load at the output.

Most will consider this value to be relatively low, thus allows good loading into the next stages.

Gain Bandwidth Product (GBP):

This is the frequency range an amplifier is able to operate within and provide gain.

Of use where the frequency employed is high and minimum loss of signal is needed.

Distortion Performance:

A figure of merit that shows the approximate output signal from the defined input in terms of amplification.

The smaller the distortion, the better the signal is for reproduction.

Power Handling and Efficiency:

This is the limit of a certain system configuration to efficiently manage power while preventing overheating.

This parameter directly changes the amount of used energy and the reliability of the circuit.

These parameters can serve as reasons why the common emitter configuration forms the basic structure of many amplifiers in different electronic methods, thus providing adaptability with dependable performance in various settings.

The Role of the Transistor in a Common Emitter

Here is a list of prominent parameters with their description in a common emitter configuration:

• Definition: This refers to resistance that a source would have when it attempts to connect with the input terminal.
• Typical Values: Starts from a couple of hundred ohms and goes up to some kilo ohms.
• Importance: Influence the quality of transmission of the input signal to a transistor.
• Definition: The resistance seen from the output terminal.
• Typical values: Ranges from KΩ to MΩ which is usually greater than the input impedance.
• Importance: Limit the driving capability to further stages or loads.
• Definition: The ratio of the output voltage to input voltage.
• Formula: \( Av = -\frac{\beta R_C}{R_E + r_e} \) (where \( \beta \) is the current gain, \( R_C \) is the collector resistor, \( R_E \) is the emitter resistor, and \( r_e \) is the intrinsic emitter resistance).
• Typical values: Dependant on configuration but usually volume from 10 to 200.
• Importance: Illustrates the level of amplification in relation to the previous stage.
• Define: Ratio of output current at the collector to input current at the base.
• Definal: the ratio of output current at the collector to input current at the base.
• Current Gain (\( \beta \))
• Typical range: 20 to 400.
• Importance: Factors the overall efficiency of the signal being amplified.
• Definition: Voltage gain multiplied by current gain, illustrating the transistor’s amplification ability.
• Typical Values: High, but depends on the operating conditions.
• Importance: Measurement of the capability to amplify power.
• Definition: The effective operating range of frequencies the transistor can work with.
• Typical Range: 10 Hz up to few MHz.
• Importance: Guarantees the amplifier performs accurately for the entire frequency range without distortion.
• Definition: The deviation of the waveform of the output signal relative to the amplified input.
• Types: Harmonic, phase, and amplitude distortion.
• Importance: Enhancing fidelity and accuracy of signals relies on the reduction of distortion.
• Definition: The ability of the configuration to perform within specific limits irrespective of temperature variations.
• Importance: Dependability and avoidance of thermal runaway ensures reliability.

These parameters together apply in determining the performance of the common emitter configuration in different electronic systems and broaden its use as a critical design element.

Common Emitter Versus Other Configurations:

The common emitter configuration sets itself apart from other configurations, like the common base or common collector, with regard to gain properties and the input-output relationships of a system:

Common Emitter: demanding increased input, the output responds with increased current, therefore providing high current gain.

Common Base: Current gain is very low.

Common Collector: Current gain is nearly one.

Common Emitter: Yield significant voltage gain.

Common Base: Higher voltage gain than other configurations.

Common Collector: Voltage gain is about unity.

Common Emitter: Marks moderate values for input and output impedance.

Common Base: Input impedance is low and output impedance is high.

Common Collector: Low output impedance and high input impedance.

These differences amplify the common emitter configuration when compared to common base and collector configurations, which have specific functions in dealing with signals and impedance matching.

How Does a Common Emitter Amplifier Work?

Operation of the Emitter Resistor in the Circuit

In a typical common emitter amplifier circuit, the emitter resistor performs two critical tasks. Primarily, it offers bias stability by controlling the emitter current which, in return, helps in avoiding thermal runaway situations. Additionally, it also helps to regulate the overall voltage gain of the amplifier by controlling the gain factor of the amplifier. Its role ensures performance consistency and enhances the amplifier’s dependability amid diverse dynamic conditions.

Analyzing the Voltage Gain of a Common Emitter Configuration

In a common emitter amplifier configuration, voltage gain is a function of the load resistor, internal characteristics of the transistor, and the emitter resistor. The voltage gain (Av) can be expressed mathematically as follows:

Av = -Rc / Re’

where Rc is the collector resistance, and Re’ is assumed to be the emitter resistance external to the emitter and the emitter intrinsic resistance which is part of the transistor. The negative alternate sign indicates the phase shift polarity meaning the output signal is inverse to the input signal by 180 degrees.

In order to maximize gain, the emitter resistance is usually bypassed with a capacitor to allow the AC signal to ignore the resistor’s effect while preserving the stability of the DC bias. However, increasing Rc or decreasing Re’ must be done carefully so that the amplifier does not lose stability, level of efficiency, or the amount of distortion. These design factors are to be integrated with the thermal stability and the impedance for optimal performance.

What are the Key Components of a Common Emitter Amplifier Circuit?

The Value of Biasing in the Design of a Circuit

In a common emitter amplifier circuit, biasing is crucial in aiding predictability and stable operation. The following factors are very important:

Quiescent Point (Q-Point): This is the operating stage of a transistor when it is in the active region. The voltage for the Q-point is set so that the transistor is not pushed into saturation or cutoff, which would loss linearity in amplification.

Voltage Divider Bias: One of the most simple methods that uses two resistors (R1 and R2) to create a base voltage divider network is voltage divider biasing. For instance:

R1= 10 kΩ

R2= 2.2 kΩ

These values were selected due to the base current (Ib) that was intended and the type of transistor to be used.

Emitter Resistor (Re): Negative feedback is used by the emitter resistor to stabilize thermal changes. A common value could be:

Re= 470 Ω

The higher Re is set to, the better temperature stability will be achieved at the expense of overall gain.

Collector Resistor (Rc): This resistor defines the output voltage swing and contributes to the overall voltage gain. For example:

Rc= 2.2 kΩ

Greater values of Rc, the better the voltage gain at the cost of output dynamic range.

Coupling Capacitors (C1 and C2): These Components enable alternating current signals to go through while blocking the direct current bias. For these capacitors, a good estimation is:

C1, C2 = 10 µF to 100 µF

The effectiveness of the amplifier depends on its frequency response.

Power Supply (Vcc): The supply voltage provides energy to the circuit. Common values might include:

Vcc = 12V, 18V, or 24V

Increased Vcc allows larger voltage swing outputs.

These components allow the circuit design goals to be achieved ie. proper stabilisation, gain, and optimized performance. From a theoretical perspective one must respect Kirchhoff’s laws, node voltage’s and current flow to optimize circuit performance.

Significance of Capacitors in Common Emitter Circuits

By contributing to its functioning and stability, capacitors in a common emitter circuit help in achieving multiple goals. Their roles can be presented in the following categorized way:

Coupling Capacitors (Cc):

Function: Allow AC signals, while blocking DC components.

Typical Value Range: 0.1 µF – 10 µF.

Importance: Allows multi-stage amplifiers to prevent DC biasing in subsequent stages.

Bypass Capacitor (Ce):

Function: Connected in parallel to the emitter resistor to stabilize gain by AC shunting bypassing to ground.

Typical Value Range: 10 µF – 100 µF.

Importance: Reducing AC impedance in the circuit enhances the voltage gain.

Decoupling Capacitor:

Function: Stabilizes and reduces the supply voltage noise.

Typical Value Range: 0.1 µF – 1 µF.

Importance: Suppresses variation of power supplied to the circuit which could induce undesired behaviour.

Filter Capacitor:

Function: Reduces supply ripples by smoothing rectified outputs.

Typical Value Range: 47 µF – 470 µF.

Importance: Provides the circuit a consistent and precise DC supply.

With optimized performance, a common emitter circuit can be made ensuring reliable operation by careful selection of specific capacitor types with regard to valued performance parameters.

Understanding Input and Output Impedance

In the context of deep signal flow diagrams, optimizing these impedance values is very important for consistent signal level transfer and enhanced circuit performance. Excessively low input impedance may excessively load the source, whereas high output impedance can result in poor power transfer to the load. Ensuring correct impedance level adjustment with maximum connected devices configurable results in minimum efficiency loss, signal flow attenuation, and a high dynamic range, which is fundamental when designing reliable electronic circuits.

How to Design a Common Emitter Amplifier?

Guidelines to Determine Bias Voltage and Values of Resistors

Choosing a Transistor:

An NPN transistor with the appropriate gain, frequency, and power capabilities would be ideal. For example, 2N3904 or BC547 can be used in case of low power applications.

Operating Point (Q-Point):

Determine a reasonable Q-point so that the transistor remains in the active region.

The parameters of the Q-point are:

Collector Current (I_C): For many designs, this is usually between 1 mA to 10 mA.

Collector-Emitter Voltage (\(V_{CE}\)): Most commonly set to half the supply voltage to maximize signal swing.

Voltage Divider Biasing:

Determine the base voltage \(V_B\) that will set the correct bias for the transistor.

Two resistors \(R_1\) and \(R_2\) can be used to form a voltage divider network:

V_B = V_{CC}\cdot\frac{R_2}{R_1+R_2}

Emitter Resistor (\(R_E\)):

Operating point of the transistor can be adjusted using \(R_E\).

Propose a value of \(R_E\) using:

\[R_E = \frac{V_E}{I_E}\]

Where \(V_E = V_B – 0.7\) (this is because of a base-emitter voltage drop) and \(I_E \approx I_C\).

Collector Resistor (\(R_C\)):

The value of \(R_C\) should give the desired value of voltage drop while providing a satisfactory value for load impedance.

Calculate \(R_C\) using the formula:

\[R_C = \frac{V_{CC} – V_{CE} – V_E}{I_C}\]

Coupling Capacitors (\(C_{in}\) and \(C_{out}\)):

A capacitor can now be used at the input and output to block DC but allow AC signal to flow.

Choosing parameters under the lowest signal frequency conditions \((f)\):

C = \frac{1}{2\pi f R}

where \(R\) is the related resistance.

Bypass Capacitor \(C_E\):

C_E = \frac{1}{2\pi f R_E}

Provide adequate voltage and current for the amplifier circuit.

Commonly used: 9V, 12V, or by the design.

Adjusting these accurately will enable the common emitter amplifier to perform the desired amplification with reliable performance and low distortion.

Design Engineering For The Highest Voltage Gain Possible

For optimal performance in the design of a common emitter amplifier, the following additional parameters are taken into consideration.

Input Parameters:

Expected range: \(1k\Omega – 10k\Omega\)

Depends on \(R_1, R_2\) and base resistance of the transistor.

Input Signal Frequency \((f)\):

Should be appropriate to the application (for example, audio within 20 Hz – 20 kHz).

Collector Current \((I_C)\):

Example: between \(1mA -10mA\)

Depends on \(V_{CC}, R_C\) and expected gain.

Collector-Emitter Voltage \((V_{CE})\):

Aim for the center for max signal swing – typically \(\frac{1}{2} V_{CC}\).

Base Resistors \((R_1,R_2)\):

Sets the bias; keeps the transistor stable.

Controls voltage gain \((A_v)\).

Usual values: \(1k\Omega – 10k\Omega\).

Emitter Resistor (\(R_E\)):

Enhances stability and linearity on the range.

Typical value range: \(100\Omega – 1k\Omega\).

Capacitor Values:

Coupling capacitors (\(C_{in}\), \(C_{out}\}):

Will impede DC while permitting an AC signal.

Calculate appropriate capacitor value using \(C = \frac{1}{2\pi f R}\) for the desired frequency \(f\).

Bypass Capacitor (\(C_E\)):

Gain enhancement by short-circuiting AC signals across \(R_E\).

Depend on \(f\) and \(R_E\) most typical values.

\[A_v \approx \frac{R_C}{R_E}\] (Without bypass capacitor \(C_E\)).

With \(C_E\), then \(A_v\) will rise greatly as emitter degenerative effects are bypassed.

Power Supply \((V_{CC})\):

Common levels include 9V, 12V, or 15V, depending on application.

Must deliver sufficient current along with voltage stability.

By carefully examining and defining these parameters, the common emitter amplifier can be tailored to optimize its performance for specific applications.

Considerations for Power Supply Selection

In regard to the common emitter amplifier, the following parameters are important when choosing the power supply:

Voltage Level \((V_{CC})\):

Should not exceed the maximum ratings of the transistor and swing that will be produced at the output. For instance,

Low power pd devices will be powered by a 9V supply.

12V or 15V will enable a significantly higher output, providing extra headroom.

Existing Capability:

Make sure that the current supply can provide enough current to energize the amplifier and any connected load, for example:

Example 1: An amplifier driving a high-impedance load (example: headphones) uses up to 10mA.

Example 2: Speakers tend to be a low-impedance load. They can draw current Greater than 50mA.

Ripple and Noise Specifications:

A lower ripple of voltage (\<10 \, \text{mV}_{\text{rms}} \) noise is recommend to avoid distortion on the output signal.

Linear regulated supplies or well filtered Switch mode supplies will commonly achieve the standard.

Load Regulation Example:

Changes in output voltage with constant power supply and varying load conditions (range is about 0-1%-2%).

Requires protection of the circuit ensuring constant supply output during continuous operation or temperature changes.

Considering these aspects with accurate data for design calculations enhances the performance and reliability of the common emitter amplifier circuit.

What are the Limitations of a Common Emitter Amplifier?

Knowing Current Gain and Its Limiting Factors

There is a certain amount of current gain a transistor receives, but the amount may change based on the temperature and manufacturing differences.

Typical range for β: 20-200 (Depending on which model of the transistor it is).

The voltage gain is dependent on the load resistance and the internal components of the transistor.

Due to parasitic capacitance, the high frequency performance capabilities may reduce.

The voltage gain is relatively low causing some loading effects in certain circuitry.

Does not sharply distort signal but causes some loss of integrity which may require buffering stages to fix.

When lowered to a moderate level, the signal distortion will become more prominent thus enhancing certain signals over others and causing an imbalance between various signals.

Equalization at low impedance levels causes restricted output when connected in a parallel configuration.

Irreversible requires slew rate reduction devices in order to prevent a reaction.

The feedback capacitance \(C_{bc}\) caused additional capacitance which shifts the bandwidth towards the higher end of the lower frequencies.

More OFB capacitance the lower the bandwidth limit

Temperature changes may hinder the operating point or gain, skipping certain steps in the process.

Net result causes reactiveness range of frequencies not greatly limited.

Needs bias stabilization methods in order to help recover.

Excess nonlinear traits of a transistor’s active region can distort harmonically thus severely distorting the signal.

Internal capacitance impose restrictions thus limiting bandwidth cap greatly depended on the application, shifting the ranges to tens of kHz to below a few megahertz.

Often critical limit is the roll off at high frequencies.

The maximum power handling capability of a system is limited by the thermal limits of the transistors, as well as the efficiency of the heat dissipation mechanisms employed.

Requires proper heatsinking for applications with high power levels.

These issues need to be resolved clearly during the design phase if the common emitter amplifier is to be optimized for the intended use.

Difficulties With Output Impedance

Output impedance of common emitter amplifiers is dependent on numerous factors including the configuration of the circuit and the relationship between the load and the amplifier’s internal resistance. It primarily results from the characteristics of the active transistor and the resistive or reactive components external to the amplifier. To minimize unneeded signal attenuation and allow for maximum power transfer, engineers strive to match the amplifier’s output impedance to the load’s impedance. To ensure precise performance without distortion, and to take full advantage of the amplifier’s capability, careful analysis and effective impedance matching is essential during the design stage.

Resolving Issues with Signal Distortion

There are various causes of signal distortion which can include nonlinearities in the active device, improper biasing, or an impedance mismatch. For example, let’s consider the harmonic distortion caused by an amplifier circuit. The total harmonic distortion (THD) metric is widely accepted as a measure of distortion quantification. In a basic class A amplifier, for instance, and THD would likely approximate 1% under ideal constraints. It is essential to note, however, that component tolerances and biasing deceptions can drastically affect THD in real-life conditions.

IMD is another critical aspect which intermodulation distortion(IntMD) which applies most intensely to an intermultiplier system with multiple-frequency signals entering the device of each unit. For instance, let’s say an amplifier is provided with 1 MHz and 1.2MHz frequencies. IMD may add additional frequencies of 0.8MHz and 2.2MHz which may ruin the pure signal by its distortion. This is why engineers try to solve these difficulties by designing feedback-dominant linearizing circuits which perform better under feedback.

Frequently Asked Questions (FAQs)

Q: What is a Common Emitter Amplifier?

A: A common emitter amplifier is a variant of a bipolar junction transistor (BJT) circuit with a single emitter common to both input and output sides. This configuration provides considerable voltage amplification. It gets preference in many transistor amplifier circuits owing to its considerable gain.

Q: How does a BJT function in an amplifier circuit?

A: A bipolar junction transistor (BJT) is an amplifier whose action is based on the control of current flow from the collector to the emitter by the base terminal. With the right current to the base of a transistor, it can turn on a much larger current from collector to emitter effectively, which means enhancing the input signal.

Q: What role does the emitter voltage play in a Common Emitter Amplifier?

A: The emitter voltage in a common emitter amplifier determines the operating point of the transistor which is quite important. It determines the biasing of the transistor which has a direct effect on the amplification and linearity of the circuit. The performance of the amplifier is dependent on the proper emitter voltage.

Q: What factors affect the input impedance of a Common-Emitter Amplifier?

A: In a common-emitter amplifier, the input impedance is determined by the influence of the base-emitter junction, the required base currents flow, and the connected resistors at the base terminal. Higher input impedance is always preferred to avoid loading the previous stage of the circuit.

Q: How does emitter degeneration improve amplifier performance?

A: Emitter degeneration is accomplished adding a resistor to the emitter terminal. This helps in maintaining the gain and linearity of the amplifier circuit. There is improved performance due to lower distortion, increased input impedance, and higher reliability of the amplifier circuit.

Q: What is the difference between a Common-Eemitter Amplifier and a Common-Collector Amplifier?

A: The emitter configuration is used in a common-emitter amplifier because it provides voltage amplification, while a common-collector amplifier or an emitter follower will provide current amplification but no voltage gain, therefore, acts as a current source with a load impedance. The common-emitter configuration is used to perform voltage amplification, while the common-collector is primarily to match impedance and buffer the signal.

Q: How do you design a basic common emitter amplifier circuit?

A: Selecting the components which include the bipolar junction transistor, biasing resistors, coupling and bypass capacitors, and designing the circuit includes the calculation of the biasing network and the value of the emitter degeneration resistor in a manner that allows the transistor to work in the active region for maximum amplification.

Q: What are the common applications of a Transistor Used as a Common-Emitter Amplifier?

A: With its capability to amplify signals, a transistor that functions as a common-emitter amplifier is widely used in audio signal amplifiers, RF signal amplifiers, and various other circuits that require signal processing.

Q: Why is the base current required in a BJT Common Emitter Amplifier?

A: Each transistor has specific operating conditions that must be satisfied in order for the transistor to function, base current being one of them. In a BJT common emitter amplifier, base current is necessary to turn on conduction in the transistor. A small input current (in this case base current) enables to control larger current flow between collector and emitter which in turn allows the transistor to work as an effective voltage amplifier. The base current also allows the amplifier’s operating point to be set, ensuring the amplifier works well.

Reference Sources

1. A SiGe BiCMOS D-Band LNA with Gain Boosted by Local Feedback in Common-Emitter Transistors

• Authors: Guglielmo De Filippi, Lorenzo Piotto, Andrea Bilato, A. Mazzanti
• Published in: 2023 IEEE Radio Frequency Integrated Circuits Symposium (RFIC)
• Publication Date: June 11, 2023
• Summary: This paper discusses a D-band low noise amplifier (LNA) designed using common-emitter transistors with local reactive feedback to enhance gain while maintaining stability. The design achieves a gain of 22.8 dB, operates between 130–165 GHz, and has a noise figure (NF) as low as 5 dB with a power consumption of 40 mW.
• Methodology: The authors implemented a common-emitter configuration with local feedback to shift the transistor into a conditionally stable operating region, allowing for a gain beyond its maximum available gain (MAG). The design was fabricated in a 55 nm BiCMOS technology, and performance was validated through measurements(Filippi et al., 2023, pp. 133–136).

2. Stacked Common-Base vs Common-Emitter mmWave PA Cells and 68–105 GHz Broadband Asymmetrical PA in 250nm InP HBT

• Authors: Zheng Liu, T. Sharma, K. Sengupta
• Published in: IEEE Access
• Publication Date: 2023
• Summary: This study compares stacked common-emitter (SCE) and stacked common-base (SCB) topologies for millimeter-wave power amplifiers (PAs). The SCB configuration demonstrated higher gain and better linearity compared to SCE at higher frequencies, achieving a measured gain of 11.8 dB and a peak power-added efficiency (PAE) of 34%.
• Methodology: The authors designed and tested both PA configurations in 250 nm InP HBT technology, analyzing their performance under various load conditions and measuring key parameters such as gain, PAE, and linearity(Liu et al., 2023, pp. 14487–14499).

3. Pengukuran Daya Listrik Menggunakan Penguat Common Emitter dan Komparator

• Authors: Arif Wahyu Wibowo, Ibrahim Nawawi, Dwi Novianto
• Published in: AVITEC
• Publication Date: January 24, 2023
• Summary: This research presents a design for a single-phase electric power measurement tool using a common emitter amplifier and comparator. The system measures voltage, current, frequency, and power factors, achieving an average error percentage of less than 3% across various parameters.
• Methodology: The study utilized ZMPT101B and SCT013-005 as voltage and current transducers, respectively. The output from these transducers was amplified using a common emitter amplifier, and phase differences were measured using a comparator circuit(Wibowo et al., 2023).

Electrical network

Resistor