CommonEmitter Transistor Amplifier Design
Key Takeaways

Due to its high efficiency and positive gain greater than unity, the most commonly used transistor amplifier is the commonemitter transistor amplifier.

When a commonemitter transistor amplifier without emitter degeneration is designed, the value of resistor RC is chosen to match the amplifier gain requirements. The gain of this amplifier is directly proportional to the resistor RC value.

The merit of a commonemitter degeneration amplifier with a bypassed emitter resistor with a parallel resistor design is that the DC biasing of the amplifier is not dependent on the RE1 value, so the designer can set the RE1 value once the DC bias is fixed.
Amplifiers are critical to electronic circuits
Transistor amplifiers are circuits that are used to amplify weak audio, DC, or AC signals, and have a wide range of applications. When amplifying AC signals using a transistor amplifier, both voltage and current can be amplified simultaneously.
There are three configurations of transistor amplifiers:

Commonemitter amplifiers

Commoncollector amplifiers
If the aim is to increase the amplitude of an AC signal, a commonemitter transistor circuit is designed. Commonemitter configurations are the most widely used type of transistor amplifier, due to their highefficiency and positive gain greater than unity.
Let’s take a closer look at commonemitter transistor amplifiers and discuss some things designers should consider during the commonemitter transistor amplifier design process.
CommonEmitter Transistor Amplifier Design Criteria
Before discussing how to design a commonemitter transistor amplifier, it is important to understand the types of commonemitter amplifiers available. Irrespective of the configuration, an input signal is given to the base and output is collected from the collector terminal in all types of commonemitter amplifiers. The emitter terminal remains common to base and collector.
In a commonemitter without emitter degeneration, the bypass capacitor C_{B1} makes the ground connection of the emitter, so this configuration can also be called a grounded emitter. When this transistor amplifier is designed, the value of resistor R_{C} is chosen to match the amplifier gain requirements. The gain of this amplifier is directly proportional to the resistor R_{C} value.
In commonemitter configurations without a bypass capacitor, the bias stability and gain of the amplifier depend on resistor R_{E}. This transistor amplifier design gives more importance to the R_{C} and R_{E} values, as the gain can be controlled using them.
A commonemitter degeneration amplifier with a bypassed emitter resistor with series emitter resistor has a bypass capacitor that connects the resistor R_{E1} to the ground for highfrequency signals and bias stability. Even though the gain of the amplifier is dependent on R_{C} and R_{E1}, designers usually keep R_{C} constant and R_{E1} as a variable for gain control.
In a commonemitter degeneration amplifier with a bypassed emitter resistor with a parallel resistor, the R_{E1} value is considerably smaller than R_{E}, making the low impedance path for highfrequency signals through the bypass capacitor. In this configuration, the gain is controlled by keeping R_{C} constant and varying R_{E1}. The merit of this design is that the DC biasing of the amplifier is not dependent on the R_{E1} value, so the designer can set the R_{E1} value once the DC bias is fixed.
The Steps Required for CommonEmitter Transistor Amplifier Design
Let’s examine the steps involved in designing a commonemitter transistor amplifier without emitter degeneration. In this transistor amplifier specification, some parameters such as bias voltage, collector current, input resistance, the input AC signal, load resistance, gain, and output voltage can be given according to how the amplifier is designed. Next, let’s consider the given values: bias voltage V_{CC}, collector current I_{C}, input resistance R_{in}, and load resistance R_{L}.
Step 1: Determine R_{C}
To calculate the value of RC, we can use equation (1), below. The values of V_{CC }and I_{C} are known. For symmetrical output, the maximum possible value of voltage V_{CE} is 0.5V_{CC}. So, by substituting these known values and rearranging the equation, we can obtain equation (2), allowing us to calculate R_{C}.
Step 2: Determine the ‘Q’ Point
Once the values of V_{CE} and I_{C} are obtained, the Q point can be found from the output characteristics of the transistor. From the output characteristics of the transistor, find the base current curve on which the coordinate (0.5V_{CC}, I_{C}) lies. The base current required for this bias point is obtained.
Step 3: Determine R_{E}
The emitter resistor R_{E} is usually set as 10% of the resistor R_{C}:
Step 4: Determine Emitter Voltage V_{E}
Using the I_{B} and I_{C} values, the emitter current I_{E} can be calculated with the following equation:
Step 5: Determine Base Voltage V_{B}
Step 6: Determine R_{B1} and R_{B2}
The resistors RB1 and RB2 should be designed so that the base current IB flowing in the circuit corresponds to that of the Qpoint base current. The Thevenin equivalent circuit of the voltage divider is formed by RB1 and RB2. VBB is the Thevenin equivalent voltage, RB is the Thevenin equivalent resistance, and Rib is the input resistance looking into the base of the transistor.
Step 7: Calculate Thevenin Resistance R_{B}
The input resistance R_{in} can be written as equation (9). The resistance R_{ib} can be calculated using equation (10). From the known values of R_{in} and R_{ib}, the resistance R_{B} can be derived from equation (9).
The base voltage can be given as:
Step 8: Calculate R_{B1} and R_{B2}
From equations (7), (8), and (11), the resistors R_{B1} and R_{B2} can be calculated.
The bypass capacitor C_{E1} is selected so that it obeys equation (12), where X_{CE} is the reactance of the bypass capacitor C_{E1}:
Step 9: Determine CC1 and CC2
The coupling capacitors can be calculated using the following equations:
The variants of commonemitter transistor amplifiers with or without degeneration can be employed to satisfy amplifier requirements in electronic applications. Basic commonemitter transistor amplifier design can be carried out by following steps 1 through 9, provided the values of V_{CC}, I_{C}, R_{in}, and R_{L} are known. Depending on the parameters given in amplifier specifications, various equations are derived from the amplifier circuit diagram, which supports the design of the amplifier components.
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