Common-Emitter Transistor Amplifier Design

Key Takeaways

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

• When a common-emitter 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 common-emitter 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:

1. Common-base amplifiers

2. Common-emitter amplifiers

3. Common-collector amplifiers

If the aim is to increase the amplitude of an AC signal, a common-emitter transistor circuit is designed. Common-emitter configurations are the most widely used type of transistor amplifier, due to their high-efficiency and positive gain greater than unity.

Let’s take a closer look at common-emitter transistor amplifiers and discuss some things designers should consider during the common-emitter transistor amplifier design process.

Common-Emitter Transistor Amplifier Design Criteria

Before discussing how to design a common-emitter transistor amplifier, it is important to understand the types of common-emitter 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 common-emitter amplifiers. The emitter terminal remains common to base and collector.

In a common-emitter 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 common-emitter 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 common-emitter 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 high-frequency 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 common-emitter 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 high-frequency 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 Common-Emitter Transistor Amplifier Design 

Let’s examine the steps involved in designing a common-emitter 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 Q-point 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 common-emitter transistor amplifiers with or without degeneration can be employed to satisfy amplifier requirements in electronic applications. Basic common-emitter 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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