Understanding Transistor Amplifier Configurations and Voltage Gain

Transistors can be configured in different ways to build amplifier circuits. The most common configurations for bipolar junction transistors (BJTs) are Common Emitter (CE), Common Base (CB), and Common Collector (CC), also known as the emitter follower.

Each configuration has distinct characteristics in terms of voltage gain, current gain, input impedance, and output impedance. The voltage gain (\(\text{A}_\text{v}\)) of an amplifier is defined as the ratio of the output voltage to the input voltage. A higher voltage gain means the amplifier significantly increases the voltage signal.

Voltage Gain Characteristics of Common Transistor Configurations

Let's look at the typical voltage gain for the standard configurations:

• Common Emitter (CE): This configuration is widely used for voltage amplification. It typically provides a high voltage gain and a moderate current gain. There is a 180-degree phase shift between the input and output signals.
• Common Base (CB): The Common Base configuration offers a high voltage gain similar to the Common Emitter, but its current gain is close to unity (around 1). It's often used for impedance matching or high-frequency amplification. There is no phase shift between input and output.
• Common Collector (CC): Also known as the emitter follower, this configuration is primarily used for current amplification and impedance matching. Its most notable characteristic regarding voltage is that the output voltage is almost equal to the input voltage, meaning its voltage gain is approximately 1 (\(\text{A}_\text{v} \approx 1\)). More accurately, the voltage gain is slightly less than 1. This configuration provides a high current gain and a high input impedance, along with a low output impedance. There is no phase shift.

The "Emitter Coupled" configuration usually refers to a differential amplifier (like a long-tailed pair) which can provide voltage gain, but comparing the standard single-stage configurations, the Common Collector stands out for its characteristically low voltage gain.

Why Common Collector Has the Lowest Voltage Gain

In the Common Collector configuration, the input signal is applied to the base, and the output is taken from the emitter, with the collector common to both input and output (effectively connected to the power supply which is AC ground). The emitter voltage follows the base voltage very closely due to the feedback action of the emitter resistance.

The voltage gain (\(\text{A}_\text{v}\)) for an ideal Common Collector configuration is given by:

\(\text{A}_\text{v} = \frac{\text{R}_\text{E}}{\text{r}_\text{e}' + \text{R}_\text{E}}\)

Where \(\text{R}_\text{E}\) is the emitter resistance and \(\text{r}_\text{e}'\) is the dynamic resistance of the emitter diode. Since \(\text{r}_\text{e}'\) is usually small compared to \(\text{R}_\text{E}\) in a practical circuit, \(\text{A}_\text{v}\) is close to 1 but always slightly less than 1.

Compared to the high voltage gains typically achieved by Common Emitter and Common Base configurations (which can be much greater than 1), the Common Collector configuration has the lowest voltage gain.

Based on the characteristics, the Common Collector configuration exhibits the lowest voltage gain among the standard transistor amplifier configurations.

Revision Table: Transistor Configurations Summary

Additional Information: Applications and Uses

Knowing the characteristics helps in choosing the right configuration for specific applications:

• Common Emitter: Suitable for general-purpose voltage amplification stages in multi-stage amplifiers, audio amplifiers.
• Common Base: Used in applications requiring good high-frequency response, such as RF amplifiers, or as a current buffer.
• Common Collector (Emitter Follower): Excellent as a buffer stage due to its high input impedance and low output impedance, used for driving low-impedance loads without significant voltage loss, voltage regulation circuits.

The Common Collector configuration's low voltage gain is not a drawback in its intended applications; rather, it's a feature that makes it ideal for current amplification and impedance buffering.