Amplifier Configuration Basics

An amplifier configuration refers to how a transistor (like a BJT or FET) is connected within a circuit to amplify signals. The input signal, output signal, and power supply are connected to different transistor terminals (base, emitter, collector for BJT; gate, source, drain for FET). Each configuration has unique characteristics regarding voltage gain, current gain, input impedance, and output impedance.

Common Amplifier Configurations Explained

There are three fundamental amplifier configurations for bipolar junction transistors (BJTs):

• Common Base Configuration: In this setup, the base terminal is common to both the input and output signals. This configuration provides a high voltage gain but a low current gain (approximately 1). It also features a low input impedance and a high output impedance. It's often used in high-frequency applications.
• Common Emitter Configuration: Here, the emitter terminal is common to both the input and output signals. This is the most versatile and widely used configuration because it offers a good balance: high current gain, moderate voltage gain, moderate input impedance, and moderate output impedance. Its characteristics make it suitable for a broad range of amplification tasks.
• Common Collector Configuration: With the collector terminal common to both input and output, this configuration provides a high current gain but a very low voltage gain (close to 1). It has a high input impedance and a low output impedance, making it ideal for impedance matching applications, often referred to as an emitter follower.

Why Common Emitter is Most Popular

The common emitter configuration is the most frequently chosen for general-purpose amplification due to its excellent combination of properties:

• High Current Gain: It significantly boosts the current of the input signal. The current gain is typically denoted by $ \beta $ or $ A_i $.
• Moderate Voltage Gain: It provides a substantial increase in signal voltage. The voltage gain is typically denoted by $ A_v $.
• Versatility: It works well across a wide range of frequencies and signal levels, making it suitable for audio amplifiers, radio frequency amplifiers, and many other applications.
• Input/Output Impedance Balance: While not extreme like the other configurations, its moderate input ($ Z_{in} $) and output ($ Z_{out} $) impedances are often practical for cascading amplifier stages.

In contrast, the common base configuration is limited by its low current gain and impedance, and the common collector is limited by its low voltage gain, making them suitable only for specific niche applications rather than general amplification.

Conclusion on Widely Used Configuration

Considering the desirable characteristics for general signal amplification, the common emitter configuration stands out as the most widely used amplifier setup in electronics.