BJT Amplifier Configurations

Hello Alex,

Welcome back! In our last lesson, we delved into the internal physics of the Bipolar Junction Transistor, establishing how its unique structure allows a small base current ($I_B$) to control a much larger collector current ($I_C$), a relationship defined by the current gain, $\beta$.

This lesson builds directly on that foundation. Now that we know how a BJT works, we'll explore the three fundamental ways to connect it in a circuit to create an amplifier. You'll discover that the choice of configuration dramatically changes the amplifier's characteristics, making each one suitable for very different tasks. This is a core concept in practical circuit design.

Approximate time to complete: 45-50 minutes.

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1. The Three Amplifier Topologies

A transistor is a three-terminal device (Emitter, Base, Collector). When building an amplifier, we need an input port and an output port, each requiring two connections. This means one of the transistor's three terminals must be "common" to both the input and output circuits. This simple choice gives rise to the three fundamental BJT amplifier configurations:

1. Common-Emitter (CE)
2. Common-Collector (CC)
3. Common-Base (CB)

The "common" terminal is typically connected to a fixed potential, like ground or a power supply rail, which acts as a stable reference for AC signals. Each configuration leverages the BJT's current-gain property in a unique way, resulting in distinct characteristics for voltage gain, current gain, input impedance, and output impedance.

To start, let's get a quick overview of these three topologies.

Watch the introduction to familiarize yourself with the names and the basic concept.

• Introduction to Transistor Configurations (0:00 - 0:49): This segment sets the stage for what we'll be covering.

The choice of configuration is a fundamental design decision. Do you need to boost voltage? Match a high impedance source to a low impedance load? Or amplify a signal at very high frequencies? The answer will point you to one of these three topologies.

Let's examine each one in detail.

2. The Common-Emitter (CE) Configuration

This is the most widely used amplifier configuration due to its ability to provide significant voltage and current gain simultaneously, leading to a very high power gain.

• Input: Applied to the Base.
• Output: Taken from the Collector.
• Common: The Emitter is common to both input and output.

Key Characteristics:

• High Voltage Gain: A small change in base voltage causes a large change in collector current, leading to a large change in the voltage across the collector resistor.
• High Current Gain: The current gain is approximately equal to the transistor's $\beta$.
• High Power Gain: The product of high voltage and high current gain.
• Phase Inversion: The output signal is 180° out of phase with the input signal. This is a crucial characteristic. As the input voltage increases, $I_B$ and $I_C$ increase. This increased current causes a larger voltage drop across the collector resistor $R_C$, pulling the collector's voltage down.
• Moderate Input and Output Impedance: They are neither exceptionally high nor low, making this configuration a good general-purpose choice.

Now, let's see this in action. The following video provides an excellent explanation and a practical demonstration on an oscilloscope.

• Common Emitter Amplifier (00:19 - 03:47): Pay close attention to the scope demonstration showing both the voltage gain and the 180° phase inversion. Notice how bypassing the emitter resistor with a capacitor dramatically increases the gain, a common technique in amplifier design.

3. The Common-Collector (CC) Configuration

This configuration is also known as an Emitter Follower. The name is a hint to its primary behavior: the emitter voltage "follows" the base voltage.

• Input: Applied to the Base.
• Output: Taken from the Emitter.
• Common: The Collector is common to both input and output.

Key Characteristics:

• Unity Voltage Gain: The voltage gain is approximately 1 (or slightly less). The output signal voltage is nearly identical to the input signal voltage.
• High Current Gain: Similar to the CE configuration, the current gain is high (specifically, $\beta +1$).
• High Input Impedance: The circuit presents a high impedance to the input source, meaning it draws very little current.
• Low Output Impedance: The circuit can drive loads that require significant current without the output voltage dropping.
• No Phase Inversion: The output signal is in phase with the input signal.

The combination of high input impedance, low output impedance, and unity voltage gain makes the Common-Collector configuration an excellent buffer amplifier or impedance matching circuit. It's used to connect a high-impedance source to a low-impedance load, preventing the load from "loading down" the source and attenuating the signal.

Let's watch the practical demonstration of the Emitter Follower.

• Common Collector (Emitter Follower) Amplifier (03:47 - 05:59): Observe on the oscilloscope how the output signal's amplitude and phase perfectly track the input signal.

4. The Common-Base (CB) Configuration

While less common in general audio circuits, the Common-Base configuration is very important in high-frequency applications, like the radio circuits you studied in your Radiophysics degree.

• Input: Applied to the Emitter.
• Output: Taken from the Collector.
• Common: The Base is common to both input and output.

Key Characteristics:

• High Voltage Gain: Similar to the CE configuration.
• Unity Current Gain: The current gain is approximately 1 (or slightly less). This is because the output current is $I_C$ and the input current is $I_E$, and we know from the last lesson that $I_C\approx I_E$. The current gain is effectively $\alpha$.
• Low Input Impedance: This is a defining feature. The circuit presents a very low impedance at the emitter terminal.
• High Output Impedance: Similar to the CE configuration.
• No Phase Inversion: The output signal is in phase with the input signal.

Given your background, the applications of this configuration are particularly relevant. Its low input impedance is perfect for matching to low-impedance sources like antennas (often 50Ω or 75Ω). Furthermore, grounding the base provides excellent isolation between the input (emitter) and output (collector), which improves stability at high frequencies and mitigates unwanted oscillations.

Watch the final segment of the video for a demonstration and discussion of its RF applications.

• Common Base Amplifier (05:59 - 10:46): The video explicitly mentions its use in RF circuits and introduces the concept of the "Miller effect," which limits the high-frequency performance of CE amplifiers. The CB configuration avoids this issue, making it superior for RF amplification. The discussion of the "cascode" configuration (a CE stage followed by a CB stage) is a preview of advanced amplifier design techniques.

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5. Summary of Configurations

The best way to remember the differences is to see them side-by-side.

Characteristic	Common-Emitter (CE)	Common-Collector (CC)	Common-Base (CB)
Voltage Gain	High ($>1$)	Unity ($\approx 1$)	High ($>1$)
Current Gain	High ($\approx \beta$)	High ($\approx \beta$)	Unity ($\approx \alpha$)
Power Gain	Very High	Moderate	Low-Moderate
Input Impedance	Moderate	High	Low
Output Impedance	Moderate	Low	High
Phase Shift	180° (Inverting)	0° (Non-inverting)	0° (Non-inverting)
Primary Use	General-purpose amplifier	Buffer, Impedance matching	RF/High-frequency amp.

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Conclusion

In this lesson, you've learned how a single device, the BJT, can be used to build amplifiers with vastly different properties simply by changing which terminal is common to the input and output.

Here are the key takeaways:

• Common-Emitter: The workhorse configuration, providing high voltage and current gain. Its output is inverted.
• Common-Collector (Emitter Follower): An impedance-matching hero. It has a high current gain, high input impedance, and low output impedance, making it ideal as a buffer. Its voltage gain is 1.
• Common-Base: The high-frequency specialist. It offers high voltage gain and excellent performance at RF due to its low input impedance and good input-output isolation. Its current gain is 1.

The ability to choose the right configuration is a key skill for any circuit designer.

In our next lesson, we will address a crucial practical detail: BJT Biasing. To make any of these amplifiers work correctly, we must first establish a stable DC operating point (the "Q-point"). This process ensures the transistor stays in the active region and faithfully amplifies the AC signal without distortion.