Identifying the Emitter Follower Amplifier
The question asks to identify which type of amplifier is commonly known as an emitter follower.
Amplifiers using bipolar junction transistors (BJTs) can be configured in several ways, depending on which terminal is used as the common terminal for both input and output signals. The three basic configurations are common-emitter, common-collector, and common-base.
Understanding the Common-Collector Amplifier (Emitter Follower)
The common-collector amplifier configuration is characterized by the input signal being applied to the base and the output signal being taken from the emitter. The collector terminal is connected to the power supply (AC ground), making it the common terminal.
This configuration is called an "emitter follower" because the output voltage at the emitter closely follows the input voltage applied at the base. This means the voltage gain of a common-collector amplifier is approximately equal to 1 ($\approx 1$).
Key characteristics of the common-collector amplifier (emitter follower) include:
- High Input Impedance: It presents a high impedance to the source, drawing very little current from the input.
- Low Output Impedance: It has a low impedance at the output, allowing it to drive low-impedance loads effectively.
- Voltage Gain: The voltage gain ($A_v$) is slightly less than, but very close to, 1. This is why the output voltage "follows" the input voltage.
- Current Gain: It provides significant current gain.
- Phase Relationship: The output signal is in phase with the input signal (no phase inversion).
Due to its high input impedance and low output impedance, the common-collector configuration is primarily used as a buffer stage to provide impedance matching between a high-impedance source and a low-impedance load. It can also be used as a current amplifier.
Comparing Amplifier Configurations
Let's briefly look at the other configurations mentioned:
- Common-Emitter Amplifier: Input at base, output at collector, emitter common. Offers voltage and current gain, medium input and output impedance, and 180-degree phase inversion.
- Common-Base Amplifier: Input at emitter, output at collector, base common. Offers voltage gain but no current gain, low input impedance, high output impedance, and no phase inversion.
- Common-Drain Amplifier: This refers to a Field-Effect Transistor (FET) configuration, analogous to the common-collector BJT amplifier. It is also known as a "source follower".
Based on the characteristics, the common-collector amplifier is uniquely known as the emitter follower because its output at the emitter closely follows the input voltage.
Therefore, the amplifier called an emitter follower is the common-collector amplifier.
| Configuration | Input Terminal | Output Terminal | Common Terminal | Voltage Gain ($A_v$) | Input Impedance | Output Impedance | Phase Shift | Primary Use |
|---|---|---|---|---|---|---|---|---|
| Common-Emitter | Base | Collector | Emitter | High (> 1) | Medium | Medium | 180° | Voltage Amplifier |
| Common-Collector (Emitter Follower) | Base | Emitter | Collector | Approx. 1 | High | Low | 0° | Buffer, Impedance Matching |
| Common-Base | Emitter | Collector | Base | High (> 1) | Low | High | 0° | High Frequency, Impedance Matching (Low to High) |
Revision Table: BJT Amplifier Configurations
| Configuration | Alias | Voltage Gain | Current Gain | Input Impedance | Output Impedance |
|---|---|---|---|---|---|
| Common-Emitter | Inverting Amplifier | High | High | Medium | Medium |
| Common-Collector | Emitter Follower | $\approx 1$ | High | High | Low |
| Common-Base | Non-inverting Current Buffer | High | $\approx 1$ | Low | High |
Additional Information: Emitter Follower Applications
The emitter follower (common-collector amplifier) is widely used in electronic circuits because of its unique properties, particularly its impedance transformation capability.
- Buffer Stages: Used to connect a circuit with high output impedance to a circuit with low input impedance without significant signal loss or loading effects.
- Output Stages: Can be used as the final stage in an amplifier chain to drive low-impedance loads like speakers or cables.
- Voltage Regulators: Sometimes used in series pass regulators as the pass transistor stage.
- Current Amplifiers: Although voltage gain is close to unity, the current gain is high, making it useful for applications requiring significant current driving capability.
Understanding the common-collector configuration, or emitter follower, is crucial for analyzing and designing various analog circuits.
Important Questions from Miscellaneous-Engineering
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2. Uses of active heating systems put restrictions on the building design to make possible the flow of heat naturally.
3. Active heating systems employ mechanical devices to circulate the working fluid for transportation of heat.
Which of the above statements are correct?
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