The other name for the common collector amplifier is -
Emitter follower
Understanding the Common Collector Amplifier
The question asks for another name for the common collector amplifier configuration. Let's explore this amplifier type.
A common collector amplifier is a type of bipolar junction transistor (BJT) amplifier. In this configuration:
- The input signal is applied to the base terminal.
- The output signal is taken from the emitter terminal.
- The collector terminal is common to both the input and output circuits (often connected to a fixed DC voltage supply, which is AC ground).
Key Characteristics of a Common Collector Amplifier
This amplifier configuration has several important characteristics:
- Voltage Gain: The voltage gain is very close to 1 (unity). This means the output voltage waveform at the emitter closely follows the input voltage waveform applied at the base.
- Current Gain: It has a high current gain.
- Input Impedance: It has high input impedance.
- Output Impedance: It has low output impedance.
Because the output voltage at the emitter effectively "follows" the input voltage at the base due to the near-unity voltage gain, this configuration is also known by a descriptive name that highlights this behavior.
Why is it Called Emitter Follower?
The primary reason the common collector amplifier is called an emitter follower is its voltage gain characteristic. The voltage at the emitter terminal (output) closely replicates or "follows" the voltage applied at the base terminal (input), usually with a small DC offset.
Let's consider the relationship between the base voltage ($V_B$) and the emitter voltage ($V_E$). For a silicon BJT operating in the active region, $V_B \approx V_E + 0.7V$. This means $V_E \approx V_B - 0.7V$. If the base voltage changes, the emitter voltage changes by approximately the same amount, minus the constant base-emitter voltage drop ($V_{BE}$), which is around 0.7V. Thus, the emitter voltage follows the changes in the base voltage.
Analyzing the Options
Let's look at the provided options:
- Base follower: This is not a standard term for an amplifier configuration.
- Emitter follower: This name accurately describes the behavior of the common collector amplifier where the emitter voltage follows the input base voltage.
- Collect follower: This is not a standard term for an amplifier configuration.
- Impedance follower: While the common collector configuration can be used as an impedance buffer (converting high impedance to low impedance), "Impedance follower" is not its standard common name. The term "voltage follower" is sometimes used for similar concepts in op-amps, but "Emitter follower" is specific to the BJT common collector setup.
Based on the characteristics and common terminology in electronics, the other name for the common collector amplifier is the emitter follower.
Revision Table: BJT Amplifier Configurations
| Configuration | Input Signal | Output Signal | Common Terminal | Voltage Gain ($A_v$) | Current Gain ($A_i$) | Input Impedance ($R_{in}$) | Output Impedance ($R_{out}$) | Phase Shift |
|---|---|---|---|---|---|---|---|---|
| Common Emitter | Base | Collector | Emitter | Moderate to High | Moderate to High | Moderate | Moderate | 180° |
| Common Base | Emitter | Collector | Base | Moderate to High | Low (<1) | Low | High | 0° |
| Common Collector (Emitter Follower) | Base | Emitter | Collector | ≈ 1 | High | High | Low | 0° |
Additional Information: Applications of Emitter Follower
The common collector or emitter follower configuration is widely used for specific purposes due to its unique characteristics:
- Buffering: Its high input impedance and low output impedance make it ideal for use as a buffer amplifier. This allows a high-impedance source to drive a low-impedance load without significant signal attenuation or distortion.
- Impedance Matching: It can effectively match a high-impedance source to a low-impedance load.
- Voltage Regulation: Emitter followers are sometimes used in voltage regulator circuits.
Understanding the different BJT amplifier configurations and their properties is fundamental in electronics.
Important Questions from Configuration of BJT
BC147 is the transistor used for:
- What is the relationship between the common-emitter current gain ($\beta$) and the common-base current gain ($\alpha$) of a bipolar junction transistor (BJT)?
Which of the following is NOT true for a common collector transistor?
Match List I with List II:
List I
(Bias Configuration of BJT)
List II
(Stability factor equation)
(A) Fixed Bias Configuration (I) S(V BE ) = \(\rm −\frac{\beta/R_E}{\beta+R_{TH}/R_E}\) (B) Emitter Bias Configuration (II) S(V BE ) = −β/R E (C) Voltage Divider Configuration (III) S(V BE ) = \(\rm −\frac{\beta/R_C}{\beta+R_E/R_C}\) (D) Feedback Bias Configuration (IV) S(V BE ) = \(\rm −\frac{\beta/R_E}{\beta+R_B/R_E}\) Choose the correct answer from the options given below :
During normal working of transistor as amplifier, the emitter junction is _______.
