Which of the following is NOT true for a common collector transistor?
Understanding the Common Collector Transistor Configuration
The common collector (CC) configuration is one of the three basic single-stage bipolar junction transistor (BJT) amplifier configurations, typically used as a voltage buffer. In this configuration, the input signal is applied to the base, and the output signal is taken from the emitter. The collector terminal is connected to the power supply (VCC), which acts as an AC ground, hence the name "common collector".
Let's examine the typical characteristics of a common collector configuration to determine which of the given statements is NOT true.
Key Characteristics of a Common Collector Configuration
- Input Impedance: High. The input impedance is typically much higher than that of common emitter or common base configurations. This is because the input resistance at the base is multiplied by the current gain ($\beta+1$).
- Output Impedance: Low. The output impedance from the emitter is typically much lower than the common emitter configuration. This configuration can drive low-impedance loads effectively.
- Voltage Gain: Close to unity (approximately 1). The output voltage at the emitter closely follows the input voltage at the base (hence also called an emitter follower). The voltage gain is slightly less than 1.
- Current Gain: High. The current gain is approximately equal to $\beta+1$, which is high. The output current at the emitter is the sum of the base current and the collector current.
- Power Gain: Moderate. Power gain is the product of voltage gain and current gain. Since the voltage gain is close to unity and the current gain is high, the power gain is moderate, not typically described as "high" in comparison to the common emitter configuration which is known for its high power gain.
Analyzing the Given Statements for Common Collector Transistors
Let's evaluate each statement based on the characteristics discussed:
- Input impedance is high: This statement is TRUE. The common collector configuration is known for its high input impedance.
- It can be used for impedance matching: This statement is TRUE. Due to its high input impedance and low output impedance, the common collector is excellent for impedance matching, particularly for coupling a high-impedance source to a low-impedance load.
- Output impedance is low: This statement is TRUE. The common collector configuration provides a low output impedance, allowing it to drive low-resistance loads.
- Power gain is high: This statement is NOT TRUE. While the current gain is high, the voltage gain is close to 1. Therefore, the power gain ($\text{A}_p = \text{A}_v \times \text{A}_i$) is moderate, not typically classified as "high" like that of a common emitter amplifier.
Based on the analysis, the statement that is NOT true for a common collector transistor is that the power gain is high. Its primary advantage lies in impedance buffering rather than power amplification.
Revision Table: Common Collector Characteristics
| Characteristic | Common Collector (Emitter Follower) |
|---|---|
| Input Signal | Base |
| Output Signal | Emitter |
| Common Terminal | Collector (AC Ground) |
| Voltage Gain ($\text{A}_v$) | < 1 (close to unity) |
| Current Gain ($\text{A}_i$) | High ($\approx \beta+1$) |
| Power Gain ($\text{A}_p$) | Moderate |
| Input Impedance ($\text{Z}_{in}$) | High |
| Output Impedance ($\text{Z}_{out}$) | Low |
| Phase Shift (Input vs. Output) | $0^\circ$ |
Additional Information: Transistor Configurations Comparison
Comparing the common collector configuration with the other two basic BJT configurations, common emitter (CE) and common base (CB), provides further clarity on their uses:
- Common Emitter: High voltage gain, high current gain, high power gain, medium input impedance, medium output impedance. Used primarily for voltage and power amplification. Input and output are $180^\circ$ out of phase.
- Common Base: High voltage gain, current gain < 1, high output impedance, very low input impedance. Used primarily for voltage amplification at high frequencies and as a current buffer. Input and output are in phase.
- Common Collector: Voltage gain < 1, high current gain, moderate power gain, high input impedance, low output impedance. Used primarily for impedance matching or buffering. Input and output are in phase.
Each configuration has distinct characteristics that make it suitable for different applications in electronic circuits.
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)?
The other name for the common collector amplifier is -
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 _______.
