Question:medium
The application of a CC configured transistor is _________.
The application of a CC configured transistor is _________.
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Remember the core configurations and their primary uses:
- Common Emitter (CE): High Voltage and Current Gain (General Amplification).
- Common Base (CB): High Voltage Gain, Constant Current (High-Frequency RF Applications).
- Common Collector (CC): Unity Voltage Gain, High Current Gain (Impedance Matching Buffer Stage).
Updated On: Jul 4, 2026
- voltage multiplier
- level shifter
- rectification
- impedance matching
Show Solution
The Correct Option is D
Solution and Explanation
Understanding the Concept:
A Common Collector (CC) transistor configuration, commonly known as an emitter follower, features unique impedance properties. It presents a very high input impedance and a very low output impedance. Because of this large disparity between input and output characteristics, voltage gain is nearly unity ($\approx 1$), and the output voltage closely tracks the input voltage. This configuration does not serve well for typical voltage amplification but is perfectly suited to bridge connections between a high-impedance source circuit and a low-impedance load circuit, minimizing signal loss caused by loading effects.
Step 1: Analyzing the Impedance Characteristics of a CC Configuration
In a Common Collector configuration, the input signal is applied across the base-collector junction, while the output is extracted across the emitter-collector junction. The input impedance ($Z_{in}$) is given by: \[ Z_{in} \approx \beta \cdot R_E \] where $\beta$ is the current gain (which is typically high, e.g., $>100$) and $R_E$ is the emitter resistance. This results in an exceptionally high input impedance, typically in the range of hundreds of kilo-ohms ($\text{k}\Omega$). Conversely, the output impedance ($Z_{out}$) looking back into the emitter terminal is given by: \[ Z_{out} \approx \frac{R_S}{\beta} + r_e \] where $R_S$ is the source resistance and $r_e$ is the intrinsic emitter dynamic resistance. This results in a very low output impedance, typically a few ohms or tens of ohms.
Step 2: Assessing Applications Based on Characteristics
When a circuit with high output resistance needs to drive a small load resistance, directly connecting them creates a massive voltage drop across the internal resistance of the source, causing signal attenuation. By introducing a CC configuration between them:
• The high input impedance draws minimal current from the preceding stage, preventing signal loading.
• The low output impedance ensures that it can supply sufficient current to a low-resistance load without the terminal voltage dropping significantly. Therefore, its primary role is to match the impedance of two mismatched stages to ensure efficient signal transmission, a technique known as impedance matching.
Step 3: Disproving the Incorrect Alternatives
• Voltage Multiplier: These are specialized diode-capacitor networks designed to step up DC voltage from an AC source; transistors are not configured this way for basic multiplier units.
• Level Shifter: While shifting DC levels can be done with various combinations, the defining fundamental, textbook application of a pure CC stage is impedance buffering.
• Rectification: This refers to converting AC to DC, which is uniquely the domain of diodes, not a CC linear amplifier stage.
Step 1: Analyzing the Impedance Characteristics of a CC Configuration
In a Common Collector configuration, the input signal is applied across the base-collector junction, while the output is extracted across the emitter-collector junction. The input impedance ($Z_{in}$) is given by: \[ Z_{in} \approx \beta \cdot R_E \] where $\beta$ is the current gain (which is typically high, e.g., $>100$) and $R_E$ is the emitter resistance. This results in an exceptionally high input impedance, typically in the range of hundreds of kilo-ohms ($\text{k}\Omega$). Conversely, the output impedance ($Z_{out}$) looking back into the emitter terminal is given by: \[ Z_{out} \approx \frac{R_S}{\beta} + r_e \] where $R_S$ is the source resistance and $r_e$ is the intrinsic emitter dynamic resistance. This results in a very low output impedance, typically a few ohms or tens of ohms.
Step 2: Assessing Applications Based on Characteristics
When a circuit with high output resistance needs to drive a small load resistance, directly connecting them creates a massive voltage drop across the internal resistance of the source, causing signal attenuation. By introducing a CC configuration between them:
• The high input impedance draws minimal current from the preceding stage, preventing signal loading.
• The low output impedance ensures that it can supply sufficient current to a low-resistance load without the terminal voltage dropping significantly. Therefore, its primary role is to match the impedance of two mismatched stages to ensure efficient signal transmission, a technique known as impedance matching.
Step 3: Disproving the Incorrect Alternatives
• Voltage Multiplier: These are specialized diode-capacitor networks designed to step up DC voltage from an AC source; transistors are not configured this way for basic multiplier units.
• Level Shifter: While shifting DC levels can be done with various combinations, the defining fundamental, textbook application of a pure CC stage is impedance buffering.
• Rectification: This refers to converting AC to DC, which is uniquely the domain of diodes, not a CC linear amplifier stage.
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