Transistor Biasing u0026 Small-Signal Amplifiers Fundamentals Quiz Quiz

Explore essential concepts of transistor biasing and small-signal amplifiers through this focused quiz. Assess your understanding of fundamental principles such as biasing techniques, stability, input/output signals, and amplifier configurations relevant to electronic circuit design.

1. Effect of Bias Instability

   Which of the following consequences is likely if a bipolar junction transistor (BJT) biasing arrangement is unstable in a common-emitter amplifier circuit?

   1. Automatic thermal stabilization without extra components
   2. Significant distortion or cutoff during amplification
   3. Unchanged collector current under temperature variations
   4. Higher output impedance leading to improved gain

   Explanation: An unstable biasing arrangement can cause the transistor to drift out of the active region, resulting in significant signal distortion or even cutoff. Unchanged collector current is incorrect because instability often causes collector current to fluctuate with temperature. Higher output impedance does not result from bias instability but from the amplifier configuration itself. Automatic thermal stabilization is not achieved without added design features, so the last option is incorrect.

2. Purpose of Emitter Resistance

   Why is an emitter resistor frequently included in the biasing network of a transistor amplifier, such as in a voltage-divider configuration?

   1. To lower the voltage gain to zero
   2. To eliminate the need for a coupling capacitor
   3. To increase the collector current directly
   4. To stabilize the operating point against temperature changes

   Explanation: Adding an emitter resistor provides negative feedback, which helps stabilize the transistor's operating point by counteracting changes in current caused by variations in temperature. Increasing collector current is not the purpose of the emitter resistor—if anything, it may slightly reduce it. Lowering voltage gain to zero is incorrect, as the resistor does reduce gain but does not nullify it. The resistor also does not replace or remove the need for coupling capacitors.

3. Small-Signal Parameter Definition

   In small-signal analysis, what does the parameter 'rπ' (r-pi) represent in the hybrid-pi model of a BJT?

   1. A fixed supply voltage source
   2. The output capacitance of the transistor
   3. The collector-emitter leakage current
   4. The base-emitter dynamic resistance

   Explanation: In the hybrid-pi model, 'rπ' refers to the small-signal (dynamic) resistance between the base and emitter terminals, crucial for analyzing input characteristics. Collector-emitter leakage current is represented by a different parameter and not by rπ. A fixed supply voltage is unrelated to this model parameter. Output capacitance exists in the transistor model but is not referred to as rπ.

4. Voltage Gain in CE Amplifiers

   Suppose a common-emitter (CE) amplifier exhibits a high voltage gain when a bypass capacitor shorts the emitter resistor at signal frequencies. What is the primary reason for this increase?

   1. The collector resistance becomes negligible
   2. Introduction of additional phase shift at the output
   3. Removal of AC negative feedback across the emitter resistor
   4. The bypass capacitor increases DC bias voltage

   Explanation: The bypass capacitor effectively shorts the emitter resistor for AC signals, removing AC negative feedback and resulting in increased voltage gain. The bias voltage is not affected by the capacitor during AC analysis. Introducing additional phase shift is not the main effect; phase shift in CE amplifiers stems from the amplifier's configuration. The collector resistance does not become negligible, so that option is incorrect.

5. Common-Base Amplifier Characteristic

   Which is a distinguishing characteristic of a common-base transistor amplifier, as compared to common-emitter and common-collector amplifiers?

   1. Unity voltage gain with high output resistance
   2. Maximum current gain among all configurations
   3. Inverted output and highest power gain
   4. Very low input resistance and high voltage gain

   Explanation: Common-base amplifiers are known for their very low input resistance and relatively high voltage gain, making them suitable for high-frequency applications. They do not provide an inverted output; the output is in phase with the input. Unity voltage gain and high output resistance describe a common-collector (emitter-follower) amplifier. Maximum current gain is achieved in the common-emitter configuration, not common-base.