Transistor Biasing and Small-Signal Analysis Essentials Quiz Quiz

Explore fundamental concepts of transistor biasing and small-signal analysis with these targeted questions designed to reinforce your understanding of amplifier stability, biasing circuits, and signal parameters. Gain insight into core techniques and key calculations important for electronics and circuit design studies.

1. Purpose of Biasing

   Why is biasing necessary in a common-emitter bipolar junction transistor amplifier circuit?

   1. To set a stable operating point and prevent distortion
   2. To increase the collector resistance
   3. To make the output waveform square-shaped
   4. To reduce leakage current to zero

   Explanation: Biasing ensures that the transistor operates within the desired region of its characteristic curves, usually the active region, to provide linear amplification with minimal distortion. Increasing collector resistance is unrelated to establishing the operating point. Completely eliminating leakage current is impractical and not the main role of biasing. Making the output square-shaped describes signal shaping rather than setting the transistor bias.

2. Voltage Divider Bias Analysis

   In a voltage divider bias circuit using an NPN BJT, what mainly determines the emitter current if the base-emitter voltage is 0.7 V?

   1. The collector resistor value only
   2. The frequency of the input signal
   3. The resistor values in the voltage divider and the emitter resistor
   4. The breakdown voltage of the transistor

   Explanation: The resistor values in the voltage divider set the base voltage, and combined with the emitter resistor, they establish the emitter current given a fixed base-emitter voltage. The collector resistor affects voltage gain and collector voltage but not the emitter current directly. The transistor's breakdown voltage is unrelated to normal biasing. The input signal frequency doesn't determine bias current in a DC bias analysis.

3. Effect of Emitter Degeneration

   What is the main effect of adding an emitter resistor to a common-emitter amplifier's small-signal model?

   1. It doubles the input impedance and output current
   2. It eliminates all signal distortion
   3. It causes the transistor to enter cutoff
   4. It increases linearity and reduces voltage gain

   Explanation: An emitter resistor, known as emitter degeneration, stabilizes the gain and increases the amplifier's linearity, but it also reduces the voltage gain. It does not necessarily double the impedance or current, as the changes depend on resistor values and circuit design. It does not cause cutoff unless biased incorrectly. While it can reduce distortion, it cannot eliminate all types of signal distortion.

4. Small-Signal Parameter Importance

   When analyzing a small-signal model of a BJT amplifier, why is the transconductance (gm) parameter important?

   1. It directly sets the collector supply voltage
   2. It defines only the maximum allowable base current
   3. It determines the physical size of the transistor
   4. It relates the output current to the input voltage for gain calculation

   Explanation: Transconductance (gm) is crucial as it quantifies the sensitivity of the output current to a small input voltage, directly affecting voltage gain calculations in amplifier models. It does not influence the collector supply voltage, which is determined by the circuit. It does not set the base current's maximum value nor dictate the physical size of the transistor, both of which are unrelated to small-signal analysis.

5. Key Assumption in Small-Signal Analysis

   What key assumption is made when performing small-signal analysis of a transistor amplifier?

   1. Leakage current dominates all transistor currents
   2. The base-emitter junction never conducts
   3. Signal variations are small enough to keep the transistor operating in the linear region
   4. The circuit is operating at maximum power output

   Explanation: Small-signal analysis assumes input signals are sufficiently small that the transistor remains biased in the linear (active) region and circuit elements respond linearly. Maximum power operation is not an assumption for small-signal models. The base-emitter junction must conduct for amplifier action, and leakage current is minor compared to the typical operating currents and not the basis for this analysis.