MOSFET Biasing and Amplifier Configurations Quiz Quiz

Explore key concepts of MOSFET biasing techniques and amplifier configurations with this quiz designed for electronics students and enthusiasts. Assess your grasp of circuit operation, bias stability, and the differences between common amplifier setups.

1. MOSFET Biasing Fundamentals

   Which biasing technique best ensures thermal stability in an n-channel MOSFET amplifier circuit using a voltage divider at the gate?

   1. Voltage-divider bias
   2. Collector-base bias
   3. Source-follower bias
   4. Fixed-bias configuration

   Explanation: The voltage-divider bias technique uses resistors to provide a stable gate voltage that is less affected by temperature variations, leading to improved thermal stability in n-channel MOSFETs. Source-follower bias refers to a specific amplifier configuration and not a biasing technique. Fixed-bias configurations are generally more sensitive to temperature changes. Collector-base bias is related to bipolar junction transistors, not MOSFETs.

2. Common Source Amplifier Characteristics

   What is the primary advantage of using a common source (CS) amplifier configuration with an n-channel MOSFET?

   1. Inverting output with low impedance
   2. High input resistance and voltage gain
   3. Unity voltage gain
   4. Negligible phase shift between input and output

   Explanation: A common source amplifier provides high input resistance and significant voltage gain, making it suitable for many amplification tasks. Unity voltage gain refers to the common drain (source follower) amplifier, not the CS configuration. While the CS amplifier does invert the signal, its output impedance is moderate, not particularly low, and there is a significant 180-degree phase shift, not negligible.

3. Source Follower Configuration

   In a scenario where a MOSFET is set up as a source-follower, what is the main application advantage of this configuration?

   1. Guarantees thermal runaway prevention
   2. Ensures maximum voltage gain with minimum current
   3. Provides voltage buffering with high input and low output impedance
   4. Maximizes power gain with phase inversion

   Explanation: The source follower, also known as the common drain amplifier, is ideal for voltage buffering due to its high input impedance and low output impedance, allowing it to drive heavy loads effectively. It does not maximize power gain nor does it provide phase inversion—the output is in phase. While thermal stability is a feature, it's not guaranteed by this configuration alone. Maximum voltage gain is not its primary characteristic.

4. Bias Point and Load Line

   When setting the MOSFET bias point in an amplifier, what does the intersection of the load line and the transfer characteristic represent?

   1. A breakdown voltage condition
   2. The cutoff region
   3. The quiescent or operating point (Q-point)
   4. The point of maximum frequency response

   Explanation: The intersection of the load line and the MOSFET’s transfer characteristic on a graph defines the Q-point, or quiescent point, where the device operates in the absence of a signal. This is not related to maximum frequency response, which involves bandwidth considerations. Breakdown voltage refers to a destructive voltage limit, not the bias point. The cutoff region is an off state far from the typical Q-point.

5. Effect of Source Resistor in Biasing

   How does adding a resistor in the source lead of an n-channel MOSFET bias circuit improve stability?

   1. It provides negative feedback, reducing sensitivity to parameter variations
   2. It increases the gate current, ensuring higher load current
   3. It bypasses the drain current directly to ground
   4. It decreases the threshold voltage so the device turns on sooner

   Explanation: A source resistor introduces negative feedback, which stabilizes the operating point by making the bias less sensitive to changes in device parameters or temperature. Increasing the gate current is incorrect because MOSFET gates draw negligible steady-state current. The source resistor does not decrease threshold voltage; instead, it slightly increases the voltage needed to turn the device on. It does not bypass the drain current; it merely sets the source voltage.