Question

Which of the following statements is/are TRUE with respect to ideal MOSFET-based DC-coupled single-stage amplifiers having finite load resistors?

• A. The common-gate amplifier has an infinite input resistance
• B. The common-source amplifier has an infinite input resistance
• C. The input and output voltages of the common-source amplifier are in phase
• D. The input and output voltages of the common-drain amplifier are in phase

Hint:

The common-gate amplifier has low input resistance.
The common-source amplifier has a finite input resistance, not infinite.
The common-drain amplifier has input and output voltages in phase, unlike the common-source amplifier which has a phase inversion.

Answer 1

The Correct Option is B, D

Let’s evaluate each of the given options based on the characteristics of MOSFET amplifiers with finite load resistors:

1. Common-gate amplifier:
A common-gate amplifier typically has a low input resistance, not infinite. It’s usually used for high-frequency applications where the input resistance is dominated by the source impedance, and thus, Option (A) is incorrect.

2. Common-source amplifier:
The common-source amplifier is widely used in amplification and has a finite input resistance (not infinite). The input resistance of a common-source amplifier depends on the MOSFET parameters and the resistive load. Therefore, Option (B) is correct because, in practice, the common-source amplifier has a high but finite input resistance.

3. Common-source amplifier’s voltage phase relationship:
In a common-source amplifier, the input and output voltages are out of phase (180° phase shift). This makes Option (C) incorrect because the common-source amplifier inverts the signal.

4. Common-drain amplifier:
The common-drain amplifier (also called a source follower) has an input and output voltage in phase. The common-drain amplifier provides unity gain and does not invert the signal, so Option (D) is correct.

Thus, the correct answers are (B) and (D).