Question

Which of the following statements is correct about the small-signal voltage gain of single-stage MOSFET amplifiers? Consider the statement: "Both common source and common gate amplifiers are inverting amplifiers."

Hint:

Remember the phase relationships for the three basic single-stage MOSFET amplifiers: \begin{itemize} \item \textbf{Common Source (CS):} Inverting (180° phase shift). \item \textbf{Common Gate (CG):} Non-inverting (0° phase shift). \item \textbf{Common Drain (CD) / Source Follower:} Non-inverting (0° phase shift). \end{itemize} Only the Common Source configuration inverts the signal.

Answer 1

Step 1: Analyze the Common Source (CS) Amplifier
In a CS amplifier, the input signal is applied to the gate and the output is taken from the drain. The small-signal voltage gain is \(A_v = -g_m (R_D || r_o)\). The negative sign indicates a phase shift of \(180^\circ\). Thus, CS is an inverting amplifier.
Step 2: Analyze the Common Gate (CG) Amplifier
In a CG amplifier, the input is applied to the source and the output is taken from the drain. The small-signal voltage gain is \(A_v = g_m (R_D || r_o)\). There is no negative sign, meaning the output is in phase with the input (\(0^\circ\) phase shift). Thus, CG is a non-inverting amplifier.
Step 3: Evaluate the Statement
Because the Common Gate amplifier is non-inverting, the statement "Both... are inverting" is factually incorrect.