For an ideal MOSFET biased in saturation, the magnitude of the small signal current gain for a common drain amplifier is

The correct option is infinite.

Let us analyze the small-signal behavior of a common-drain (source follower) amplifier using an ideal MOSFET biased in saturation to understand why the magnitude of its small-signal current gain is infinite.

First, recall the basic structure of a common-drain amplifier. In this configuration, the input signal is applied to the gate terminal, the output is taken from the source terminal, and the drain terminal is connected to a constant DC voltage (which acts as an AC ground for small-signal analysis).

Next, we examine the input of the ideal MOSFET. The gate of an ideal MOSFET is insulated from the channel by a layer of silicon dioxide (SiO₂). Consequently, no physical current flows into the gate terminal under DC or low-frequency AC small-signal conditions. Thus, the small-signal input current ($i_{\mathrm{in}}$) entering the gate is zero:
$i_{\mathrm{in}}=0$
This corresponds to an infinite input impedance at the gate of the MOSFET.

On the other hand, the small-signal output current ($i_{\mathrm{out}}$) is the current flowing through the load connected at the source terminal. Due to the field-effect operation, a change in the gate-to-source voltage ($v_{\mathrm{gs}}$) modulates the channel charge, producing a non-zero small-signal drain-to-source current controlled by the transconductance ($g_m$):
$i_{\mathrm{out}}=g_m{v}_{\mathrm{gs}}$
Since the transistor is biased in saturation and operating normally, this small-signal output current is non-zero ($i_{\mathrm{out}}\ne 0$).

The small-signal current gain ($A_i$) of an amplifier is defined as the ratio of the small-signal output current to the small-signal input current:
$A_i=\frac{i_{\mathrm{out}}}{i_{\mathrm{in}}}$
Substituting the values we obtained:
$A_i=\frac{i_{\mathrm{out}}}{0}\to \infty$
Therefore, the magnitude of the small-signal current gain for a common-drain amplifier using an ideal MOSFET is infinite.