220-A3.3

Activity: MOS Frequency Response

• Instructions: This activity is structured as a tutorial with an activity at the end. Should you have any questions, clarifications, or issues, please contact your instructor as soon as possible.
• At the end of this activity, the student should be able to:

1. Plot the frequency response of NMOS and PMOS transistors.

MOS Transition Frequency

Figure 1: The MOS transistor small signal model.

At high frequencies, the MOS gate-to-source capacitance, $C_{GS}$ and the gate-to-drain capacitance, $C_{GD}$ limits the MOSFET current gain at high frequencies, and thus, needs to be included in the two-port model, as shown in Fig. 1. The short-circuit current gain can be expressed as:

(1)

We then define the transition frequency, $f_{T}$ as the frequency when magnitude of the short-circuit current gain is equal to 1, or:

(2)

Figure 2: The NMOS Transition Frequency.

Figure 3: The Short-Circuit NMOS Current Gain.