Small-signal Operation of MOSFET

Transistor is usually biased in its saturation region for linear applications

• For small signal variations near constant bias point,
                               first-order Taylor approximation can be used
  
• Drain current incremental (small signal, ac) component of NMOS transistor

                  where

                              g_m , g_mb ... Transconductances - represented by voltage-controlled current sources (VCCSs)

                              g_d ... Drain conductance (incremental)

Low-frequency equivalent circuit of a MOSFET

High-frequency equivalent circuit of a MOSFET

Capacitors representing the incremental variations of stored charges
         with changing electrode voltages
         must be added to the low-frequency model

C_gd : Gate-to Drain Capacitance

C_gs : Gate-to Source Capacitance

C_sb : Source-to Substrate Capacitance

C_db : Drain-to Substrate Capacitance

C_gb : Gate-to Substrate Capacitance

T model

Alternate low-frequency model

Can often result in simpler equations

Used for a quick analysis

When using the T model, the gate current is assumed to be 0 (i_D = i_S)

Body effect is not modelled here