Electrical Engineering ⇒ Topic : Transconductance gm

Transconductance

The ratio of change in drain current I_D at constant drain source voltage V_DS, i.e.,

                                                      .......... (1)

Its unit is mho or siemen.

The transfer characteristics of JFET are non-linear. If the location of operating point changes, the value of g_m also changes. Generally, the JFET manufacturers provide the value of g_m at V_GS = 0 V, i.e., g_mo.

If g_mo is known for a JFET, the value of g_m at V_GS # V, can be determined from the equation.

The gate current is represented by

                                             .............. (2)

From Eq. (1), we have

Substituting Eq. (2) in Eq. (1), we get

Therefore, g_m can be expressed as:

                            ........... (3)

This can also be written as:

                              ............... (4)