Parameters of JFET:

JFET has certain parameters which determine the performance. Such Parameters of JFET are (i) ac drain resistance, (ii) transconductance, (iii) amplification factor, and (iv) dc drain resistance, as explained below:

1. AC Drain Resistance: It is defined as the ratio of change in drain-source voltage to change in drain current at constant gate-source voltage and is denoted by r_d.

i.e. AC drain resistance,

It is also called the dynamic drain resistance.

Referring to the output or drain characteristic shown in Fig. 13.8 (b), it is clear that in the active region the change in drain current, I_D is very small for change in drain-source voltage, V_DS because the characteristic curves are almost flat. Hence ac drain resistance of a JFET is very large ranging from 10 kΩ to 1 MΩ.

2. Transconductance: The control that the gate-source voltage has over the drain current, I_D is measured by transconductance. It is denoted by g_fs. It may be defined as the ratio of change in drain current to the change in gate-source voltage at constant drain-source voltage.

i.e. Transconductance,

It is also called the forward transconductance (g_fs) or forward transadmittance (Y_fs). It is measured in mA/volt or millisiemens.

The transconductance measured at I_DSS is denoted by g_mo.

Mathematical Expression For Transconductance:

Differentiating both sides of Shockley equation

Substituting V_GS = 0 in above equation, we get

From Eqs. (13.14) and (13.15) we have

3. Amplification Factor: It is defined as the ratio of change in drain-source voltage to the change in gate-source voltage at constant drain current and is denoted by μ.

i.e. Amplification factor,

Amplification factor of a JFET indicates how much more control the gate-source voltage has over drain current in comparison to the drain-source voltage.

Amplification factor,

Amplification factor μ of a FET may be as high as 100.

4. DC Drain Resistance: It is also called the static or ohmic resistance of the channel and is defined as ratio of drain-source voltage and drain current. It is denoted by R_DS