**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}