Parameters of JFET or Specifications of JFET

During purchasing a JFET for a particular application we need to check the specifications of the device. These specifications are provided by manufacturers. The followings are the parameters used to specify a JFET and these are

• Gate Cut Off Voltage (V_GS(off))
• Shorted Gate Drain Current (I_DSS)
• Transconductance (g_mo)
• Dynamic Output Resistance (r_d)
• Amplification Factor (μ)

Gate Cut Off Voltage

At a fixed drain voltage, the drain current (I_D) of a JFET depends on the gate to source voltage (V_GS). If the gate to source voltage decreases from zero in n channel JFET, the drain current also gets decreased accordingly. The relation between gate to source voltage and drain current is given below. After a certain gate to source voltage (V_GS), the drain current I_D becomes zero. This voltage is known as Cut Off Gate Voltage (V_GS(off)). This voltage numerically equals the pinch-off drain to source voltage (V_p). In the case of p channel JFET if we increase the gate terminal voltage from zero the drain current gets decreased and after a certain gate to source voltage, the drain current becomes zero. This voltage is the cut off gate voltage for p channel JFET. It is the gate cut off voltage for p channel JFET.

Shorted Gate Drain Current

When the gate terminal is grounded (V_GS = 0) and positive drain to source voltage (V_DS) is being increased slowly in case of n channel JFET, the drain current gets increased linearly. But after pinch-off voltage (V_p), the drain current would not be increased further and gets a constant value. This is the maximum drain current that can flow through the channel when the gate terminal is in ground potential. This current is fixed for a JFET and this is called shorted gated drain current and generally denoted by I_DSS.

Transconductance

Transconductance is the ratio of change in drain current (δI_D) to change in the gate to source voltage (δV_GS) at a constant drain to source voltage (V_DS = Constant).

This value is maximum at V_GS = 0. This is denoted by g_mo. This maximum value (g_mo) is specified in a JFET data sheet. The transconductance at any other value of gate to source voltage (g_m) can be determined as follows. The expression of drain current (I_D) is

By partial differentiating the expression of drain current (I_D) in respect of gate to source voltage (V_GS)

At V_GS = 0, the transconductance gets its maximum value and that is

Therefore, we can write,

Dynamic Output Resistance

This is the ratio of change of drain to source voltage (δV_DS) to the change of drain current (δI_D) at a constant gate to source voltage (V_GS = Constant). The ratio is denoted as r_d.

Amplification Factor

The amplification factor is defined as the ratio of change of drain voltage (δV_DS) to change of gate voltage (δV_GS) at a constant drain current (I_D = Constant).

There is a relation between transconductance (g_m) and dynamic output resistance (r_d) and that can be established in the following way.