Characteristics Parameters of JFET

Characteristics Parameters of JFET

The important characteristics parameters of JFET are as follows:

• Transconductance (g_m)

• Input resistance and capacitance

• Drain to source resistance (r_d)

• Amplification factor (μ)

• Power Dissipation (P_D).

• Transconductance

• The transconductance, g_m, is the change in the drain current for given change in gate to source voltage with the drain to source voltage constant as shown in Fig. 5.7.1.

• Looking at Fig. 5.7.1, we can say that it is the slope of the transfer characteristic. Since the slope varies, g_m also varies. g_m has a greater value near the top of the curve than it does near the bottom. The transconductance g_m is defined as

• The transconductance g_m is also called mutual conductance. The practical unit for g_m is mS (millisiemen) or mA/V. For given g_m, we can calculate an approximate value for g_m at any point on the transfer characteristic curve using following equation.

            ………... (5.7.2)

•  where g_mo is the value of g_m for V_GS = 0, and is given by,

 g_mo = ‒2I_DSS / V_P

       ………. (5.7.3)

• This can be proved as given below. We know that,

• Differentiating this equation with respect to V_GS we get,

Ex. 5.7.1: For JFET, if I_DSS = 20 mA, V_GS(off) = − 5 V, and g_mo = 4 mS or mA/V. Determine the transconductance for V_GS = ‒ 4 V, and find I_D at this point.

Solution:

From equation (5.7.2) we have,

 = 4×10^‒3 × 0.2 = 0.8 mS

We have,

 = 20×10^‒3 × 0.04 = 0.8 mA

• Input Resistance and Capacitance

• We know that a JFET operates with its gate source junction reverse‒biased. Therefore, the input resistance at the gate is very high. This high input resistance is one advantage of the JFET over the bipolar transistor. (Recall that a BJT operates with a forward biased base‒emitter junction). JFET data sheets often specify the input resistance by giving a value of the gate reverse current, I_GSS at a certain gate to source voltage, V_GS. The input resistance can then be determined using the following equation, where the vertical lines indicate an absolute value.

R_IN = | V_GS / I_GSS |

• For example, the 2N3909 data sheet lists a maximum I_GSS of 10 nA for V_GS =  10 V at 25 °C. Therefore,

R_IN = | 10V / 10nA | = 1000 ΜΩ

• From data sheet we can also observe that I_GSS is 1.0 μA for VGS = 10 V at 100 °C. This shows that I_GSS increases with temperature. Here,

 R_IN = | 10V / 1μΑ | = 10 ΜΩ

Key Point: The input resistance decreases with increase in temperature.

• The input capacitance, C_iss is a result of the JFET operating with a reverse biased p‒n junction. Recall that a reverse biased p‒n junction acts as a capacitor whose capacitance depends on the amount of reverse voltage. For example, the 2N3909 has a maximum C_iss of 32 pF for V_GS = 0.

• Drain to Source Resistance

• From the drain characteristic, the important parameter of JFET, drain resistance r_d, can be calculated. Fig. 5.7.2 shows the drain characteristics of n‒channel JFET.

 Fig. 5.7.2 Drain characteristics of n‒channel JFET

• The drain resistance r_d is the a.c. resistance between drain and source terminals when the JFET is operating in the saturation region. It is the reciprocal of the slope of the drain characteristic in the saturation region. It is given by,

• Since the characteristics in the saturation region is almost flat, r_d is not easily determined from the characteristics. Values of r_d range from about 50 kΩ to several 100 kΩ. Since r_d is usually the output resistance of the JFET, it may also be expressed as an output admittance |Yos| = 1/r_d.

• Amplification Factor

• The amplification factor, denoted by μ is defined as,

Amplification factor μ = 

 μ = r_d ×g_m

        …………. (5.7.5)

• Power Dissipation (P_D)

• The power dissipation in JFET is given by,

 P_D = I_D × V_DS

Review Questions

1. Sketch a typical transfer characteristic for an n‒channel JFET and show how the transconductance g_m can be derived from the transfer characteristic.

2. Define the following parameters of JFET :

1) Transconductance

2) Drain resistance

3) Amplification factor

4) Power dissipation.