Semiconductor Devices - JFET Biasing

There are two methods in use for biasing the JFET: Self-Bias Method and Potential Divider Method. In this chapter, we will discuss these two methods in detail.

Self-Bias Method

The following figure shows the self-bias method of n-channel JFET. The drain current flows through R_s and produces the required bias voltage. Therefore, R_s is the bias resistor.

Therefore, voltage across bias resistor,

$$V_s = I_{DRS}$$

As we know, gate current is negpgibly small, the gate terminal is at DC ground, V_G = 0,

$$V_{GS} = V_G - V_s = 0 - I_{DRS}$$

Or $V_{GS} = -I_{DRS}$

V_GS keeps gate negative w.r.t. to the source.

Voltage Divider Method

The following figure shows voltage spanider method of biasing the JFETs. Here, resistor R₁ and R₂ form a voltage spanider circuit across drain supply voltage (V_DD), and it is more or less identical to the one used in transistor biasing.

The voltage across R₂ provides necessary bias −

$$V_2 = V_G = frac{V_{DD}}{R_1 + R_2} imes R_2$$

$= V_2 + V_{GS} + I_D + R_S$

Or $V_{GS} = V_2 - I_{DRS}$

The circuit is so designed that V_GS is always negative. The operating point can be found using the following formula −

$$I_D = frac{V_2 - V_{GS}}{R_S}$$

and $V_{DS} = V_{DD} - I_D(R_D + R_S)$