Biasing the FET

BIASING THE FET

For the proper functioning of a linear FET amplifier, it is necessary to maintain the operating point Q stable in the central portion of the pinch off region. The Q point should be independent of device parameter variations and ambient temperature changes. This can be achieved by suitably selecting the gate to source voltage (V_GS) and drain current (I_D) which is referred to as biasing.

An FET may have combination of self bias and fixed bias to provide stability of the quiescent drain current against device and temperature variations.

Self-Bias

Fig. 2.9 shows the self bias circuit for N-channel FET. When the drain voltage V_DD is applied, a drain current I_D flows even in the absence of gate voltage (V_G). The voltage drop across resistor R_S produced by the drain current is given by V_S = I_D R_S. This voltage drop reduces the gate to source reverse voltage required for FET operation. The resistor R_S, the feedback resistor prevents any variation in FET drain current.

When drain current increases, the voltage drop across R_S increases. The increased voltage drop increases the reverse gate to source voltage, which decrease the effective width of the channel and hence, reduces the drain current. Now the reduced drain current decreases the gate to source voltage which, in turn increase the effective width of the channel thereby increasing the value of drain current.

Voltage Divider Bias

Fig. 2.10 (a) shows the voltage divider bias circuit and it's Thevenin's equuivalent is shown in Fig. 2.10(b). Resistors R₁ and R₂ connected on the gate side forms a voltage divider. The gate voltage

The bias line satisfies the equation V_GS - V_GG - I_D R_S

The drain to ground voltage, V_d = V_DD - I_D R_D

If the gate voltage V_GG is very large as compared to gate to source voltage V_GS. The drain current is approximately constant. In practice, the voltage divider bias is less effective with JFET than BJT.

This is because, in BJT, V_BE = 0.7 V (silicon) with only minor variations from one transistor to another But in a JFET, the VGs can vary several volts from one JFET to another.