Biasing the Field Effect Transistor

Current Source Bias:

This is another method to produce solid Q point. The aim is to produce the drain current which is independent of V_GS. Voltage divider bias and self bias attempt to do this by swamping out of variations in V_GS.

Using two power supplies:

The current source bias is used to make ID constant which is shown in the figure 1.

Figure 1

Bipolar transistor is emitter biased; its collector current can be given by

I_C = (V_EE - V_BE ) / R_E.

Because the bipolar transistor behaves like a current source, it forces drain current to equal the bipolar collector current.

I_D = I_C

As I_C  is constant, both the Q points have the same value of drain current. The current source quite effectively wipes out the influence of V_GS. However, V_GS  is different for each Q point, it no longer influences value of the drain current.

Using One power supply:

When the positive supply is available only, the circuit shown in the figure 2, can be taken in use to set up a constant drain current.

In this case, bipolar transistor is voltage divider biased. By assuming a stiff voltage divider, emitter and collector currents are constant for all the bipolar transistors. This force the FET drain current equal the bipolar collector current.

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