MOSFET Biasing

There are four biasing methods for MOSFET: –

1- Drain to gate bias

2-Voltage divider bias

3-Fixed bias

4- Self bias

Drain to gate bias configuration

• DC Analysis-
• Gate current , I_G =0
• So, we have voltage drop across resistance R_G = V_RG = 0
• Therefore, we get a direct connection between drain and source i.e.
• V_D = V_G
• So V_DS = V_GS
• Drain to gate bias always enables MOSFET in saturation region
• For output circuit, we have: V_DS = V_DD – I_DR_D

Voltage divider bias configuration:

DC – ANALYSIS:

Using voltage divider, gate voltage is obtained by:

Applying KVL is loop 1, we get

V_G – V_GS – I_DR_S = 0

VGS = V_G – I_DRS …. (1)

Assume that MOSFET is in saturation, so we have I_D = Kn (V_GS– V_TN)2

By solving the quadratic equation, determine the value of V_GSor I_D, then apply KVL in

source to drain loop

V_DD – I_DR_D – V_DS – I_SR_S = 0

V_DS = V_DD – I_D (R_S + R_D)

If V_DS> V_GS –V_TN, then the transistor is indeed biased in saturation region, as we have assumed.

However, if V_DS < V_DS (sat), then transistor is biased in the non saturation region

Therefore from equation (1):

V_GS = V_G – I_DR_S

Fixed bias configuration

DRAWBACK OF FIXED BIAS:
It is a dual battery design which makes it expensive and more space occupied bias Configuration

Self bias configuration:

DC ANALYSIS:
0 = V_GS + I_DR_S
V_GS = – I_DR_S

(I_D)_Q=-(V_GS)_Q

For dc analysis, we draw the circuit as:

So, the source voltage is
V_S = V_G – V_GS
Assume that transistor is in saturation. So, we have