MOSFET Biasing

Because of its low power consumption and high input impedance, MOSFET is frequently utilized in electronic circuits. MOSFETs are more efficient than Bipolar Junction Transistors (BJTs) since they do not need a lot of input current. Because the gate and channel are isolated in MOSFETs, there is very little gate current and less power loss.

Biasing is the process of giving the MOSFET the proper DC voltages and currents to function in the required region, usually the saturation region for amplifier applications. The circuit will operate steadily and behave as intended with proper biasing.

Typical methods for biasing MOSFETs include:

• Gate bias circuit (fixed bias)
• Drain-to-gate bias circuit
• Voltage divider bias circuit
• Self-bias circuit

Gate Bias Circuit

The gate voltage in the gate bias circuit is fixed by an external voltage source. There is no voltage drop across the gate resistor in a MOSFET since the gate current is zero (I_G = 0). As a result, the applied bias voltage equals the gate voltage.

Given that the source is typically grounded:

V_G = V_{GS}

KVL application in the drain circuit:

V_{DS} = V_{DD} - I_D R_D

In order to properly bias in the saturation region:

V_{DS} \geq (V_{GS} - V_{TN}), \quad V_{GS} \geq V_{TN}

The MOSFET can be biased in the desired region by choosing the right gate voltage and drain resistance settings.

Drain-to-Gate Bias Circuit Configuration

A separate gate supply is not required thanks to the drain-to-gate bias circuit. Here, automatic biasing is achieved by connecting the gate to the drain via a resistor.

There is no voltage drop across the gate resistor since I_G = 0. Hence:

V_G = V_D

Therefore:

V_{DS} = V_{GS}

KVL application in the circuit:

V_{DS} = V_{DD} - I_D R_D

DC Analysis

In this setup:

• Voltage across R_G = 0
• Gate current I_G = 0
• V_G = V_D
• V_{GS} = V_{DS}

The MOSFET functioning is naturally stabilized by this biasing technique, which usually maintains it in the saturation area.

Voltage Divider Bias Circuit

The gate voltage in this circuit is set using a voltage divider network (R_1 and R_2), which improves control and stability over earlier techniques.

Given that I_G = 0, the gate voltage is determined by:

V_G = \left( \frac{R_2}{R_1 + R_2} \right) V_{DD}

Source voltage:

V_S = I_D R_S

Thus:

V_{GS} = V_G - V_S

KVL application in the drain circuit:

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

In order to function properly in the saturation region:

V_{DS} \geq (V_{GS} - V_{TN}), \quad V_{GS} \geq V_{TN}

This technique is frequently utilized in real-world circuits and offers good bias stability.

Self-Bias Circuit Configuration

The source resistor in the self-bias circuit is used to bias the gate, which is coupled to ground via a resistor.

Given that I_G = 0:

V_G = 0

KVL application from gate to source:

V_{GS} = - I_D R_S

DC Analysis

From the relationship mentioned above:

I_D = - \frac{V_{GS}}{R_S}

This demonstrates that in this configuration, V_{GS} is negative.

• Negative V_{GS} for an enhancement MOSFET indicates no channel creation and the device stays off.
• Therefore, self-bias is inappropriate for MOSFET enhancement.
• The channel for the Depletion MOSFET is already present, thus the device can function correctly.