MOSFET Bias Circuits

Hello friends, I hope you all are doing great. In today’s tutorial, we will have a look at MOSFET Bias Circuits. The MOSFET is type of FET and stands for metal oxide field effect transistor used as amplifier and switch in different circuit configurations. In digital and analog circuits MOSFET is commonly used than BJT. There are 2 further main types of MOSFET: E-MOSFET and D-MOSFET.

The main benefit that it provides over the BJT is that it needs a very less amount of current at the input for controlling of a load. In today’s post, we will have a detailed look at its different basing methods and their related circuits with the detailed. So let’s get started with MOSFET Biasing Circuits.

Table of Contents

mosfet biasing Circuits

E-MOSFET Bias Configuration

Since for E-MOSFET the value of VGS should be larger than the threshold value VGS(th), so zero biasing cannot be used.

In below figure you can see 2 configurations of E-MOSFET biasing.

In these configurations n channels, MOSFET is used.
Two biasing configurations are shown first is voltage divider and second one drain feedback bias.
The benefit of the use of this 2 configurations is to make more positive at the gate than by the source through the amount VGS(th).
The equation for voltage divider bias configuration is given here.

VGS=[R2/(R1 + R2)]VDD

VDS=VDD – IDRD

In this equation ID will be equal to K(VGS – VGS(th))²we have discussed this expression in last tutorial about MOSFET.
In drain feedback bias circuitry shown in figure the gate current is very less and so there will be no voltage drop about the resistance RG. So VGS = VDS.

D-MOSFET Bias Configuration

As we know D-MOSFET can operate with both positive and negative values of VGS voltage.
The basic method of biasing is to make VGS=0 so ac voltage at gate changes the gate to source voltage over this zero voltage biasing point.
Zero bais configuration for MOSFET is shown in below figure.
As VGS is zero and ID=IDSS as denoted. The drain to source voltage will be.

VDS = VDD – IDSSRD

The main function of RG is to sustain an ac voltage input by separating it from the ground as shown in figure denoted as b.
As there is no dc gate current there will be no effect of RG on zero gate-to-source bias.

How to bias a MOSFET

For biasing of MOSFET is voltage given at the source and gate pins of this transistor. The connected voltage on the MOSFET helps the current to move the drain towards to source. Current flow based on gate voltage and MOSFET structure

mosfet biasing circuits are

Fixed bias:
In this biasing technique, a voltage supply is given to the gate and source. It is a less stable type of biasing and simple considered
Voltage divider bias:
In this circuit reference voltage is produced with the creation of a voltage divider. The gate pin of MOSFET is connected with a reference voltage. It is a complicated structure but considered as fix
Self-bias:
It produces its own bias voltage.