# MOSFET Bias Circuit and Equations

• baby_1
In summary, the conversation discusses designing an amplifier with Mesfet and the difficulties in determining the DC bias due to the lack of equations and information in the datasheet. A suggestion is given to measure the MOSFET and use an equation to solve for Vgs(th) and K coefficient, but it is noted that this may not be accurate in the real world due to process spread and deviation from theoretical equations. The question is raised about the accuracy of using MOSFET or JFET equations for Mesfet.
baby_1
Hello
I'm starting to design my amplifier with Mesfet.I calculated my stable amplifier with scattering parameters,however I don't know how do dc bias because I don't have any dc equations for Mesfet and and I can't extract dc information from datasheet (because datasheets only define IDSS (without threshold voltage such as FETs equation) and Vp (without define K coefficient such as Mosfet's equation).

You can always measure your MOSFET and then you will know Vp and K coefficient.

https://www.physicsforums.com/threa...vds-for-mosfet-amplifier.660440/#post-4206467

Or simple take two measurements Id1 for Vgs1 and Id2 for Vgs2 .
And use this equation to solve for Vgs(th) = Vp = Vt

$$\Large Vt =\frac{V_{GS1} \sqrt{ \frac{I_{D2}}{I_{D1}}} - V_{GS2} } { (\sqrt{ \frac{I_{D2}}{I_{D1}}} - 1) }$$

And K = Id1/(Vgs1 - Vgs(th))^ 2

But this equations are not very useful in real world because MOSFET show great process spreader. Also real world MOSFET characteristic is far from theoretical quadratic equation.

baby_1
Thanks dear Jony130
as you know In Mesfets' datasheet we have only IDSS and VP parameters.does we have any equation for Mesfets? because it doesn't clear when we should use Mosfet equation or JFET equations for MESFET.
and does them accurate for Mesfet?

## What is a MOSFET Bias Circuit?

A MOSFET bias circuit is a circuit that is used to establish and maintain the proper operating conditions for a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). It ensures that the MOSFET operates within its specified parameters and provides the desired output.

## What are the main components of a MOSFET Bias Circuit?

The main components of a MOSFET bias circuit include a DC power supply, resistors, and capacitors. The DC power supply provides the necessary voltage for the circuit, while the resistors and capacitors are used to set the bias voltage and to stabilize the circuit.

## What is the purpose of a bias voltage in a MOSFET Bias Circuit?

The bias voltage in a MOSFET bias circuit is used to set the operating point of the MOSFET. It ensures that the MOSFET is biased in the linear region, allowing it to amplify the input signal without distortion.

## What are the different types of bias circuits for MOSFETs?

There are two main types of bias circuits for MOSFETs: fixed bias and self-bias. Fixed bias circuits use a fixed voltage source to establish the operating point, while self-bias circuits use the MOSFET's own characteristics to set the operating point.

## What are the equations used in a MOSFET Bias Circuit?

The two main equations used in a MOSFET bias circuit are the drain current equation (ID = β(VGS - VTH)²) and the drain-to-source voltage equation (VDS = VDD - ID(RD + RDS)). These equations are used to determine the bias voltage and current for the MOSFET.

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