Where Did I Go Wrong in Calculating Avo and Gv for a MOSFET Amplifier?

In summary, for a Common-Source amplifier with a MOSFET having µnCox = 400µA/V2, W/L = 10, and VA = 10V, biased at ID = 0.2 mA and using RD = 6 kΩ, the values of Rin, Avo, and Ro are not mentioned. However, the value of Ro is found to be 5.35kΩ, and the value of Avo is -15.81 V/V. The overall voltage gain, Gv, is calculated to be -0.940 V/V with a load resistance of 10kΩ. To achieve a 0.2V peak sine-wave signal at the output
  • #1
francisg3
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A Common-Source amplifier utilizes a MOSFET with µnCox = 400µA/V2, W/L = 10, and VA = 10V. It is biased at ID = 0.2 mA and uses RD = 6 kΩ. Find Rin, Avo and Ro. Also, if a load resistance of 10kΩ is connected to the output, what overall voltage gain Gv is realized? Now, what if a 0.2V peak sine-wave signal is required at the output, what must the peak amplitude of vsig be?


Below I have included the general model of the MOSFET along with what I believe is the equivalent pi-model.

I have found Ro to be RD//ro = (50*6)/(56) = 5.35kΩ (that part is correct according to the answer key.
where ro= VA/ID = 10/0.2 = 50kΩ

What I am having trouble with is the Avo and Gv.

I have the following:

ID = 0.5*Kn*(W/L)*(Vov^2)
subsituting values I get:
200 = 0.5*(400)*(10)*Vov^2
therefore Vov = 0.3162 Volts

gm = Kn*Vov
=(0.4)*(10)*(0.3162)

therefore Avo = -gm*(RD//ro)
= -(0.3162)*(5.3535kΩ)
= -6.76 (the answer is -2.14 V/V


Where did I go wrong? Any help would be appreciated!
 

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  • #2




Hello,

Thank you for your detailed response. It seems like you have the correct values for Ro and Vov. However, I believe your calculation for Avo is incorrect.

To find Avo, we need to use the small-signal equivalent circuit of the MOSFET, which includes the transconductance parameter (gm) and the output resistance (ro). The formula for Avo is Avo = -gm*ro.

In your calculation, you used the parallel combination of RD and ro, which is correct for finding Ro, but not for Avo. To find Avo, we need to use the value of ro, which is 50kΩ in this case.

Therefore, the correct calculation for Avo would be:
Avo = -gm*ro
= -(0.3162)*(50kΩ)
= -15.81 V/V

As for Gv, we need to take into account the load resistance of 10kΩ. Therefore, the overall voltage gain would be:
Gv = Avo/(1 + Avo*RL)
= (-15.81)/(1 + (-15.81)*(10kΩ))
= -0.940 V/V

To find the peak amplitude of vsig, we can use the formula:
vsig = Vpeak/Gv
= (0.2)/(0.940)
= 0.213 V

I hope this helps clarify the calculations for Avo and Gv. Let me know if you have any further questions. Keep up the good work!
 

FAQ: Where Did I Go Wrong in Calculating Avo and Gv for a MOSFET Amplifier?

What is a MOSFET amplifier configuration?

A MOSFET amplifier configuration is a type of electronic circuit using a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) as the amplifying device. It is commonly used in audio and power amplifiers due to its high input impedance, low output impedance, and high gain.

What are the different types of MOSFET amplifier configurations?

There are three main types of MOSFET amplifier configurations: common source, common drain (also known as source follower), and common gate. Each type has its own unique characteristics and is used for different purposes.

What are the advantages of using a MOSFET amplifier configuration?

MOSFET amplifiers have several advantages over other types of amplifiers. These include high input impedance, low noise, low distortion, and high bandwidth. They also have a simple circuit design, making them easy to implement in electronic devices.

What are the limitations of a MOSFET amplifier configuration?

One limitation of MOSFET amplifiers is their low output impedance. This means that they may not be suitable for driving low impedance loads, such as speakers. They also require a higher supply voltage compared to other amplifiers, which can be a limitation in some applications.

How do I choose the right MOSFET amplifier configuration for my project?

The choice of MOSFET amplifier configuration depends on the specific requirements of your project. Factors to consider include the desired gain, input and output impedance, frequency response, and power requirements. It is important to carefully analyze these factors and choose the configuration that best suits your needs.

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