Biasing circuit with a MOSFET: am I missing something

In summary, in order for both transistors to be saturated, the output voltage must be at least 4V, given a fixed VB of 5V. This is because the drain of M1 cannot be at 1.4V due to the fixed VB of 5V.
  • #1
Granger
168
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Homework Statement



I have the following circuit with MOSFET (cascode amplifier).

[![enter image description here][1]][1] [1]: https://i.stack.imgur.com/BAqQh.png
upload_2018-2-27_21-52-16.png

Information given:
##V_t=1V##
##I_1=20\mu A##
##k=10 \mu AV^{-2}##

Knowing that ##V_b## is 5V determine the minimum value of the output voltage so that both transistors are saturated.

Homework Equations


3. The Attempt at a Solution [/B]

I'm kinda confused on how to proceed on this.
First of all, don't we need VDD? How to relate VB with this?
I started by assuming both transistors had the current I_1 (I assume this is correct).
Then I used the MOSFET equation for saturation ##i_D=k(v_ {GS}-Vt)^2## to determine ##v_{GS}## in both transistors. obtaining 2.4 V.
Then I applied the boundary condition for saturation ##v_{DS}=v_{GS}-V_t## and so obtained for both transistors ##v_{DS}=1.4V##
So the minimum value of ##v_O## is 2.8V.

This is incorrect, the answer should be 4V. I'm really not understanding how to proceed... Can someone give me an hint on how to do this? Thanks!
 

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  • #2
What you're missing is that the drain of M1 cannot be at 1.4 V. This is because VB is fixed at 5V. The drain of M1 is instead at 5V - VGS2. Otherwise you're on the right track.
 

Related to Biasing circuit with a MOSFET: am I missing something

1. What is a biasing circuit with a MOSFET?

A biasing circuit with a MOSFET is a type of electronic circuit that uses a metal-oxide-semiconductor field-effect transistor (MOSFET) to control the flow of current in a circuit. It is used to maintain a constant voltage or current level in the circuit, allowing for stable and reliable operation.

2. How does a biasing circuit with a MOSFET work?

In a biasing circuit with a MOSFET, the MOSFET acts as a variable resistor, allowing the flow of current to be controlled by changing the gate voltage. By adjusting the gate voltage, the MOSFET can be biased to operate in either its linear or saturation region, resulting in a stable output voltage or current.

3. What are the advantages of using a MOSFET in a biasing circuit?

MOSFETs have several advantages in biasing circuits, including high input impedance, low output impedance, and low power consumption. They also have a wide range of voltage and current handling capabilities, making them suitable for a variety of applications.

4. Are there any potential drawbacks to using a biasing circuit with a MOSFET?

One potential drawback of using a biasing circuit with a MOSFET is the need for careful design and calibration. Improper biasing can lead to instability or even damage to the circuit. Additionally, MOSFETs can be sensitive to temperature and other environmental factors, requiring additional precautions in certain applications.

5. How do I choose the appropriate MOSFET for my biasing circuit?

Choosing the right MOSFET for a biasing circuit depends on several factors, including the desired output voltage or current, the input voltage and current, and the operating temperature. It is important to select a MOSFET with appropriate specifications for these factors to ensure reliable and efficient operation of the circuit.

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