MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE) PDF Download

Introduction

In this article, you will find the Circuits Analysis and Applications of Diodes, BJT, FET and MOSFET which will cover the topics such as Basics of MOSFET, Drain Current equation for Triode and Saturation Region, Operating Condition of MOSFET, MOS Transconductance, Different biasing methods of MOSFET.


Basics of MOSFET

  • A Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is a field effect transistor (FET with an insulated gate) where the voltage determines the conductivity of the device. It is used for switching or amplifying signals. 
  • The ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. MOSFETs are now even more common than BJTs in digital and analog circuits.
    MOSFET Structure
    MOSFET Structure

Question for MOSFET Biasing & Amplifiers
Try yourself:Which of the following terminals does not belong to the MOSFET?
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Drain Current Equation

  • MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE) 
    where μn = mobility of electron
    Cox = Capacitance of oxide layer
    Cox = εox/tox, εr = 3.9 for SiO2
  • εox = 3.9ε0
    = 3.9 x 8.85 x 10-12 F/m
    = 3.45 x 10-11 F/m
    W/L → aspect ratio
    VDS = drain to source voltage

There are three possible regions for the working of the MOSFET:

  1. Triode Region
  2. Cut-off Region
  3. Saturation Region

1. Triode Region

  • VDS < VGS – Vt, if VDS (mV)
  • ID = μnCax(W/L)(VGS-VT)VDS
  • RDS = VDS/ID = MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)

2. Current Saturation

  • VDS ≥ VGS – Vt
  • (VDS)Sat = VGS – Vt
  • ID = 1/2(μnCox(W/L)(VGS-Vt)2
  • 1/2nCox(W/L)=kn Transconductance parameter) mA/V2
    → gm should be more, so kn should be more μn → faster, gain → higher.
    → A good MOSFET should have high value of kn
  • ID = kn(VGS-Vt)2
    ∴gm = ∂ID/∂Vas = 2kn(VGS-Vt)

Question for MOSFET Biasing & Amplifiers
Try yourself:For MOSFET is to be used as a switch then it must operate in:
View Solution

 

Operating Condition for MOSFET

Table: Operating Condition for N channel Enhancement type MOSFET.
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)


Table: Operating Condition for P channel Enhancement type MOSFET.
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)


Table: Operating Condition for N channel depletion type MOSFET.
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)


Table: Operating Condition for P channel depletion type MOSFET.
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)


MOS Transconductance

  • As a voltage-controlled source, a MOS transistor can be characterized by its transconductance
    MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)
  • Various dependencies of gm:
    MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)


Different biasing methods of MOSFET

There are four biasing methods for MOSFET:

  1. Drain to gate bias
  2. Voltage divider bias
  3. Fixed bias
  4. Self-bias

1. Drain to Gate Bias Configuration
Drain to gate bias Configuration is shown below:
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)
DC Equivalent of above circuit:
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)

DC Analysis:

  • Gate current, IG =0
  • So, we have voltage drop across resistance RG = VRG = 0
  • Therefore, we get a direct connection between drain and source i.e. VD = VG
    VDS = VGS
  • Note: Drain to gate bias always enables MOSFET in saturation region
    For output circuit, we have VDS = VDD – IDRD

2. Voltage Divider Bias Configuration
Voltage Divider Configuration is show below:
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)
DC Equivalent:
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)


DC – ANALYSIS:

  • Using voltage divider, gate voltage is obtained by:
    MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)
  • Applying KVL is loop 1, we get
    VG – VGS – IDRS = 0
    VGS = VG – IDR…. (1)
  • Assume that MOSFET is in saturation, so we have ID = Kn (VGS – VTN)2
  • By solving the quadratic equation, determine the value of VGS or ID, then apply KVL in source to drain loop
    VDD – IDRD – VDS – ISRS = 0
    VDS = VDD – ID (RS + RD)
  • If VDS > VGS – VTN, then the transistor is indeed biased in saturation region, as we have assumed.
  • However, if VDS < VDS (sat), then transistor is biased in the non saturation region
    Therefore from equation (1)
    VGS = VG – IDRS
    MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)

3. Fixed bias Configuration

Fixed bias Configuration is shown below:
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE) 
DC Equivalent:
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)
(In DC model, RG is short and input impedance is very high i.e. (IG ≃ 0))

Drawback of fixed bias: It is a dual battery design which makes it expensive and more space occupied bias Configuration.


4. Self-bias Configuration
Self Bias Configuration is shown below:
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)
DC Equivalent:
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE)

DC ANALYSIS:

0 = VGS + IDRS
VGS = – IDRS
MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE) 

The document MOSFET Biasing & Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE) is a part of the Electronics and Communication Engineering (ECE) Course Analog Circuits.
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