Examcrazy Logo
HOME  SITEMAP CONTACT US LOGIN
HOME Engineering AIEEE GATE IES DRDO-SET BSNL-JTO
MBA in India CAT How to Prepare for Exams Technical Freshers Jobs
  Follow us|  twitter  Orkut  facebook
MOSFET Tutorials
   Enhancement Type MOSFET Operation, P-Channel, and CMOS
   MOSFET Circuit Symbols, iD-vDS Characteristics
   MOSFET as an Amplifier. Small-Signal Equivalent Circuit Models
   MOSFET Small-Signal Amplifier Examples
   Biasing MOSFET Amplifiers. MOSFET Current Mirrors
   Biasing MOSFET Amplifiers. MOSFET Current Mirrors
   Common Source Amplifier
   Common Source Amplifier with Source Degeneration.
   CMOS Common Source Amplifier
   MOSFET Common Gate Amplifier
   CMOS Common Gate Amplifier
Free Electronics Tutorials
   Diode Tutorials
   BJT Tutorials
   MOSFET Tutorials
   Electronics II Tutorials
   Applied Electromagnetics Tutorials
   Microwave Tutorials
GATE preparation tips
   GATE Books & How to prepare
   Objective Solving Tricks
   Other GATE links
   IES exam preparation
   All about DRDO-SET
More Engineering Links
   Directory of coaching Institutes
   Govt engg college rankings
   Private engg college rankings
   Admission notifications for Mtech/PhD
   All Engineering Colleges in India
MOSFET as an Amplifier. Small-Signal Equivalent Circuit Models

As with the BJT, we can use MOSFETs as AC small-signal amplifiers. An example is the so-called conceptual MOSFET amplifier shown in

This is only a “conceptual” amplifier for two primary reasons:
1. The bias with VGS is impractical. (Will consider others later.)
2. In ICs, resistors take up too much room. (Would use another triode-region biased MOSFET in lieu of RD.)
To operate as a small-signal amplifier, we bias the MOSFET in the saturation region. For the analysis of the DC operating point,
we set vgs =0 so that from (4.22) with l=0

From the circuit
VDS=VDD-IDRD
For operation in the saturation region
VGD £ Vt ÞVGS-VDS £ Vt
Or
V DS ³ VGS-Vt
where the total drain-to-source voltage is

Similar to what we saw with BJT amplifiers, we need make sure that (3) is satisfied for the entire signal swing of vds. With an AC signal applied at the gate
VGS=VGS+Vg Substituting (4) into (4.20)


The last term in (6) is nonlinear in vgs, which is undesirable for a linear amplifier. Consequently, for linear operation we will require that this term be small:

If this small-signal condition (7) is satisfied, then the total drain current is approximately the linear summation

where

From this expression (9) we see that the AC drain current id is related to vgs by the so-called transistor transconductance, gm:

which is sometimes expressed in terms of the overdrive voltage

Because of the VGS term in (10) and (11), this gm depends on the bias, which is just like a BJT. Physically, this transconductance gm equals the slope of the iDvGS characteristic curve at the Q point:


In the case that the MOSFET has a non-zero channel-length modulation coefficient (i.e., 0 .. ), then the drain current is given from (4.22) to be

Using (13) in (12) then

Lastly, it can be easily show that for this conceptual amplifier in

Consequently, Am gv , which is the same result we found for a similar BJT conceptual amplifier [see (5.103)].
MOSFET Small-Signal Equivalent Models
For circuit analysis, it is convenient to use equivalent smallsignal models for MOSFETs – as it was with BJTs. The MOSFET acts as a voltage controlled current source. The control voltage is vgs and the output current is iD, which gives rise to this small-signal p model:

Things to note from this small-signal model include:
1. ig =0 and vgs ¹0Þ. infinite input impedance.
2. ro models the finite output resistance. Practically speaking, it will range from 10 k Mbeta
3. From (10) we found (l .=0 )

Alternatively, it can be shown that

which is similar to g mV = 1|Vtfor BJTs. One big difference from BJTs is 25 Vt mV while V eff =0.1 V or greater. Hence, for the same bias current gm is much larger for BJTs than for MOSFETs. A small-signal T model for the MOSFET is shown in

Notice the direct connection between the gate and both the dependent current source and 1/gm. While this model is correct, we’ve added the explicit boundary condition that ig = 0 to this small-signal model. It isn’t necessary to do this because the currents in the two vertical branches are both equal to ggs vm , which means ig =0 . But adding this condition ig =0 to the small-signal model in Fig. 4.40a makes this explicit in the circuit calculations. (The T model usually shows this direct connection while the p model usually doesn’t.)
MOSFETs have many advantages over BJTs including:
1. High input resistance
2. Small physical size
3. Low power dissipation
4. Relative ease of fabrication.
One can combine advantages of both technologies (BJT and MOSFET) into what are called BiCMOS amplifiers:

Such a combination provides a very large input resistance from the MOSFET and a large output impedance from the BJT.


Discuss about MOSFET here
   START NEW THREADS
Discussion Board for MOSFET
You can discuss all your issues on MOSFET here
Thread / Thread Starter Last Post Replies Views
fourier transform
sir,
plz send me the notes for fourier transforms its very urgent.


Posted By :-
 shruthi.s
Aug 31, 12:14:03 PM 0 60013
Fourier Transform
Sir I want tutorial on Fourier Transform.........


Posted By :-
 jainpooja.1989@yahoo.com
Jul 15, 3:24:49 PM 0 63021
fourier transform
sir i want tutorial on fourier transform


Posted By :-
 apseng2005
Jul 11, 10:08:19 AM 0 59384
match filter
heloo sir ,i want a tutorial for match filter.plz send it as soon as possible it is very urgent.


Posted By :-
 richa
Jun 4, 2:25:18 AM 0 80495
Electic circuits
I want lecture notes for single phase ac & 3phase ac circuits


Posted By :-
 kalaivanisudhagar
May 21, 11:32:46 AM 1 100008
electro statics
what is the work done to move a charge? derive an expression for assembling a configuration of point charges


Posted By :-
 harismhkt
May 12, 8:28:18 AM 0 58904
electromagnetic waves
i want lcr circiut teorems derivations


Posted By :-
 harismhkt
May 12, 8:23:58 AM 0 58352
oscillators
i want oscillator frequency derivations for all. plz let me know from where i can get that


Posted By :-
 shruthi.s
May 4, 5:21:01 PM 0 59499
Equivalent circuit Models
I've got a question in one of my revision papers,

Explain the advantages of representing a transistor by means of an equivalent circuit circuit model

The only thing i can remember is that you can take complex circuits and break them down into simpler circuits which are easier to understand,

Is this the only advantage or do you have any more?

Please help!


Posted By :-
 ally79
Apr 12, 6:33:57 PM 2 126467
temperature Vs reverse satuation current
I want to know the variation of reverse saturation current with the increase in temperature for both germanium and silicon diodes


Posted By :-
 bhrahma@yahoo.co.in
Jan 28, 7:17:23 PM 0 65434
communication system
analog and digital communication system, fiber optic communication, telecommunication technique and application, mobile communication


Posted By :-
 hirakec@gmail.com
Dec 17, 10:38:27 AM 0 71618
CMOS TRANSISTOR
I WANT A TUTORIAL FOR CMOS TRANSISTORS.


Posted By :-
 maroofalamkhan
Dec 14, 3:29:22 PM 0 71261
coaching in ies in indore
what about coaching


Posted By :-
 satish21
Dec 7, 5:27:52 PM 0 75672

To start your new thread you must login here.
New user signup at ExamCrazy.com Exam Crazy
To reply/post a comment you need to login, Use your user name and password to login if you are already registered else register here

EXISTING USER LOGIN
(Members Login)
Username:
Password:
NEW USER REGISTERATION FORM
Login-Id
Email-ID
Password
Confirm-Password
Full-Name

  About us | Privacy Policy | Terms and Conditions | Contact us | Email: support@Examcrazy.com  
Copyright © 2014 Extreme Testing House, India. All rights reserved.  4476