Examcrazy Logo
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 Small-Signal Amplifier Examples

MOSFET Small-Signal Amplifier Examples.
We will illustrate the analysis of small-signal MOSFET amplifiers through two examples in this lecture.
Example N29.1 (text example 4.10). Determine Av (neglecting the effects of RG), Rin, and Rout for the circuit below given that
Vt=1.5V,KaW|L=0.25 MA /V2, and VA=50 V

The first step is to determine the DC operating point. The DC equivalent circuit is:

Since Vg VD =0 < the MOSFET is operating in the saturation mode if ID =0 So, neglecting ro and assuming operation in the saturation mode

For this DC circuit

Notice in the circuit that VDS= VGS so that this last equation Becomes ID=0.125(VDS-1.5)2
Also, by KVL VDS=15-RDID=15-10,000 ID
Substituting (2) into (1) ID=1.25*10-4(15-10000ID-1.5)2
Solving this equation gives
ID=1.06 mA Þ VDS=4.4 V (=Vgs ID=1.72 mA Þ VDS=2.2 V (=Vgs
This latter result is not consistent with the assumption of operation in the saturation mode since Vt < VGS = 1.5V. So the proper solution for ID is the first ( ID =1.6 mA).
Next, we construct the small-signal equivalent circuit. Well use the p small-signal model of the MOSFET with ro included:

To compute the small-signal voltage gain, we start at the output (assuming RG is extremely large RG >> rt0|| RD|| RL ) V0-gmVgs(r0||RD||RL)
At the input notice that vgs = Vi. Therefore

Notice that the assumption RG|| r0|| RD|| RL is met and hugely exceeded since 10 MO >> 4,521 O. For the input resistance Rin calculation, we cannot set 0 gs vgs =0 and subsequently open circuit the dependent current source since this would artificially force Rin =0 . Rather, we need to determine ii as a function of vi and use this in the definition:

The dependent current source will remain in these calculations. Proceeding, at the input of the small-signal equivalent circuit shown above


Consequently, using this expression we find that

Lastly, to determine the output resistance, we can set 0 gs v = in the small-signal equivalent circuit above, which will open circuit the dependent current source leading to the equivalent circuit:

from which we see that Rout =||R0 ||RD|| r =4 =5.2 kO
Example N29.2 (text exercise 4.23). Determine the following quantities for the conceptual MOSFET small-signal amplifier of Fig. 4.34 given that VDD = 5 V, RD = 10 kO, and VGS = 2 V.

The MOSFET characteristics are Vt =1 V, kn ' =20 ?A/V2 2, W/L= 20, and . l = 0.
(a) Determine ID and VD. We see from the circuit that Vt >Vt . Therefore, the MOSFET is operating in the saturation or triode mode. Well assume saturation. In that case
Lets check if the MOSFET is operating in the saturation mode:
(c) Determine gm. Using (4.61)
(c) Determine the voltage gain Av. We begin by first constructing the small-signal equivalent circuit

Directly from this circuit,

(e) If ( ) 0.2sin vgs =0.2 V, find vd and the max/min vD.
Therefore, vd = - 0.8 (wt)V Hence VD|max=VD+Vd=3+0.8=3.8V
While VD|max=VD+Vd=3-0.8=2.2V
e) Determine the second harmonic distortion. From (4.57) or
(6) in the previous lecture notes, the drain current is given as


Substituting sin vgs =0.2 sin into this equation gives
iD(wt)= I D+80*10-6sin (wt) t+8*10-6 sin 2(wt) Using the trigonometry identity sin2 =1 /2- (wt) 1 /2cos (wt). this last expression becomes
iD=200+80 sin (wt)+4-4 cos (2wt) uA iD=204+80 sin (wt)-4 cos (2wt) uA The first term in iD is ID, the DC current. We see that there is a slight shift upward in value by 4 mA.
The third term in iD is the second harmonic term because it varies with time at twice the frequency of the input signal. The second harmonic distortion is
4 \80*100%=5%

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

Posted By :-
Aug 31, 12:14:03 PM 0 47887
Fourier Transform
Sir I want tutorial on Fourier Transform.........

Posted By :-
Jul 15, 3:24:49 PM 0 50926
fourier transform
sir i want tutorial on fourier transform

Posted By :-
Jul 11, 10:08:19 AM 0 47070
match filter
heloo sir ,i want a tutorial for match filter.plz send it as soon as possible it is very urgent.

Posted By :-
Jun 4, 2:25:18 AM 0 68158
Electic circuits
I want lecture notes for single phase ac & 3phase ac circuits

Posted By :-
May 21, 11:32:46 AM 1 78890
electro statics
what is the work done to move a charge? derive an expression for assembling a configuration of point charges

Posted By :-
May 12, 8:28:18 AM 0 46402
electromagnetic waves
i want lcr circiut teorems derivations

Posted By :-
May 12, 8:23:58 AM 0 46004
i want oscillator frequency derivations for all. plz let me know from where i can get that

Posted By :-
May 4, 5:21:01 PM 0 46831
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 :-
Apr 12, 6:33:57 PM 2 99580
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 :-
Jan 28, 7:17:23 PM 0 52903
communication system
analog and digital communication system, fiber optic communication, telecommunication technique and application, mobile communication

Posted By :-
Dec 17, 10:38:27 AM 0 59437

Posted By :-
Dec 14, 3:29:22 PM 0 58303
coaching in ies in indore
what about coaching

Posted By :-
Dec 7, 5:27:52 PM 0 62845

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

(Members Login)

  About us | Privacy Policy | Terms and Conditions | Contact us | Email: support@Examcrazy.com  
Copyright 2014 Extreme Testing House, India. All rights reserved.  3308