Transcript Small Signal MOSFET Ch-4-6

Small Signal Model
MOS Field-Effect Transistors
(MOSFETs)
1
Quiz No 3 DE 27 (CE)
20-03-07
Rout.
(a)
(b)
(c)
Draw small signal model (4)
Find expression for Rout (2)
Prove vo/vsig = (β1α2RC)/(Rsig+rπ) (4).
A traveler once came to the mosque to see the prophet.
After greeting the prophet he was asked where he was
from. The traveler replied that he came from very far
just to get a few questions answered. Following is the
dialogue between the traveler and the prophet.
Traveler: I do not want adhaab to be written in my account.
Prophet: behave well with your parents
Traveler: I want to be known amongst people as an
intelligent person.
Prophet: fear Allah always.
Traveler: I want to be counted amongst Allah's favorites.
Prophet: recite Quran every morning and evening.
Traveler: I want my heart to always be enlightened.
(roshan and munawer)
Prophet: never forget death
Traveler: I never want to be away from Allah's blessing.
Prophet: always treat fellow creatures well.
Traveler: I never want to be harmed by my enemies.
Prophet: always have faith in only Allah.
Traveler: I never want to be humiliated.
Prophet: becareful of your actions.
Traveler: I wish to live long.
Prophet: always do sila-rahm. (Goodness towards blood relations)
Traveler: I want my sustenance to increase.
Prophet: always be in wudhoo.
Traveler: I wish to stay free of adhaab in the grave.
Prophet: always wear pure (paak) clothes.
Traveler: I never want to burn in hell.
Prophet: control your eyes and tongue.
Traveler: how do I get my sins forgiven?
Prophet: always ask forgiveness from Allah with a lot of humility.
Traveler: I want people to respect me always.
Prophet: never extend your hands of need at people.
Traveler: I want to always be honored.
Prophet: never humiliate or put down anyone.
Traveler: I don't want to be squeezed by fishare qabr.(Squeezing in the
grave)
Prophet: recite sura e mulk often.
Traveler: I want my wealth to increase.
Prophet: recite sura e waqia every night.
Traveler: I want to be safe and at peace on day of judgment.
Prophet: do zikr of Allah from dusk to night.
Traveler: I want to be in full attention and concentration during namaaz.
Prophet: always do wudhoo with concentration and attention
Enhancement-type NMOS transistor:
Figure 4.2 The enhancement-type NMOS transistor with a positive voltage applied to the gate. An n channel is induced at
the top of the substrate beneath the gate.
Enhancement-type NMOS transistor:
MOSFET Analysis
iD = iS, iG = 0
Large-signal equivalent-circuit model of an n-channel
MOSFET : Operating in the saturation region.
Large-signal equivalent-circuit model of an p-channel
MOSFET : Operating in the saturation region.
Large Signal Model : MOSFET
Transfer characteristic of an amplifier
Conceptual circuit utilized to study the operation of the
MOSFET as a small-signal amplifier.
The DC BIAS POINT
To Ensure Saturation-region Operation
Signal Current in Drain Terminal
Total instantaneous voltages vGS and vD
Small-signal ‘π’ models for the MOSFE
Common Source amplifier circuit
Example 4-10
Small Signal ‘T’ Model : NMOSFET
Small Signal Models
‘T’ Model
Single Stage MOS Amplifier
Amplifiers Configurations
Common Source Amplifier (CS) :Configuration
Common Source Amplifier (CS)
• Most widely used
• Signal ground or an ac earth is at the source
through a bypass capacitor
• Not to disturb dc bias current & voltages
coupling capacitors are used to pass the
signal voltages to the input terminal of the
amplifier or to the Load Resistance
• CS circuit is unilateral –
– Rin does not depend on RL and vice versa
Small Signal Hybrid “π” Model
(CS)
Small Signal Hybrid “π” Model : (CS)
Rin  RG
R o  ro || RD
vo
vo v gs
Gv 


vsig v gs vsig
RG
v gs 
vsig
RG  Rsig
vo   g m v gs ro || RD || RL 
 RG
vo
Gv 
  g m ro || RD || RL 
v gs
 RG  Rsig




Small-signal analysis performed directly on the amplifier
circuit with the MOSFET model implicitly utilized.
Rin  RG
R o  ro || RD
 RG 
vo

  g m ro || RD || RL 


vgs
 RG  Rsig 
Common Source Amplifier (CS)
Summary
• Input Resistance is infinite (Ri=∞)
Rin  RG
• Output Resistance = RD
R o  ro || RD
• Voltage Gain is substantial
 RG 
vo

  g m ro || RD || RL 
R R 
vgs
sig 
 G
Common-source amplifier
with a resistance RS in the source lead
The Common Source Amplifier
with a Source Resistance
• The ‘T’ Model is preferred, whenever a
resistance is connected to the source terminal.
• ro (output resistance due to Early Effect) is not included, as it
would make the amplifier non unilateral & effect
of using ro in model would be studied in Chapter
‘6’
Small-signal equivalent circuit with ro neglected.
i
vg
1
 RS
gm
Small-signal Analysis.
Rin  RG
Ro  RD
Voltage Gain : CS with RS
vo
vo v gs vi
Gv 



vsig v gs vi vsig
vo   g m v gs RD || RL 
1
gm
vi
v gs 
vi 
1
1  g m RS
 RS
gm
RG
vi 
vsig
RG  Rsig
 RG  g m RD || RL  
vo


 Gv  

 R  R  1 g R
vsig
sig 
m S

 G
Common Source Configuration with Rs
• Rs causes a negative feedback thus
improving the stability of drain current of
the circuit but at the cost of voltage gain
• Rs reduces id by the factor
– (1+gmRs) = Amount of feedback
• Rs is called Source degeneration
resistance as it reduces the gain
Small-signal equivalent circuit directly on Circuit
A common-gate amplifier based on the circuit
Common Gate (CG) Amplifier
• The input signal is applied to the source
• Output is taken from the drain
• The gate is formed as a common input &
output port.
• ‘T’ Model is more Convenient
• ro is neglected
A small-signal equivalent circuit
A small-signal Analusis : CG
vi
vi
1
Rin  

ii g m vi g m
Rout  RD
A small-signal Analusis : CG
Gv 
vo vo vi
 
vsig vi vsig
vo  g m vi RD || RL 
1
gm
vsig
Rin
vi 
vsig 
vsig 
1
Rin  Rsig
1  g m Rsig
 Rsig
gm
Gv 
vo
g R || RL 
 m D
vsig
1  g m Rsig
Small signal analysis directly on circuit
The common-gate amplifier fed with a current-signal input.
Summary : CG
4. CG has much higher output Resistance
5. CG is unity current Gain amplifier or a Current Buffer
6. CG has superior High Frequency Response.
A common-drain or source-follower amplifier.
Small-signal equivalent-circuit model
Small-signal Analysis : CD
(a) A common-drain or source-follower amplifier
:output resistance Rout of the source follower.
Rout
 1  1
 ro ||   
 gm  gm
(a) A common-drain or source-follower amplifier. : Smallsignal analysis performed directly on the circuit.
Common Source Circuit (CS)
Common Source Circuit (CS) With RS
Common Gate Circuit (CG)
Current Follower
Common Drain Circuit (CD)
Source Follower
Summary & Comparison
Small Signal Model
Small Signal Model
VBE=0.7 V
β =200
K’n(W/L)=2mA/V2
Vt=1V
Figure P6.123
DC Analysis
Figure P6.123