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22-1
EE105 – Fall 2015 Microelectronic Devices and Circuits
Prof. Ming C. Wu
511 Sutardja Dai Hall (SDH)
22-2
Typical Frequency Response of Discrete MOS or BJT Amplifiers
22-3
Frequency Response of Direct-Coupled (dc) Amplifier
Gain does not fall off at low frequencies, and the midband gain AM extends down to zero frequency
22-4
Cross Section of MOSFET Showing Internal Capacitances
MOSFET has several internal capacitances, which take time to charge/discharge, limitingthe transistor speed.
Gate/Source capacitance:
Cgs =23WLCox +Cov
where Cox = εox / t [F/cm2 ]W : transistor widthL : gate lengthCov =WLovCox
Lov : overlap bet. gate/sourceor gate/drain, ~ 0.05 to 0.1L
Gate/Drain capacitance:Cgd =Cov
22-5
(Simplified) High-Frequency Equivalent-Circuit Model for MOSFET
Hybrid-pi Model T Model
Capacitances between source/body, Csb, and between drain/body, Cdb, are neglected
22-6
Unity-Gain Frequency, fT
Vgs =Ii
sCgs + sCgd
KCL: Io +Vgs
1/ sCgd
= gmVgs
Io = gmVgs − sCgdVgs ≈ gmVgs
= gmIi
sCgs + sCgd
AI =IoIi=
gmsCgs + sCgd
s = jω
IoIi=
gmω(Cgs +Cgd )
ωT =gm
Cgs +Cgd
fT : defined as frequency at which short-circuit current gain = 1fT : a figure-of-merit for transistor speed
Drain is grounded (short-circuit load)
As gate length reduces in advanced technology node, Cgs reduces and fT increases
22-7
High Frequency Response of CS Amplifier
Frequency response can be solvedby analyzing the gain of the high-frequency equivalent circuit.However, it is tedious, and doesnot show intuitive interpretation.
Vo = (Igd − gmVgs )RL' ≈ −gmVgsRL
'
Igd << gmVgs (Cgd is small)
RL' = ro || RD || RL
Igd =Vgs −Vo1/ sCgd
=Vgs + gmRL
'Vgs1/ sCgd
Igd =Vgs (1+ gmRL
' )1 / sCgd
22-8
Miller Capacitance Igd =Vgs (1+ gmRL
' )1 / sCgd
=Vgs
1/ s(1+ gmRL' )Cgd
=Vgs
1/ sCeq
Ceq = (1+ gmRL' )Cgd
Vgs =RG
Rsig + RGVsig
!
"##
$
%&&
1/ sCin
Rsig' +1/ sCin
=RG
Rsig + RGVsig
!
"##
$
%&&
1sωH
+1
ωH =1/CinRsig'
Cin =Cgs + (1+ gmRL' )Cgd
AV (s) = −RG
Rsig + RGgmRL
'!
"##
$
%&&
1sωH
+1
Large input capacitance in CS amplifier due to Miller effect, which greatly reduced the bandwidth of CS amp
22-9
Miller Theorem Miller Equivalent Circuit
If V2 = KV1
Input side:
I = V2 −V1
Z=
(K −1)V1
Z=
V1
Z(K −1)
=V1
Z1
Z1 =Z
(K −1)
Ouput side:
I = V2 −V1
Z=
(1−1/K)V2
Z=
V2Z
1−1/K
=V2
Z2
Z2 =Z
1−1/K
22-10
Open-Circuit Time Constant (OCTC) Method for High Cut-off Frequency
1. Replace all capacitance by open circuit2. Replace signal source by short circuit3. Consider one capacitor at a time, find resistance Ri "seen" by the i-th capacitor, Ci
4.
ωH ≈1CiRi
i∑
22-11
Applying OCTC to CS Amplifier
Rgs = Rsig'
Ix = gmVgs +VdRL' = gmVgs +
Vx +VgsRL'
Vgs = −IxRsig'
Rgd =VxIx= Rsig
' (1+ gmRL' )+ RL
'
RCL= RL
'
22-12
Applying OCTC to CS Amplifier
τ H = Rsig' Cgs + Rsig
' (1+ gmRL' )+ RL
'( )Cgd + RL' CL
Rearranging :
τ H = Rsig' Cgs + Rsig
' (1+ gmRL' )+ RL
'( )Cgd
= Rsig' Cgs + Rsig
' (1+ gmRL' )Cgd + RL
' Cgd + RL' CL
≈ Rsig' Cgs + (1+ gmRL
' )Cgd( )+ RL' Cgd +CL( )
Time constant from input port of
Miller Equivalent Circuit
Time constant from Output port of
Miller Equivalent Circuit
22-13
High-Frequency Response of CG Amplifier
Rsig ||1gm
+RLgmro
!
"#
$
%&
Note: Cgd and CL can be lumped togetheer since they are in parallel.Rgd = RL || Ro ≈ RLRo = ro + (1+ gmro )Rsig
τ H = Rgd Cgd +CL( )+ RgsCgs
= RL Cgd +CL( )+ Rsig ||1gm
+RLgmro
!
"#
$
%&Cgs
fH =12π
1τ H
• No Miller effect since both capacitance are grounded • The dominant term is likely to be (1/gm)Cgs, which is small à High fH à Common-Gate is a broadband amplifier
22-14
Impedance Transformation of Common Gate Amplifier
Impedance transformation:
Look into Drain : Rs amplified by gmro( )Look into Source : RL reduced by gmro( )
From Lecture Slide: 17-5
22-15
More Detailed Analysis of Resistances in OCTC