Common-Source, Common-Gate, and Common-Drain Amplifiers
ELE 724
Chapter 1: Building Blocks of
CMOS Analog Integrated Circuits
Lecture 2 Review Notes
Fei Yuan
September 19, 2015
Fei Yuan: ELE 724 Chapter 1: Building Blocks of CMOS Analog Integrated Circuits Lecture 2 Review Notes
Common-Source, Common-Gate, and Common-Drain Amplifiers
Intrinsic Capacitances of MOSFETs
Figure: Intrinsic capacitances of MOSFETs.
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Intrinsic to field-effect based operation of MOSFETs.
Cutoff : Gate-substrate capacitance due to positive charge at G and negative
charge (irons) in gate voltage induced depletion region of substrate :
Cg = Cox WL.
Triode : Gate-channel capacitance from S to D due to positive charge at G and
negative charge (free electrons) in inversion layer. Cgs = Cgd = 12 Cox WL.
Saturation : Gate-channel capacitance at S only due to positive charge at G and
negative charge (free electrons) in tapped inversion layer. Cgs = 32 Cox WL and
Cgd ≈ 0.
Miller effect functions as capacitance amplification. For example, for CS,
C1 = Cgd (1 + gm Av ). In this case, C1 cannot be neglected despite small Cgd .
Fei Yuan: ELE 724 Chapter 1: Building Blocks of CMOS Analog Integrated Circuits Lecture 2 Review Notes
Common-Source, Common-Gate, and Common-Drain Amplifiers
Parasitic Capacitances of MOSFETs
Figure: Parasitic capacitances of MOSFETs.
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pn-Junction
qcapacitances (CJ ) : pn-junction capacitances at S/B and D/B :
CJ = CJo / 1 + VφR where CJo is junction capacitance at zero reverse biasing
voltage, VR is the revise biasing voltage of pn-junction, and φ ≈ 0.5V is built-in
potential of pn-junction.
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CJ is nonlinear as its value varies with VR .
For namometer MOSFETs, CJ is comparable to intrinsic capacitances −→ introduce
significant distortion !
Overlap capacitances (Cov ) : Caused by fabrication. They are linear.
Fringe capacitances (Cf ) : Linear, significant for namometer MOSFETs,
comparable to intrinsic capacitances.
Cov and Cf can be absorbed into Cgs and Cgd , leaving CJ the only parasitic
capacitances.
Fei Yuan: ELE 724 Chapter 1: Building Blocks of CMOS Analog Integrated Circuits Lecture 2 Review Notes
Common-Source, Common-Gate, and Common-Drain Amplifiers
Common-Source (CS) Amplifiers
Figure: Common-source amplifiers.
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Since vin = vGS while vGS > VT = 0.4V (for IBM 1‘30 nm), CC is attractive for
amplifying signals with a large dc offset voltage.
Cgs is floating −→ replaced with two single-ended capacitors C1 = Cgs (1 + gm ro )
at the gate (capacitance amplification or Miller effect) and
C2 = Cgs [1 + (gm ro )−1 ] ≈ Cgd at the drain.
Input pole : Located at ωin = 1/(Rs Cin ) where input capacitance Cin = Cgs + C1 .
Output pole : Located at ωout = 1/(Rout Cout ) where output resistance
Rout = ro1 ||ro2 and output capacitance Cout = Cdb + C2 ≈ C2 .
Fei Yuan: ELE 724 Chapter 1: Building Blocks of CMOS Analog Integrated Circuits Lecture 2 Review Notes
Common-Source, Common-Gate, and Common-Drain Amplifiers
Common-Source (CS) Amplifiers (continued)
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Voltage transfer function
Vo (s)
= Vs (s)
s
ωin
−gm Rout
+1
s
ωout
+1
.
(1)
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A two-pole system with input pole at ωin and output pole at ωout .
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The numerator −gm Rout is the voltage gain at low frequencies where the effect of
the capacitances is negligible while the denominator accounts for the effect of the
capacitances.
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ωout < ωin typically −→ dominant pole at the output.
Fei Yuan: ELE 724 Chapter 1: Building Blocks of CMOS Analog Integrated Circuits Lecture 2 Review Notes
Common-Source, Common-Gate, and Common-Drain Amplifiers
Common-Gate (CG) Amplifiers
Figure: Common-gate amplifiers.
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Since VB = vGS + vin while vGS > VT = 0.4V (for IBM 1‘30 nm), CG is
attractive for amplifying signals with a small dc offset voltage.
Input node is a low-impedance node and output node is a high-impedance node
−→ loading effect exists at input node as Rs = 50Ω typically.
Since the gate is an AC ground, no floating capacitor in CG −→ no Miller effect
(capacitance amplification) −→ large BW (This differs distinctly from CS).
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Input pole : Located at ωin = 1/ Rs || g1 Cin where input capacitance
m
Cin ≈ Cgs .
Output pole : Located at ωout = 1/(Rout Cout ) where output resistance
Rout ≈ ro1 ||ro2 and output capacitance Cout ≈ Cgd .
Fei Yuan: ELE 724 Chapter 1: Building Blocks of CMOS Analog Integrated Circuits Lecture 2 Review Notes
Common-Source, Common-Gate, and Common-Drain Amplifiers
Common-Gate (CG) Amplifiers (continued)
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Voltage transfer function
Vo (s)
= Vs (s)
s
ωin
gm Rout
s
+1
ω
out
+1
.
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The numerator is the voltage gain at low frequencies where the effect of the
capacitance is negligible while the denominator accounts for the effect of the
capacitances.
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ωout < ωin typically −→ dominant pole at the output.
Fei Yuan: ELE 724 Chapter 1: Building Blocks of CMOS Analog Integrated Circuits Lecture 2 Review Notes
(2)
Common-Source, Common-Gate, and Common-Drain Amplifiers
Common-Drain (CD) Amplifiers
Figure: Common-drain amplifier (source follower).
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CD is also known as source follower as the voltage of the source follows the input
voltage.
Since vin = vGS + vo while vGS > VT ≈ 0.4V (for IBM 130 nm) is required, a
proper dc bias voltage at input is critical.
Cgs is the only floating element. Since Av ≈ 1, Miller capacitance at input
C1 = Cgs (1 − Av ) ≈ 0 and Miller capacitance at output C2 = Cgs (1 − A−1
v ) ≈ 0.
−→ No Miller effect (capacitance amplification) −→ Large BW.
Input pole : Located at ωin = 1/(Rs Cin ) = 1/(Rs C1 ) ≈ ∞.
Output pole : Located at ωout = 1/(Cout Rout ) ≈ 1/(C2 /gm ) ≈ ∞.
CD is an excellent voltage buffer as it possess the following key characteristics :
Av ≈ 1, Rin = ∞, a small Rout = 1/gm , and a large BW (ωin , ωout ≈ ∞).
Fei Yuan: ELE 724 Chapter 1: Building Blocks of CMOS Analog Integrated Circuits Lecture 2 Review Notes