single stage amplifier Unit 5 AMVLSI

Dr. AMBEDKAR INSTITUTE OF TECHNOLOGY
HARSHA.R Assistant Professor, ECE,UNIT 5 1
BY: HARSHA.R
ASSISTANT PROFESSOR
ECE DEPARTMENT,DR AMBEDKAR INSTITUTE OF
TECHNOLOGY,BENGALURU
Mob:9620640486
ANALOG MIXED MODE VLSI
Single Stage Amplifier
UNIT V,ECE

Ideal vs Non-ideal Amplifier
• Ideal amplifier (Fig. a)
– Large-signal characteristic is a straight line
–α1 is the “gain”, α0 is the “dc bias”
• Nonlinear amplifier (Fig. b)
– Large signal excursions around bias point
– Varying “gain”, approximated by polynomial
– Causes distortion of signal of interest

Analog Design Tradeof
• Along with gain and speed, other parameters
also important for amplifiers
• Input and output impedances decide interaction
with preceding and subsequent stages
• Performance parameters trade with each other
– Multi-dimensional optimization problem

Common-Source stage with Resistive load
Vout
• Very high input impedance at
low frequencies
•For Vin < VTH, M1 is off and
= VDD
•When Vin > VTH, M1 turns on
in saturation region, Vout
falls
•When Vin > Vin1, M1 enters
triode region
•At point A, Vout = Vin1-VTH

•For Vin > Vin1,
•If Vin is high enough to drive M1
into deep triode region so that
Vout << 2(Vin - VTH),

•Taking derivative of ID equation
in saturation region, small-
signal gain is obtained
• Same result is obtained from
small-signal equivalent circuit
•gm and Av vary for large input
signal swings according to
• This causes non-linearity

• For large values of RD, channel-length modulation of M1
becomes significant, Vout equation becomes
• Voltage gain is
• Above result is also obtained from small-signal
equivalent circuit

Diode-Connected MOSFET
• A MOSFET can operate as a small-signal resistor if its
gate and drain are shorted, called a “diode-connected”
device
• Transistor always operates in saturation
• Impedance of the device can be found from small-signal
equivalent model

Diode-Connected MOSFET
• Including body-effect, impedance “looking into” the
source terminal of diode-connected device is found as

CS Stage with Diode-Connected Load
• Neglecting channel-length
modulation, using impedance
result for diode-connected
device,
where,
•Expressing gm1 and gm2 in terms of device dimensions,
• This shows that gain is a weak function of bias currents
and voltages, i.e., relatively linear input-output
characteristic

• From large-signal analysis,
•If VTH2 does not vary much with Vout, input-output
characteristic is relatively linear.
•Squaring function of M1 (from its input voltage to its drain
current) and square root function of M2 (from its drain
current to its overdrive) act as inverse functions

•As I1 falls, so does overdrive of M2 so that
•Subthreshold conduction of M2 eventually brings Vout to
VDD, but at very low current levels, finite capacitance at
output node CP slows down the change in Vout from VDD-
VTH2 to VDD.
•In high-frequency circuits, Vout remains around VDD-VTH2
when I1 falls to small values.

HARSHA.R Assistant Professor, ECE,UNIT 5
•For Vin < VTH1, Vout = VDD –VTH2
•When Vin > VTH1, previous large-signal analysis predicts
that Vout approximately follows a single line
•As Vin exceeds Vout + VTH1 (to the right of point A), M1
enters the triode region and the characteristic becomes
nonlinear.
14
13

CS Stage with Diode-Connected PMOS device
• Diode-connected load can be
implemented as a PMOS device, free of
body-effect
• Small-signal voltage gain neglecting
channel-length modulation Gain is a
relatively weak function of device
dimensions
• Since µn ≈ 2µp, high gain requires “strong”
input device (narrow) and “weak” load device
(wide)
• This limits voltage swings since for λ = 0, we
get
• For diode-connected loads, swing is constrained by both required
overdrive voltage and threshold voltage, i.e., for small overdrive,
output cannot exceed VDD - |VTH|.

CS Stage with Current-Source Load
• Current-source load allows a high load resistance without
limiting output swing
• Voltage gain is
•Overdrive of M2 can be reduced by increasing its width,
ro2 can be increased by increasing its length
• Output bias voltage is not well-defined
•Intrinsic gain of M1 increases with L and decreases with ID

CS Stage with Active Load
• Input signal is also applied to gate of load device, making
it an “active” load
•M1 and M2 operate in parallel and enhance the voltage
gain
• From small-signal equivalent circuit,
• Same output resistance as CS stage with current-source
load, but higher transconductance
•Bias current of M1 and M2 is a strong function of PVT

CS Stage with Active Load: Supply
sensitivity
•Variations in VDD or the threshold voltages directly
translate to changes in the drain currents
• Supply voltage variations “supply noise” are amplified
too
•Voltage gain from VDD to Vout can be found to be

CS Stage with Triode Load
•Ron2 depends on µpCox, Vb and VTHP which vary with PVT
•Generating a precise value of Vb is complex, which makes
circuit hard to use
• Triode loads consume lesser voltage headroom than
diode connected devices since Vout,max =V
• A MOS device biased in the
deep triode region acts as a
resistive load in a CS stage
•Vb is sufficiently low to ensure
M2 is in deep triode region for
all output voltage swings
•Voltage gain is Av = -gm1Ron2, where Ron2 is the MOS ON
resistance given by
HARSHA.R Assistant Professor, ECE,UNIT 5

CS Stage with Source Degeneration
•Degeneration resistor RS in series with source terminal
makes input device more linear
–As Vin increases, so do ID and the voltage drop across
RS
–Part of the change in Vin appears across RS rather
than gate-source overdrive, making variation in ID
smoother
•Gain is now a weaker function of gm

• Nonlinearity of circuit is due to nonlinear dependence of
ID upon Vin
•Equivalent transconductance Gm of the circuit can be
defied as

•gm is the transconductance of M1
•Small-signal voltage gain Av is then given by

•Same result for Gm is obtained from small-signal
equivalent circuit, by noting that
•As RS increases, Gm becomes a weaker function of gm and
hence ID
• For , , i.e.,
•Most of the change in Vin across RS and drain current
becomes a “linearized” function of input voltage

•Including body-effect and channel-length modulation, Gm
is found from modified small-signal equivalent circuit

Large-signal behavior
RS=0
•ID and gm vary with Vin as
derived in calculations in
Chapter 2
RS≠0
• At low current levels,
turn-on behavior is similar
to when RS=0 since
and hence
•As overdrive and gm
increase, effect of RS
becomes more significant

• Small-signal derived previously can be written as
• Denominator = Series combination of inverse
transconductance + explicit resistance seen from source
to ground
• Called “resistance seen in the source path”
• Magnitude of gain = Resistance seen at the drain/ Total
resistance seen in the source path

• Degeneration causes increase in output resistance
•Ignoring RD and including body effect in small-signal
equivalent model,
•ro is boosted by a factor of {1 + (gm+gmb)RS} and then
added to RS
•Alternatively, RS is boosted by a factor of {1 + (gm+gmb)ro}
and then added to ro

•Compare RS = 0 with RS > 0
•If RS = 0, and
•If RS > 0, and , obtaining negative gmV1
and gmbVbs
•Thus, current supplied by VX is less than VX/ro and hence
output impedance is greater than ro

Intuitive understanding of increased output impedance
•Apply voltage change ΔV at output and measure resulting
change ΔI in output current, which is also the change in
current through RS
•Resistance seen looking into the source of M1 is
(gm + gmb)
•Voltage change across RS is
1/

Intuitive understanding of increased output impedance
•Change in current across RS is
•Output resistance is thus

• To compute gain in the general case including body effect
and channel-length modulation, consider above small-
signal model
• From KVL at input,
• KCL at output gives

•Since voltage drops across rO and RS must add up to Vout,
• Voltage gain is therefore

Thank you
Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or
distribution without the prior written consent of McGraw-Hill Education.
BY: HARSHA.R
ASSISTANT PROFESSOR
ECE DEPARTMENT,DR AMBEDKAR
INSTITUTE OF TECHNOLOGY,BENGALURU
Mob:9620640486

single stage amplifier Unit 5 AMVLSI

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single stage amplifier Unit 5 AMVLSI