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Department of Electronics Engineering
Sahand University of Technology
NMOS inverter with an n-channel enhancement-mode mosfet with
the gate connected to the drain
Farshad Gozalpoor
Professor : Dr Najafi Aghdam
winter 93
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Abstract
 We will analyze the dc voltage transfer characteristics of
this inverter in this figure.
 We will also define and develop the noise margin of this
digital circuit in terms of the inverter voltage transfer
curve.
 We will then determine the impact of the body effect
on the dc voltage transfer curve and the logic levels.
 A transient analysis of the NMOS inverter will
determine the propagation delay time in NMOS logic
circuits.
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Diode connected transistor
The drain current is zero.
A nonzero drain current is induced in the device.
We can see that the following condition is satisfied :
A transistor with this connection always operates in the saturation region
when not in cutoff.
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Some definitions




VIH = minimum HIGH input voltage
VIL = maximum LOW input voltage
VOH= minimum HIGH output voltage
VOL = maximum LOW output voltage
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HINT:
In the following analysis, we neglect the body effect and we assume all
threshold voltages are constant. These assumptions do not seriously affect the
basic analysis or the inverter characteristics.
VTC computations
The driver is cut off and the drain currents are zero.
M1 : Off
M2 : Sat
Assumptin :
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M1: Sat
M2: Sat
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Computing Vin1 and Vo1 :
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c)
M1 : Triode
M2 : Sat
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Assumption:
Vin=1.5
K1=20
VO+ = 2.069
K2=10
VO - = 0.31
Vth=0.41v
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Computing VIH :
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Analysis of the transient behavior of the gate
Drain diffusion capacitances of the NMOS
transistors, the capacitance of the connecting
wires, and the input capacitance of the fan-out
gates
 A fast gate is built either by keeping
the output capacitance small or by
decreasing the on-resistance of the
transistor.
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Switching Threshold
We solve the case where the supply voltage
is high so that the devices can be
assumed to be velocity-saturated
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Where
For large values of VDD
 It is considered to be desirable for VM to be located around the middle of
the available voltage swing.
 To move VM upwards, a larger value of r is required.
 Increasing the strength of the NMOS, on the other hand, moves the
switching threshold closer to GND.
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Design Technique — Maximizing the noise margins
 To balance the driving strengths of the transistors and maximize the noise margins and
obtain symmetrical characteristics use this equation:
Example: Switching threshold
 VM is sensitive to variations in the device ratio
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 We have sweeped the W of load transistor
to see the effect of change in transistors
size ratio on the VM
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 This means that small variations of the ratio do not disturb the
transfer characteristic that much.
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The usage of shifting the transient region of the VTC by changing VM
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Another example for VTC
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Scaling the Supply Voltage
 Continuing technology scaling forces the supply voltages to reduce at
rates similar to the device dimensions.
 At the same time, device threshold voltages are virtually kept
constant.
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 At a voltage of 0.2 V the width of the transition region measures only 10%
of the supply voltage while it widens to 25% for 2.5 V
 So, given this improvement in dc characteristics
 why do we not choose to operate all our digital circuits at these low
supply voltages?
 reducing the supply voltage has a positive impact on the energy
dissipation, but is detrimental to the performance on the gate.
 The dc-characteristic becomes increasingly sensitive to variations
in the device
 parameters such as the transistor threshold, once supply voltages
and intrinsic voltages become comparable.
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DELAY DEFINITIONS
t
 t PHL
t P  PHL
2
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First-Order Analysis
Integrate the capacitor (dis)charge current:
 i the (dis)charging current
 v the voltage over the capacitor
 v1 and v2 the initial and final voltage
 Equivalent resistance when (dis)charging a capacitor:
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 Equivalent resistance, averages the resistance of the device over the interval:
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Example
Assume that a driver with a source resistance of 10 kW is used to drive a 10 cm
long, 1 mm wide Al1 wire with total lumped capacitance for this wire equals 11 pF
The operation of this simple RC network is described by the following ordinary
differential Equation:
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When applying a step input, the transient response of this circuit is
Where:
 The time to reach the 50% point: t = ln(2)t = 0.69t
 To get to the 90% point: t= ln(9)t = 2.2t
 the propagation delay of such a network for a voltage step at the input is
proportional to the time-constant of the network.
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 the propagation delay of such a network for a voltage step at the input is
proportional to the time-constant of the network:
 The overall propagation delay of the inverter:
Example: Propagation Delay of a 0.25 mm Inverter
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supply voltage equals to 2.5 V
The (W/L) ratios of the transistors to be 1.5 for the NMOS1, and 4.5 for the NMOS2.
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 make the parameters governing the delay explicit
Propagation delay of inverter as a
function of supply voltage
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The propagation delay of a gate can be minimized in the following ways:
 Reduce CL: three major factors contribute to the load capacitance: the internal
diffusion capacitance of the gate itself, the interconnect capacitance, and the fanout.
 Increase the W/L ratio of the transistors. This is the most powerful and effective
performance optimization tool in the hands of the designer.
 Increase VDD
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 If symmetry and reduced noise margins are not of prime concern, it is
possible to speed up the inverter by reducing the width of the PMOS
device
 The load capacitance of the first gate:
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 By setting:
 Sizing Inverters for Performance
 Cint is associated with the diffusion capacitances of the NMOS and PMOS
transistors as well as the gate-drain overlap (Miller) capacitance
 Cext is the extrinsic load capacitance
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 tp0 = 0.69 ReqCint represents the delay of the inverter only loaded by its own
intrinsic capacitance (Cext = 0), and is called the intrinsic or unloaded delay
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 The intrinsic delay of the inverter tp0 is independent of the sizing of the gate, and
determined by technology and inverter layout.
 Making S infinitely large yields the maximum performance gain, eliminating the impact
of external load, and reducing the delay to the intrinsic one.
Sizing A Chain of Inverters:
 Determining the optimum sizing of a gate when embedded in a real environment.
  is a proportionality factor, which is only a function of technology
 Delay if the inverter is function of the ratio between its external load capacitance and
input capacitance. This ratio is called the effective fanout f.
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 The goal is to minimize the delay through the inverter chain
 the optimum size of each inverter is the
geometric mean of its neighbors sizes
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 With Cg,1 and CL given:
 Choosing the Right Number of Stages in an Inverter Chain
 The optimum value can be found by differentiating the minimum delay expression by
the number of stages, and setting the result to 0.
For  = 0:
Optimal number of stages equals N = Ln(F)
Effective fan out of each stage is set to f = 2.71828 = e
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 Optimum effective fanout f as a function
of the self-loading factor  in an inverter
chain
 Normalized propagation delay (tp/(tpopt)
as a function of the effective fanout f for
1.
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The rise/fall time of the input signal
 propagation delay of a minimum-size
inverter as a function of the input
signal slope
 Example: Delay of Inverter embedded in Network
 an expression for the delay of the stage-2
inverter
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 Power, Energy, and Energy-Delay
 The power dissipation of an inverter is dominated by dynamic dissipation resulting
from charging and discharging capacitances.
 Dynamic Power Consumption
 Dynamic Dissipation due to Charging and Discharging Capacitances
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 This means that only half of the energy supplied by the power source is stored on CL
 Example: Capacitive power dissipation of inverter
 the value of the load capacitance was
determined to equal 6 fF
 For a supply voltage of 2.5 V
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 switching activity
f : maximum possible event rate of the inputs
P01 : probability that a clock event results in a 0 1
event at the output of the gate.
CEFF = P01CL is called the effective capacitance and represents the average
capacitance switched every clock cycle
Example: Switching activity
 Power consuming transitions occur 2 out
of 8 times, which is equivalent to a
transition probability of 0.25 (or 25%).
Clock and signal waveforms
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 Example: Transistor Sizing for Energy Minimization
 Degrees of freedom are size factor f of the
inverter and supply voltage Vdd of the circuit.
 f = 1 and Vdd = Vref.
inverter driving an external load
capacitance Cext, while being
driven by a minimum sized gate.
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 Sizing of an inverter for energy-minimization
Required supply voltage as a function of Energy of scaled circuit (normalized with
the sizing factor f for different values of respect to the reference case) as a function
of f. Vref = 2.5V, VTE = 0.5V.
the overall effective fanout F
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 Static Consumption
 Example: Impact of threshold reduction on performance and static power dissipation
 0.25 mm CMOS technology
 slope factor S for this device equals 90
mV/decade
 The off-current of the transistor for a
VT of approximately 0.5V equals 10-11A
 Reducing the threshold with 200 mV to
0.3 V multiplies the off-current of the
transistors with a factor of 170!
Decreasing the threshold increases
the subthreshold current at VGS = 0.
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 Putting It All Together:
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 Some of remaining simulations
Input Charatristics
Input
Output
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Input Charatristics
Input
Output
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Ring Oscillator
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Transient simulation
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Thanks for your
attention
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