UECC012-VLSI DESIGN
Third Year ECE – Vth Semester
UNIT – II
RC Delay Model, Linear Delay Model
Timing Analysis, Elmore’s constant
Ref: Neil H. E. Weste – P.No: 150
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Unit-II
Combinational Logic Circuits
• MOSFETs as switches, Basic Logic Gates in CMOS
• Examples of Combinational Logic Design
• Pass transistor Logic, Transmission gates
• RC Delay Model, Linear Delay Model
• Logical Effort of Paths
• Timing Analysis, Delay Models, Elmore’s constant
• Static and dynamic CMOS design
• Power dissipation, Low power design principles.
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9/22/2020
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Elmore delay is a simple approximation to the delay through an RC network in
an electronic system. It is often used in applications such as logic
synthesis, delay calculation, static timing analysis, placement and routing,
since it is simple to compute (especially in tree structured networks, which are
the vast majority of signal nets within ICs) and is reasonably accurate. Even
where it is not accurate, it is usually faithful, in the sense that reducing the
Elmore delay will almost always reduce the true delay, so it is still useful in
optimization.
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The Miller effect in mosfet accounts for an increase in input capacity at the input gate caused by the gain of the
mosfet stage. There is also a miller effect associated with bipolar transistor and vacuum tube. This effect, name
for John Milton Miller, occurs only in inverting amplifier stages.
Let Cm be the miller capacitance, Cf be the feedback capacitance and A, the voltage gain of the inverting
amplifier. We have then:
Cm= Cf (1+A)
The feedback capacitance comprise the gate to drain capacitance and the added (if it applies) circuit
capacitance between the output and the input of the mosfet amplifier stage.
The miller effect will affect the frequency response of the amplifier , its bandwidth , by reducing its gain at high
frequencies.
Since the miller effect is proportional to the voltage gain of the amplifier stage, to reduce it we can use two
amplifier stages that have lower gain each and also, we can buffer the current that charge the feedback
capacitance (so it charge faster) in a quite popular two stage configuration called the cascode.
Using MOSFET, it consist of a common source amplifier stage followed by a common gate stage.
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