21-09-2013, 02:56 PM
CMOS circuits
CMOS circuits.PPT (Size: 229 KB / Downloads: 21)
INTRODUCTION
Complementary metal-oxide semiconductor (CMOS) devices are chips that combine p-channel and n-channel MOSFETS in a push pull arrangement
Why CMOS
Low power consumption
Low power dissipation
Output impedance is very low, so it can drive heavier loads
MOSFETS
Two types of MOSFETS
1.Enhancement-type
2.Depletion-type
Enhancement-type is used in CMOS
In the normal condition, current between the source and drain is zero
in enhancement-type MOSFETS
Enhancement-type is also called a normally off MOSFET
Enhancement-type MOSFET
Inversion layer :The gate and p-type substrate acts as the capacitor separated by a dielectric(SiO2).When gate is positive ,it can attract enough free electrons to form a thin layer of free electrons between source and drain .This thin layer is known as n-type Inversion layer
Threshold Voltage :The minimum gate voltage that creates the n-type inversion layer is called the threshold voltage VGs,th
Propagation Delay Time
Is the time(tp) it takes for the output of a gate to change after its inputs have changed
Standard CMOS gate has a propagation delay time tp of approximately 25 to 100 ns
The propagation delay of a cascaded CMOS gates is the sum of individual propagation delay of gates
Ex: If a CMOS gate has tp of 50 ns, then total propagation delay time of a cascaded circuit consists of 3 CMOS gates is 150 ns.
Power Dissipation
1.Static power dissipation is the average power dissipation when the output is constant.The dissipation equals the product of supply voltage and leakage current, both of which are dc quantities
2.Active power dissipation is the average power dissipation whose output is changing continuously.It occurs during the transition period(low to high or high to low).This depends on the frequency at which the output is switching states
Wired Logic
Two or more outputs are tied together to a common line
Normally used in Buses
Special drivers are needed for this
These drivers are called as bus drivers
Drivers are
Open collector/drain devices
Tri-state output devices
Some General Definitions
FAN-IN : The number of inputs that gate has
FAN-OUT : Number standard loads gate drives
Standard Load : One unit gate load
Propagation delay: The time delay between change of an input and the precipitated output change.
Read-Only Memory(ROM)
Is an IC that can store thousands of binary numbers representing computer instructions and data
Can detect whether 0 or 1 is stored (Read operation)
Nonvolatile data storage (loss of power does not cause a loss data)
Can be used to generate any Boolean function instead of SOP circuit
Random assessable
Memory Addressing
Is the process of selecting one of the cells in a memory to be written into or to be read from
In order to facilitate selection, memories are generally arranged by placing cells in a rectangular arrangement of rows and columns
The control circuitry is designed such that one and only one row is activated and one and only one column line is activated, the memory cell at the intersection of these two lines is selected
Programming a PAL
The figure shows a PAL with 3 inputs and 2 outputs
The ‘X’ on the input side are fusible links, while the ‘X’ on the output side are fixed connections
With a PROM programmer , we can burn in the desired fundamental products, which are then ORed by the fixed output connections