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1) What is body effect?
In an
integrated circuit there will be several MOSFETs and in order to maintain
cut-off condition for all MOSFETs the body substrate is connected to the most
negative power supply (in case of PMOS most positive power supply). Which
causes a reverse bias voltage between source and body that effects the
transistor operation, by widening the depletion region? The widened depletion
region will result in the reduction of channel depth. To restore the channel
depth to its normal depth the VGS has to be increased. This is
effectively seen as change in the threshold voltage – Vt. This
effect, which is caused by applying some voltage to body, is known as body
effect.
2)
Explain Break Down Mechanism in diode.
If the
reverse-bias applied to a p-n junction is increased, a point will reach when
the junction breaks down and reverse current rises sharply to a value limited
only by the external resistance connected in series. This specific value of the
reverse bias voltage is called breakdown voltage (vz). After breakdown, a very
small further increases the reverse current. The breakdown voltage depends upon
the width of depletion layer. The width of depletion layer depends upon the
doping level.
3)
Explain Zener Breakdown.
The Zener
Breakdown is observed in the Zener diodes having Vz less than 5V or between 5
to 8 volts. When a reverse voltage is applied to a Zener diode, it
causes a very intense electric field to appear across a narrow depletion
region. Such an intense electric field is strong enough to pull some of the
valence electrons into the conduction band by breaking their covalent bonds
.these electrons then become free electrons which are available for conduction.
A large number of such free electrons will constitute a large reverse
current through the Zener diode and breakdown is said to have occurred due to
the Zener effect.
A current
limiting resistance should be connected in series with the Zener diode to
protect it against the damage due to excessive heating. In Zener breakdown, the
breakdown voltage depends on the temperature of P-N junction. The breakdown
voltage decreases with increase in the junction temperature.
4)
Explain Avalanche Breakdown.
The
avalanche breakdown is observed in the Zener Diodes having Vz having than
8 V. In the reverse biased condition, the conduction will take place only due
to the minority carriers. As we increase the reverse voltage applied to the
Zener diode, these minority carriers tend to accelerated. Therefore, the
kinetic energy associated with them increases. While travelling, these
accelerated minority carriers will collide with the stationary atoms and impart
some of the kinetic energy to the valence electrons present in the covalent
bonds.
Due to
this additionally acquired energy, these valence electrons will break their
covalent bonds and jump into the conduction bond to become free conduction. Now
these newly generated free electrons will get accelerated. They will knock out
some more valence electrons by means of collision. This phenomenon is called as
carrier multiplication.
5)
Compare between Zener Breakdown and Avalanche Breakdown.
Zener breakdown
|
Avalanche breakdown
|
||
This is observed in Zener
diodes having Vz 5 to 8 volts.
|
This is observed in Zener
diodes having Vzgreater than 8 volts.
|
||
The valence electrons are
pulled into conduction due to very intense electric field appearing across
the narrow depletion region.
|
The valence electrons are
pushed into conduction band due to the energy imparted by colliding
accelerated minority carries.
|
||
V-I characteristics with Zener
the break down is very sharp.
|
|
||
The breakdown voltage decreases
with increase in temperature.
|
The breakdown voltage increases
with increase in temperature.
|
6) What is sequential circuit?
These are
defined as digital circuit whose output is dependent not only on the
present input value but also on the past history of its input. The sequential
Circuits are designed using the combinational circuits along with memory
devices known as Flip-Flops. The sequential Circuits depend over the input
value as well as the stored levels.
7)
What is a flip flop?
Flip flop is one bit storage
device. Flip flop is also called latch. It stores binary value. It is the basic
building block of the digital electronic systems. These are the basically the
data storing devices which store the information of two stable states of the
system. A flip-flop stores only a single bit of data at a time.
8) Does sequential circuit contain memory element?
Yes,
sequential circuit contain memory element. A storing element is added to store
the various stable state levels information. This help in relating the feedback
data from the past to the present data.
9)
What is the application of T flip flop?
T Flip Flops can be used as follows-
·
Frequency divider
·
Counters
·
Binary Addition devices
10) How race around condition can be eliminated?
It is
essential to understand the race around condition before the development of
edge triggered flip flop. As we know that the conditions s==1 and R==1 are not
allowed in flip flop by the use of feedback correction. Under this situation
when input J and K are 1 and 1 output will change from 0 to 1. To avoid race
around condition, we use master slave flip flop. It has two different flip
flop, which are connected serially.
11)
What is a counter?
Counter is a sequential circuit
which is used to count the number of clock pulses of the circuit. It is
also sometimes used to display the number of time any event is repeated or
happening. It’s a simple counter in the digital logics and computation which
calculates the number of times an assigned event is taking place.
12) What is the difference between asynchronous counter and synchronous
counter?
Asynchronous
counter’s speed is less while synchronous counter’s speed is high. In
asynchronous glitch occurs while in synchronous, there is no problem of glitch.
In asynchronous settling time is more while in synchronous settling time is
less. Asynchronous counters are simple and straight in operation while
synchronous are complex in operation.
13)
How many types of shift register counters are there and write
their names also?
There are two types of shift register
counters which are named as-
·
Ring counter and
·
Johnson counters.
14) What is a register?
Register
is a group of flip flop or binary cells which holds the binary information.
Since a binary cell store a bit of information, n bit register has n flip flops
and is capable of storing any information of n bit.
15)
Explain about setup time and hold time
for Synchronous flip-flops, what will happen if there is setup time and hold
tine violation, how to overcome this?
For
Synchronous flip-flops, we have special requirements for the inputs with
respect to clock signal input there is Setup Time: Minimum time Period during
which data must be stable before the clock makes a valid transition. E.g. for a
positive edge triggered flip-flop having a setup time of 2ns so input data
should be Stable for 2 ns before the clock makes a valid transaction from
zero to one.
Hold Time: Minimum time period during which
data must be stable after the clock has made a valid transition. E.g. for a
posedge triggered flip-flop, with a hold time of 1 ns. Input Data (i.e. R and S
in the case of RS flip-flop) should be stable for at least 1 ns after clock has
made transition from 0 to 1 Hold time is the amount of time after the clock
edge that same input signal has to be held before changing it to make sure it
is sensed properly at the clock edge. Whenever there are setup and hold time
violations in any flip-flop, it enters a state where its output is
unpredictable: this state is known as metastable state (quasi stable state); at
the end of metastable state, the flip-flop settles down to either ‘1’ or ‘0’.
This whole process is known as metastability.
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