SESSION:33

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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 V_GS has to be increased. This is effectively seen as change in the threshold voltage – V_t. 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 V-I characteristics with the avalanche breakdown increases gradually. It is not as sharp as that with the Zener breakdown.
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.