Answer / venugopal.k

a latch is a level sensitive

that is it holds the data in the positive level or negative
level of the clock pulse...

a flip flop is edge sensitive

that is it takes the data at the edges of the clock pulse..

the disadvantage with the latch is that when ever there is
a change in the data at that level it does not accept...

Answer / sujeet singh

flip flop and latches both are sequential circuit but
difference is that latch is level triggered while flip flop
is edge triggered .it means the output will only consider on
the positive or negative going edge of the clock pulse not
on the level but on the other hand the latches can operate
on whole level (1 or 0).
sometimes some people confused that latch has not clock
pulses and flip flop have. but this is not necessary that a
latch must operate without clock. the clock may be present .


on the other hand both latch and flip flop can be
constructed with the help of NAND(7400 for TTl ) or NOR(7402
for TTL).


and flip flop(single) is used for storing 1 bit data.
also used for constructing shift registors and counters and
other sequential circuit design.



Thanks..........
Sujeet Singh Electronics and Commu Engg (A) (10-11)
HR Institute of Technology
Uttar pradesh(India)

Answer / ajay kumar

latch and ff have littile bit difference.that is latche has
no clock signal but ff has clock signal.


latch has the input changes with the output changes at any
instant time.but the ff has the input samples changes with
output changes with determined instant time.

Answer / mehak

here the same point is repeating every where plz give sm
more points so tat we can differentiate

Answer / ravi kumar

latches means is just cross coupling of inverters(nor-nor)
and it does not have any clock signals so whenever we change
i/p it changes the o/p(according gate delay) but flip flop
have clock signal so when ever we change the i/p it will
reflect the o/p in the next clock cycle(edge or level
triggering) so we use latches as temporary buffers and flip
flops as registers

Answer / jnp

a clocked latch is a ff.latches are level sensitive whwre as
ff is edge sensitive.

Answer / saipraveen

latch works without clock signal,but works with a control
signal and it is level triggered device.
whereas flip flop is a 1 bit storage element and works with
a clock signal.its a edge triggered device.
normally latches are avoided and flip flops are preferred.

Answer / venkatesh

Latch is level sensitive whereas Flip Flop is edge triggered.
Latch is asynchronous sequential circuit but FF is clocked
sequential circuit.FF is made up of Latches whereas Latch is
made up of gates.FF is the synchronous version of Latch.

Answer / ravi teja .perumalla

Both latches and flip-flops are circuits whose output
depends on the inputs and the previous outputs..

But the difference is that, a flip-flop will have a clock
signal but a latch won't. In other words, latches are used
in asynchronous circuits and flip-flops in synchronous circuits.

So, a flip-flop is nothing but a clocked latch!

Answer / shantanu

latch is logic sensitive while f/f is edge sensitive

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Sunday, April 25, 2010 BSNL TTA EXAM Control Systems BSNL TTA EXAM Control Systems 1. An open loop control system has its (a) control action independent of the output or desired quantity (b) controlling action, depending upon human judgment (c) internal system changes automatically taken care of  (d) both (a) and (b) (e) all (a),(b) and (c) 2. A servo system must have (a) feedback system (b) power amplifier to amplify error (c) capacity to control position or its derivative  (d) all of these (e) none of these 3. The major disadvantage of a feedback system may be  Inaccuracy (b) inefficiency (c) Unreliability (d) instability (e) Insensitivity 4. Properties of a transfer function (a) It is ratio of two polynomials is S and assumes zero initial conditions (b) It depends on system elements and not input and output of the system (c) Coefficients of the powers of S in denominator and numerator are all real constant. The order of denominator is usually greater than or equal to the order of numerator (d) All of these (e) It is a function which transfer one physical system into another physical system. 5. The classical analogous of a simple lever is (a) Capacitor bridge (b) transformer (c) mutual inductor (d) either of these 6. Two blocks G1(s) and G2(s) can be cascaded to get resultant transfer function as (a) G1(s) + G2(s) (b) G1(s) / G2(s) (c) G1(s) G2(s) (d) 1+G1(s) G2(s) (e) 1-G1(s)G2(s) (f) two blocks cannot be cascade 7. The principles of homogeneity and super position can be applied to  linear time invariant system (b) non-linear time invariant system (c) digital control system (d) both (a) and (b) 8. Pick up the nonlinear system (a) automatic voltage regulator (b) d.c. servomotor with high field excitation (c) temperature control of a furnaces using thermistor (d) speed control using SCR (e) all of these 9. Signal flow graph (SFG) is a (a) polar graph (b) semi log graph (c) log log graph  (d) a special type of graph for analyzing modem control system (e) a topological representation of a set of differential equations 10. Disadvantages of magnetic amplifier (a) time lag, less flexible, non-sinusoidal waveform (b) low power consumption and isolation of the active circuit (c) saturation of the core (d) all of these 11. Pick up false statement regarding magnetic amplifiers (a) The gate coil of an ideal magnetic amplifier has either zero or infinite inductance (b) Resistance of control and gate winding is very small (c) Magnetic amplifier gas dropping load characteristics (d) Magnetic amplifiers are not used to control the speed of d.c. shunt motor (e) Magnetic amplifiers can be used in automatic control of electric drivers of higher rating. 12. High power amplification is achieved by using (a) push pull amplifier (b) amplidyne (c) magnetic amplifier (d) DC amplifier (e) D.C. generator 13. Pick up false statement regarding servomotors (a) The d.c. servomotors are lighter than equivalent a.c. servomotors (b) The d.c. servomotors develops higher starting and reversing torque than equivalent a.c. servomotor. (c) A drag cup a.c. servomotor has one windings on stator and other on rotor (d) Output power of servomotors varies from 1/20 W to 100 W 14. To reduce steady state error (a) decrease natural frequency (b) decrease damping (c) increase damped frequency (d) increase time constant (f) increase gain constant of the system 15. A good factor for Mp should be (a) less than 1 (b) lying between 1.1 and 1.5 (c) more than 2.2 (d) zero (e) infinity 16. Pick up false statement. Routh-Hurwitz criterion (a) is used for determining stability of a system (b) is an algebraic procedure (c) gives the exact location of roots of the characteristic equation (d) does not indicate relative degree of stability or instability 17. Which of the following is the time domain method of determining stability of a control system (a) Bode plot (b) Nyquist plot (c) Nicholos chart (d) Routh-Hurwitz array (e) Constant M and (fy) locus (f) Root locus technique 18. The technique which gives transient response quickly as well as stability information is (a) Nyquist plot (b) Routh-Hurwitz criteria (c) Bode plot (d) Root locus plot (e) Nichols plot 19. The bandwidth can be increased by use of (a) phase lag network (b) phase lead network (c) both (a) and (b) in cascade (d) both (a) and (b) in parallel (e) none of these 20. Nyquist plot is drawn on (a) semi log graph paper (b) log log graph paper (c) polar graph paper (d) centimeter graph paper 21. If the gain margin is positive and the phase margin is negative the system is (a) stable (b) unstable (c) indeterminist 22. The Bode plot is applicable to (a) all phase network (b) minimum phase network (c) maximum phase network (d) lag lead network (e) none of these 23. The valid relation between setting time ts and rise time tr is (a) tr>ts (b) ts>tr (c) ts=tr (d) none of these 24. As a root moves further away from imaginary axis the stability (a) increases (b) decreases (c) not affected (d) none of these 25. Flat frequency response means that the magnitude ratio of output to input over the bandwidth is (a) variable (b) zero (c) constant (d) none of above 26. How many octaves are between 200 Hz and 800 Hz (a) Two octave (b) One octave (c) Four octave (d) None of above 27. Human system can be considered as (a) open loop system (b) close loop system with single feedback (c) close loop system with multivariable feedback (d) none of these 28. In a feedback system the transient response (a) Decays at constant rate (b) gets magnified (c) decays slowly (d) decays more quickly 29. Transfer function of a system is used to calculate (a) the steady state gain (b) the main constant (c) the order of system (d) the output for any given input (e) all of the above 30. Transfer function of a system is defined as the ratio of output to input in (a) Laplace transform (b) Z-transform (c) Fourier transform (d) Simple algebraic form 31. Introduction of feedback decreases the effect of (a) disturbances (b) noise signals (c) error signals (d) all the above 32. The system response of a system can be best tested with (a) unit impulse input signal (b) ramp input signal (c) sinusoidal input signal (d) exponentially decaying input signal 33. Which of the following is a closed loop system (a) electric switch (b) car starter (c) de generator (d) auto-pilot for an aircraft 34. Which of the following is used as an error detector (a) potentiometer (b) field controlled ac motor (c) amplidyne (d) armature controlled ac motor 35. The break away point of root loci are (a) open loop poles (b) closed loop poles (c) open loop zeros (d) closed loop zeros 36. Noise in a control system can be kept low by (a) reducing the bandwidth (b) attenuating such frequencies at which external signals get coupled into the system (c) both (a) and (b) (d) none of these 37. Main cause of absolute instability in the control system is (a) parameters of controlling system (b) parameters of controlled system (c) parameters of feedback system (d) error detector where the two signals are compared 38. Basically a controller is (a) a amplifier (b) a clipper (c) a comparator (d) a summer 39. A system with gain margin close to unity or a phase margin close to zero is (a) highly stable (b) highly oscillatory (c) relatively stable (d) none of these 40. Which of following elements is not used in an automatic control system (a) sensor (b) error detector (c) oscillator (d) final control element 41. 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