Analog Guide

Raja Reddy P, Indian Institute of Science

July 26, 2007

1. Calculate Vout(s)/Vin(s). plot Vo(t). calculate time constant and pole frequency.

R

CRVi(t) Vout(t)

0

V

Vi(t)

2. In the following circuit plot iL(t), VL(t)

0

V

Vi(t) R

Vi(t) Vout(t)R

L

3. Find Vo(t) in the circuit below.

8k

4k4k

2k

1p

2k2k

1k 3k0.4sin(wt)0.6sin(wt)

Vo(t)

4. A system has an SNR of 60 dB. If an uncorrelated noise of 1 mV is added in a 1V of

signal to it, then what is the SNR?

1

5. Both ckts are equivalent. Express Rp and Cp interms of Rs and Cs. Find ω range for

which these both are equivalent. Assume high quality factor.

Rs

Cs

Rp Cp

6. Calculate the maximum clock frequency.

D Q

ClkTpd = 100ps

Tcq = 1ns

Tsetup = 200ps

Thold = 300ps

7. Plot Vout.

10sin(wt) Vout(t)

8. Plot Vout with respect to the given input waveforms.

5V

5V

Vin2

1pF 1pF

Vin1

Vout

t

Vin2

0

3V

t0

3VVin1

t1

2

9. Design a divide-by-3 counter using D-flipflops. The duty cycle of the divided clock

should be 2/3.

10. What is the function of the following circuit?

E

A’

E’

A

A

E’

A’

E

Y

11. Plot the output Vout.

R R R

10R

R

Vin = 0.1sin(wt)

Vout

12. Calculate the frequency of oscillation. What is the minimum required Gm for oscil-

lation?

Gm

R C R R RC C C

Gm Gm Gm

3

13. Calculate the output impedance.

500k

1M

500k

gm = 2uA/VRds=1M

Rout

14. Both transistors are biased in saturation. Calculate “VA/Vin”. Neglect the body

effect.

A

Rd

Vin

15. Calculate the output if (i) gain Av = infinity (ii) gain Av = 10

VoutAv

1k

1k 1mA

Vin

4

16. If the following inverter biased in the middle of Vdd, what is the small signal gain?

(Answer: gmro)

VoutVin

Vdd

17. Crossection diagram of the inverter (be able to draw the contact of power supply and

ground)

18. From the crossection of the diagram, be able to draw the parasitic BJT leads to latch-

up.

19. How to prevent latch-up (do not forget guard ring, clampping circuits!)

20. Draw the layout of an inverter or NOR/NAND gate.

21. For the following source follower, what is its -3dB bandwidth? How about it stabil-

ity?

Vi

Vo

22. For the following circuits, Vdd=5v, tell me what are Vo1 and Vo2 when Vin is 5V,

3V, 2.5V and 0V.

Vdd

VinVin

Vo1 Vo2

5

23. What are the effective resistance from source to drain of the following two transis-

tors? (The value of the resistance is R). Answer: both of them are 1/gm.

(a) (b)

R

24. In the following figure, if the two resisters are equal, what is its -3dB bandwidth?

Compare its stability with that of a source follower.

Vo

Vi

R1

R2

25. For the following circuit, if the input is a rail-to-rail square wave, plot the wave after

the inverter and vo.

Vi Vo

Vdd

26. For the following circuits, What is the gain? Using what technology to improve the

matching of the input transistors? If the bias current increase, what happens to the

gain? (Hit: Decrease!!!) What happens to the bandwidth? Replace the NMOS with

npn BJT and PMOS with pnp BJT, answer the above questions.(Now gain remains

constant with increasing biasing current!)

6

Vin+

Vo+

Vin−

Vo−

Vdd

Vb1

Vb2

27. For the following circuits, answer the questions again. What are the advantages and

disadvantages of these two amplifiers?

Vin+

Vo+

Vin−

Vo−

Vdd

Vb

28. You are porbing a square wave pulse in the lab that has a ris etime of 5 ns and fall

time of 2 ns. What is the minimum bandwidth of the oscilloscope to view the signal?

Answer: The time thay it takes an RC circuit to go from 10% to 90% of its final value

is t=ln9*RC. The bandwidth of the oscilloscope larger than BW = ln9/(2*pi*2ns)

= 174MHz. Choose a 200MHz or faster oscilloscope. To reduce error, choose a

oscolloscope 3 time faster than the calculated value, or 600MHz.

29. What is the low frequency gain of the following circuits? The input is the input

current Iin. Where does the dominant pole locate? How about the pole at node 1?

7

1Iin

Vo

R C

30. For the following circuit, the threshold voltage of the transistor is 0.7V. Vb1=1v,

Vb2=2v, When Vin change from 5V to 0V, draw the current flow through the tran-

sistors VS Vin. (This question was supplied by Wang Ge)

Vb1

Vb2

M1

M2

Vin

31. For the following circuit, what is the gain of Vout/Vin? Where is the Feedback and

what is the function of feedback?

Vin Vout

Ib1 Ib2

8

32. For the following circuits, the small signal input is Iin, the small signal output is vout,

what is the small signaloutput? What is the gain?

Vout

33. Plot the Vout wave form of the following circuits:

Vout

C

R

Vin

R

Vin Vout

C

VoutVin Vin Vout

9u

R1k

C1, 1u

C2

R1, 9k

R21k

C1u

34. For the following circuit, at time 0, the switch switches from A to B, figure out the

voltage wave form at B.

C2C11V

9

35. For the following circuit, what is the voltage value at A and B? (The Vt of the tran-

sistor is 1V).

C2C1

5V

5V

36. The capacitor of the following figure is connected with two ideal MOS switches.

Switches T1 and T2 are alternately turned on with a frequency ffc. What is the

average current flowing from node 1 to node 2? What is the equivalent impedance

from node 1 to node 2?

C1

T1 T2

V2V1

37. Plot the output of the following RC circuit

Vin Vout

10

38. How the following two circuits differ in their functionality and which one is a better

reference?

M1

M3

R

M2

M4

(a) bandgap1

R

M1

M3

M2

M4

(b) bandgap2

39. Draw Vo. Initially Vo is at +5V. Vosat = 5V

+4V

−4V

t

Vin

1k

1k

Vout

Vin

40. what is the output Y in the figure shown below.

’1’1 2 21

Y

X

41. Draw the output waveform.

11

Vdd

Vdd

Vout

42. what is the minimum value of V1.

V1

43. How the diode should be doped for faster electron-hole collection at terminals.

N

P

12

44. In which region the capacitance Cgs is maximum.

S

G

45. How a pn-diode should be doped for maximum capacitance?

46. How to reduce the crowbar (short circuit current) in an inverter?

47. How the latchup and ESD robustness are affected with increased substrate doping?

48. I have not remembered this qestion exactly. Two different MOS structures are given

and asked to find about the sidewall and bottom capacitance.

49. How I1 and I2 changes with temperature

I2I1

50. Decreased drian doping results in (choose from the options below)

(a) High gate oxide reliability

13

(b) Hot carrier reliability

(c) Decreases drain contact resistance

51. what is the output voltage

3V

3V 3V

Vout

52. what is the slew rate of output voltage

Vout

100 uA

C= 1pFVin

2V

3V

53. For an NMOS in saturation, the drain current depends on (choose from the options

below)

(a) Surface mobility

(b) Bulk mobility

(c) Both

(d) None

14

54. what is the output voltage after 5 seconds

5uA 0.1pF

3V

0V

Vdd

Vdd

Vout

55. plot the output voltage

I(t)

R

Vo

C

20C

t

I(t)

56. what is Vout in the final state?

2V 2.5V

VoutVout

Initial State Final State

1V

15

57. Frequency spectrum of ideal squrewave (choose from the options below)

(a) Only C0 = 0

(b) Odd terms are zero

(c) Even terms are zero

58. Initially V1 = V2 = 1V and Vin = 0.7V. If Vin changes to 1.5V what are the final

states?

10 uA1pF

Vin

Vdd

V1

1pF

Vin

Vdd

V2

59. what is the steady state output voltage, Threshold voltage is 0.5V

6V

2V 1V 3V

Vout

60. Plot the output voltage

3V

VoutVin

0

3V

t

Vin

61. If Vth of NMOS increases,for an inverter NML and NMH will

(a) both increase

16

(b) both decrease

(c) NML increase NMH decrease

(d) NML decrease NMH increase

62. What is the equivalent value of resistance.

S2S1

Vin Vout

C

63. Impulse response of interpolation filter is.

−T

1

Amp

f+T

(a)

Amp

f

Amp

f−T

Amp

f+T

(b) (c) (d)−T +T/2/2

1/2

1

−T/2 +T/2

1/2

64. plot the output voltage

1V

2VVin Vin

CR

t

Vout

65. What is the transfer function

G1

H1

G1

H1

R C

17

66. A planar metal is used for an interconnect.Neglecting the fringing capacitance the

RC time constant will be

W

RC

W

RC

W

RC

W

RC

(a) (d)(c)(b)

67. Relationship between the time constants of figures 1,2,3

3C

3R 1.5R 1.5R

1.5C 1.5C

R

C

R R

C C

(1) (2)

(3)

68. In NMOS at weak inversion, the current is dominated by

(a) drift current

(b) diffusion current

(c) both

(d) none of the above

69. In MOS, channel is inverted when the surface potential reaches

(a)

(b)

(c)

(d)

70. Band-to-band tunneling occurs when

18

(a) both dopings are high

(b) both dopings are low

(c) one is high and other is low

(d) independent of doping

71. One question on np = n2i

72. Logic expression [(a+b′)(a′ +b)]′

(a) XOR

(b) NOR

(c) XNOR

(d) NAND

73. For real discrete signal magnitude will be even/odd?—————–, Phase will be

even/odd?——————–

74. plot the ouput voltage

Vin

Vout

RRt

Vin

75. Logic of the gate shown is

a

b

c

a

b c

Y

76. The following logic implements a (R is asynchronus reset)

(a) MOD-15 counter

19

D

R

Q D

R

Q D

R

QD

R

Q’1’

Clk

(b) MOD-8 counter

(c) MOD-11 counter

(d) None

77. Determine Vout. Give a possible application of the circuit.

Vdd

Vout

78. Plot Vout.

Vout

Vin

Vin

t

79. What is the equation of FIFO when it is empty?

80. Two clock domains 10MHz and 100MHz. One full period pulse is produced at 5th

clock cycle of 100MHz of pulse width equal to 1/100MHz. The pulse should be

received by 10MHz clock. Rig up the circuit.

20

81. There is a set of 5 bits a5, a4, a3, a2, a1. Set for mod 8 counter is

(a) a1

(b) a2, a1

(c) a3, a2, a1

(d) a4, a3, a2, a1

(e) a5, a4, a3, a2, a1

82. Which of the following circuits has higher output impedance?

Rout1 Rout2

83. Determine ib, ic, ie and Vb with β = 100 and Vbe = 0.7V for the circuit below.

5k

5k 0.1k

0.1k

Vb

Vdd

21

84. Determine gain of following circuita. Assume µn = 2µp

2/1

4/1

Vin

Vout

2/1

4/1

Vin

Vout

85. Determine Vout

Vout

Vin

R

86. Determine the gain for the circuit below.

Vin+

Vo+

Vin−

Vo−

Vdd

Vb1

Vb2

22

87. Frequency response for an Open loop Opamp was given and the opamp was operated

as an inverting amplifier with gain -2. Plot the closed loop frequency response.

88. Number of Boolean equation for an ’n’ input variable.

89. Find equation for the circuit below.

A’

C

ED

B

A’

B

C

D

E

Y

90. Size the remaining 4 transistors for the circuit below so that Iout = In.

Iin

Iin

Iout

1/1

91. Calculate R. The network is fully resistive.

23

R1

I1 I1

R2 R=?

I1+I2

92. What is the time constant

VoutVin

10c

RR C

93. Compute the gain at low and high frequency.

Iout

Vin

gmn gmn

gmp gmpC

Ibias

94. The open-loop gain bandwidth product of an opamp is 200 MHz. If it is operating in

negative feedback mode with a closed loop gain of 2, then what is the bandwidth?

95. A signal x(t) = 0.4sin(2.π .75Hz.t) + 0.6sin(2.π .25Hz.t) is sampled with a sampling

frequency of 100 Hz. What will be the frequency spectrum of sampled signal?

24

96. Calculate the current I through resistor.

1V

1k

I

97. Calculate Iout(Vin, gmn, gmp, ∆V)

Iout

Vin

Ibias

∆V

98. What is the frquency of oscillation of output voltage?

R

C

R1R2

Vout

25

99. Calculate Vout.

R

C R

C

Vs

Vout

100. What is the minimum value of resistor Rmin?.

R CRC

R

Rmin

101. Two identical opamps having 3dB frequency fo are in cascade. Find the overall 3dB

frequency.

102. Find the maximum frequency of operation of the following circuit.

26

logic

Combinational

logic

Combinational

D Q D Q

Td = 1ns min 2ns max

2ns maxTd = 1ns min

Tcq = 0 Tcq = 0

D

Clk

103. Maximum power is dissipated in an inverter when

(a) Input is 1

(b) Input is 0

(c) Input is toggling

104. In a D Flip-Flop, Q bar is connected to D. Find its function.

105. In a ring oscillator frequency depends on

1 2 3 n

(a) No of stages n

(b) Supply voltage

(c) WL

ratio of the inverters

106. Plot Vc(t).

Vc(t)

C

RI(t)

Io

to

t

I(t)

27

107. Draw the bode plots for gain and phase.

1k

A=10db

10k

1p 10p

A=10db

108. Plot the Gate Capacitance versus Gate voltage curves for MOSFETs at DC and high

frequencies.

109. Which of these is equivalent to the given RC circuit.

R

C

Vin Vout

R

C

Vin

VoutA=10dB

Ans:

110. The following circuit is a

0

1D

Clk

Q

28

(a) level 0 transparent latch

(b) level 1 transparent latch

111. Find Rin for the following circuit.

Vg

Rd

Rin

112. Find Vout.

Vout

Vint

Vin

0.20.30.4

0.1

113. Find i1, i2, Va and Vb.

2V

1k

1k

Va

1mA 1k

i2

Vb

i1

29

114. Derive the expression for gain of an inverter.

115. Plot Io Versus Vo for the following circuits.

Iin

IoutVout

Iin

VoutIout

116. Find R such that Vo/Vin is constant.

Vin Vout

1k 1u

R 0.1u

117. How many distinct boolean expressions are possible with 3 input variables?

(a) 3

(b) 8

(c) 256

118. Plot OUT for the following circuit. What is the function of the given circuit?

D Q

In

Clk

In

Clk

Out

30