# EC GATE 2008 (Test 5)

Tag: ec gate 2008
Q.1
For each of the positive edge-triggered J - K flip flop used in the following figure, the propagation delay is Δt . Which of the following wave forms correctly represents the output at Q1?
A. B. C. D. Explaination / Solution:

Since the input to both JK flip-flop is 11, the output will change every time with clock pulse. The input to clock is The output Q0 of first FF occurs after time ΔT and it is as shown below The output Q1 of second FF occurs after time ΔT when it gets input (i.e. after ΔT from t1) and it is as shown below Workspace
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Q.2
Consider the Schmidt trigger circuit shown below A triangular wave which goes from -12 to 12 V is applied to the inverting input of OPMAP. Assume that the output of the OPAMP swings from +15 V to -15 V. The voltage at the non-inverting input switches between A. -12V to +12 V
B. -7.5 V to 7.5 V
C. -5 V to +5 V
D. 0 V and 5 V
Explaination / Solution:

Let the voltage at non inverting terminal be V1, then after applying KCL at non inverting terminal side we have If V0 swings from -15 to +15 V then V1 swings between -5 V to +5 V.

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Q.3
Which of the following is true?
A. A silicon wafer heavily doped with boron is a p+ substrate
B. A silicon wafer lightly doped with boron is a p+ substrate
C. A silicon wafer heavily doped with arsenic is a p+ substrate
D. A silicon wafer lightly doped with arsenic is a p+ substrate
Explaination / Solution:

Trivalent impurities are used for making p - type semiconductors. So, Silicon wafer heavily doped with boron is a p+ substrate.

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Q.4
The measured trans conductance gm of an NMOS transistor operating in the linear region is plotted against the gate voltage VG at a constant drain voltage VD. Which of the following figures represents the expected dependence of gm on VG ?
A. B. C. D. Explaination / Solution:
No Explaination.

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Q.5
The cross section of a JFET is shown in the following figure. Let Vc be -2 V and let VP be the initial pinch -off voltage. If the width W is doubled (with other geometrical parameters and doping levels remaining the same), then the ratio between the mutual trans conductances of the initial and the modified JFET is A. 4
B. C. D. Explaination / Solution: Workspace
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Q.6
The circuit shown in the figure is used to charge the capacitor C alternately from two current sources as indicated. The switches S1 and S2 are mechanically coupled and connected as follows: Assume that the capacitor has zero initial charge. Given that u(t) is a unit step function , the voltage vc(t) across the capacitor is given by
A. B. C. D. Explaination / Solution:  Workspace
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Q.7
In the design of a single mode step index optical fibre close to upper cut-off, the single-mode operation is not preserved if
A. radius as well as operating wavelength are halved
B. radius as well as operating wavelength are doubled
C. radius is halved and operating wavelength is doubled
D. radius is doubled and operating wavelength is halved
Explaination / Solution:
No Explaination.

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Q.8
Consider the frequency modulated signal with carrier frequency of 105 Hz. The modulation index is
A. 12.5
B. 10
C. 7.5
D. 5
Explaination / Solution: Workspace
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Q.9
The pole-zero given below correspond to a A. Law pass filter
B. High pass filter
C. Band filter
D. Notch filter
Explaination / Solution: Workspace
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Q.10
A speed signal, band limited to 4 kHz and peak voltage varying between +5 V and -5 V, is sampled at the Nyquist rate. Each sample is quantized and represented by 8 bits. The number of quantization levels required to reduce the quantization noise by a factor of 4 wo
A. 1024
B. 512
C. 256
D. 64