Monday, December 8, 2014

Previous GATE Questions on Oscillators & Waveform Generators (1987 - Till Date)


1.       Figure shows an RC phase shift oscillator.  
Solve the network to find the minimum value of hfe for the transistor for oscillations to be possible. Also determine the frequency of such oscillations. Take C = 0.01 µF and hie = 2 K.
  Answer:   Answer: 56 and 5 KHz  


     8.       Match the following:
                    a)      Hartley                                 1) Low frequency oscillator
                    b)      Wein-bridge                         2) High frequency oscillator
                    c)       Crystal                                 3) stable frequency oscillator
                                                                4) Relaxation frequency oscillator
                                                                5) Negative resistance oscillator
Answer:     a-2, b-1, c-3
              Solution :

9.   Calculate the frequency at which zero transmission is obtained from the Wien-bridge shown below.

Answer:     f = 1/2πRC


9.       A high – Q quartz crystal exhibits series resonance at the frequency ωs and parallel resonance at the frequency ωp. Then
                     a.       ωs is very close to, but less than ωp
                     b.      ωs << ωp
                     c.       ωs is very close to, but greater than ωp
                     d.      ωs >> ωp
Answer:    A


30.   A transistor LC oscillator circuit is shown below.
Assume that the transistor has very high value of β(so that you may neglect rb). Derive an equation governing the circuit operation, and find the frequency of oscillation. Also state the gain condition required for oscillation to start.

This circuit is Colpitts Oscillator using BJT as amplifier. Refer any text book of EDC to know the condition for oscillations. We dont expect this type of questions will come these days....... as it is a 5 mark question in GATE 1999.


3.       Consider the following two statements :
Statement 1:  Astable Multivibrator can be used for generating square wave.
Statement 2:  Bistable Multivibrator can be used for storing binary information.
a.       Only statement 1 is correct.
b.      Only statement 2 is correct.
c.       Both the statements 1 and 2 are correct.
d.      Both the statements 1 and 2 are incorrect.
Answer:    C

5.       The following circuit shown in the figure is 
a.       Hartley oscillator with fosc = 79.6MHz
b.      Colpitts oscillator with fosc = 50.3MHz
            c.       Hartley oscillator with fosc = 159.2MHz
d.      Colpitts oscillator with fosc = 159.2MHz
Answer:    B


8.       The oscillator circuit shown in the figure has an ideal inverting amplifier. Its frequency of oscillation in Hertz is

Answer:    A

12.   An ideal saw-tooth voltages waveform of frequency of 500 Hz and amplitude 3 volts is generated by charging a capacitor of 2 µF in every cycle. The charging requires
a.       Constant voltage source of 3 volts for 1 ms
b.      Constant voltage source of 3 volts for 2 ms
c.       Constant current source of 1 mA for 1 ms
d.      Constant current source of 3 mA for 2 ms
Answer:    D


1.       In the circuit shown below, capacitors C1 and C2 are very large and are shorts at the  input frequency . vi is a small signal input. The gain magnitude |V0/Vi| at 10 M rad/sec is
a.       Maximum
b.      Minimum
c.       Unity
d.      Zero
Answer:    A

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