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

1990

1.       Figure shows an RC phase shift oscillator.  

Solve the network to find the minimum value of h_fe for the transistor for oscillations to be possible. Also determine the frequency of such oscillations. Take C = 0.01 µF and h_ie = 2 KΩ.

  Answer:   Answer: 56 and 5 KHz  

                         Solution:   https://www.youtube.com/watch?v=xupn6VebQzg

1994

     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 :  https://www.youtube.com/watch?v=wt5Z87PuxIU

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

Answer:     f = 1/2πRC
  Solution:   https://www.youtube.com/watch?v=synowLeqf5I

1998

9.       A high – Q quartz crystal exhibits series resonance at the frequency ω_s and parallel resonance at the frequency ω_p. Then

                     a.       ω_s is very close to, but less than ω_p

                     b.      ω_s << ω_p

                     c.       ω_s is very close to, but greater than ω_p

                     d.      ω_s >> ω_p

Answer:    A

Solution :   https://www.youtube.com/watch?v=s6jLBt7qH18

1999

30.   A transistor LC oscillator circuit is shown below.

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

                        Solution:   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.

2001

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

Solution:   https://www.youtube.com/watch?v=HQUPS3pY2A8

 

5.       The following circuit shown in the figure is 

a.       Hartley oscillator with f_osc = 79.6MHz

b.      Colpitts oscillator with f_osc = 50.3MHz

            c.       Hartley oscillator with f_osc = 159.2MHz

d.      Colpitts oscillator with f_osc = 159.2MHz

Answer:    B

Solution:   https://www.youtube.com/watch?v=U6V6gK9avGY

2003

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

Answer:    A

Solution:   https://www.youtube.com/watch?v=BdULnwS91ts

 

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

Solution:   https://www.youtube.com/watch?v=jA7LuPuKGFg

2011

1.       In the circuit shown below, capacitors C₁ and C₂ are very large and are shorts at the  input frequency . v_i is a small signal input. The gain magnitude |V₀/V_i| at 10 M rad/sec is

a.       Maximum

b.      Minimum

c.       Unity

d.      Zero

Answer:    A

Solution:   https://www.youtube.com/watch?v=OL8a4oM3MhM