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All questions of Operational Amplifier for GATE Physics Exam

If Av(d) = 3500 and ACM = 0.35, then CMRR is :
  • a)
    1225
  • b)
    10000
  • c)
    40 dB 
  • d)
    80 dB
Correct answer is option 'B,D'. Can you explain this answer?

Jayant Mishra answered
The solution to your question is: 

Hence, The correct answers are: 10000, 80 dB

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Ideal op-amp has infinite voltage gain because 
  • a)
    To control the output voltage 
  • b)
    To obtain finite output voltage 
  • c)
    To receive zero noise output voltage 
  • d)
    None of the mentioned
Correct answer is option 'B'. Can you explain this answer?

Vedika Singh answered
As the voltage gain is infinite, the voltage between the inverting and non-inverting terminal (i.e. differential input voltage) is essentially zero for finite output voltage.

When negative feedback is used, then the gain bandwidth product of an Op-Amp.
  • a)
    fluctuates
  • b)
    increases
  • c)
    decreases
  • d)
    stays the same
Correct answer is option 'D'. Can you explain this answer?

Jayant Mishra answered
Applying the feedback will reduce the gain but increase the bandwidth hence, the gain bandwidth product = Ax f. he remains constant for any voltage feedback amplifier.
The correct answer is: stays the same

A differential amplifier.
  • a)
    is a part of an Op-Amp
  • b)
    has one input only 
  • c)
    has two output
  • d)
    has one input and one output
Correct answer is option 'A,C'. Can you explain this answer?

Jayant Mishra answered
Any differential amplifier has two inputs and two outputs (one is Vout and other is ground potential) The correct answers are: is a part of an Op-Amp, has two output

The circuit below represent a non-inverting integrator. Find the expression for the output voltage.
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'C'. Can you explain this answer?

Pie Academy answered

Let the inverting terminal of Op-Amp is at potential ‘V and hence non-inverting terminal also appears to have the same potential ‘V due to virtual ground concept.
Input current to op-amp is zero. Hence at non-inverting terminal mode, we have


Input current to Op-Amp is zero. Hence at inverting terminal mode we have

Substituting the values and solving all the above equations, we get



The certain inverting amplifier has a closed loop gain of 25. The Op-Amp has an open-loop gain of 100000. If another Op-Amp with an open loop gain of 200000 is substituted in the configuration, the closed loop gain is?
    Correct answer is '25'. Can you explain this answer?

    The closed loop gain depends on the configuration and open loop gain is completely independent of it. Hence even if open loop gain is changed, the closed loop gain remains at 25 with the same configuration.
    The correct answer is: 25

     
    How many PN junctions is/are present in a bipolar junction transistor?
      Correct answer is '2'. Can you explain this answer?

      A bipolar junction transistor (BJT) contains 2 PN junctions. If the transistor is NPN type then it contains a P-type semiconductor sandwiched between two N-type semiconductor. If the transistor is PNP type then it contains a N-type semiconductor sandwiched between two P-type semiconductor. In both the cases there are two PN junctions.

      In order to obtain a solution of the differential equation
      involving voltage V(t) and V1 an Op-Amp circuit would require at least.
      • a)
        one Op-Amp adder
      • b)
        two Op-Amp adder
      • c)
        one Op-Amp integrator
      • d)
        two Op-Amp integrator
      Correct answer is option 'A,D'. Can you explain this answer?

      Jayant Mishra answered
      Since, it is a two degree linear differential equation, hence, two integrators are required, also one adder to calculate the sum of outputs.
      The correct answers are: two Op-Amp integrator , one Op-Amp adder

      The natural semiconductor LH0063C has a slew rate of
      • a)
        1400V/µs
      • b)
        6000V/µs
      • c)
        500V/µs
      • d)
        None of the mentioned
      Correct answer is option 'B'. Can you explain this answer?

      Chirag Verma answered
      National semiconductor LH0063C has a slew rate of 6000V/µs. Generally, the practical op-amp is available with slew rate from 0.1V/µs to well above 1000V/µs.

      The inverting input terminal of an operational amplifier (Op-Amp) is shorted with output terminal apart from being grounded. A voltage signal V1 is applied to the noninverting input terminal of the Op-Amp. Under this configuration the Op-Amp function is as :
      • a)
        voltage to current converter 
      • b)
        an oscillator 
      • c)
        an open loop inverter
      • d)
        a voltage follower
      Correct answer is option 'D'. Can you explain this answer?

      Jayant Mishra answered
      The lowest gain can be obtained from a non-inverting amplifier with feedback 1.
      When the non-inverting amplifier is configured for unit gain, it is called a voltage follower because the output voltage is equal to and in phase with the input. In other words, in the voltage follower, the output follows the input. To obtain the voltage follower from non-inverting amplifier of figure 1. simply open R1 and short R
      f
       The resulting circuit is shown is figure 2. In this figure, all the output voltages are feedback into the inverting terminal of the Op-Amp ; consequently the gain of the feedback is 1.

      The correct answer is: a voltage follower

      What is the minimum voltage required to make the PN junction of a real silicon transistor in forward biased?
        Correct answer is '0.7'. Can you explain this answer?

        0.7 volts is the minimum voltage required to make the PN junction of a real silicon transistor in forward biased. This 0.7 volt potential difference makes the PN junction between base and emitter terminal in forward biased.

        What should be the values of the component R and R2 such the frequency of the Wien bridge oscillator is 300Hz?
        (given C = 0.01 μf and R1 = 12kΩ.
        • a)
        • b)
        • c)
        • d)
        Correct answer is option 'B'. Can you explain this answer?

        Vedika Singh answered
        The frequency of oscillation f0 is exactly the resonant frequency of balanced Wien bridge and is given by 

        Assuming that the resistor are equal in value, and the capacitors are equal is value in the reactive lag of the wien bridge. At the frequency, the gain required for sustained oscillation is given by





        The correct answer is:

        The transistor gain of the circuit shown in figure is given by
        • a)
        • b)
        • c)
        • d)
        Correct answer is option 'D'. Can you explain this answer?

        Vedika Singh answered
        A and B can be measured at the same potential. From figure;


        or

        Also 


        Combing (i) and (ii)

        The correct answer is:

        The low-pass active filter shown in the figure has a cut-off frequency of 2kHz and a pass band gain of 1.5. The values of the resistors are :
        • a)
        • b)
        • c)
        • d)
        Correct answer is option 'D'. Can you explain this answer?

        Pie Academy answered
        At f = fcc (cut o ff frequency at which gain falls to  of its low frequency value)


        (i) Pass-Band gain = Amax = 1.5.
        Hence,


        The correct answer is: 

        The circuit shown is based on ideal operational amplifiers. It acts as a
        • a)
          subtractor
        • b)
          buffer amplifier
        • c)
          adder
        • d)
          divider
        Correct answer is option 'B'. Can you explain this answer?

        Pie Academy answered
        A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another, with the aim of the signal source being unaffected by (“buffered from”) whatever currents that the load may produce.
        The correct answer is: buffer amplifier

        The output voltage of a certain Op-Amp appears as shown in figure in response to a step input. What is the slew rate (in V/μs)
          Correct answer is '18'. Can you explain this answer?

          Sinjini Nair answered
          The output goes from the lower to the upper limit. Since, this response is not ideal, the limit are taken at the 90% points, as indicated. So the upper limit is + 9Vand the lower limit is - 9V. The slew rate is 

          = 18 V/μs
          The correct answer is: 18

          The difference between the bandwidth of the amplifiers given below when both the Not answered Op-Amps have an open loop gain of 100dB and a unity-gain bandwidth of 3MHz (in KHz) is :
            Correct answer is '19.5'. Can you explain this answer?

            For non-inverting amplifier, closed loop gain is

            For the inverting amplifier, the closed loop gain is.

            Using the absolute value of Acl the closed-loop bandwidth is

            Therefore difference in their bandwidth is
            = 63.8 - 44.3
             = 19.5 kHz
            The correct answer is: 19.5

            In the given circuit using ideal Op-Amps, the output voltage (in mV) is :
              Correct answer is '-200​'. Can you explain this answer?

              Partho Gupta answered

              Hence, V1= 2x106 x100x103 = 200x10-3V
              Since, gain of second stage is -1
              ⇒ Vout = - 200mV
              The correct answer is: -200

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