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All questions of Operational Amplifier for GATE Physics Exam

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

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.

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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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.

    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 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:

    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?

    Pie Academy 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 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?

    The given information is incomplete. The slew rate of a semiconductor is typically expressed in volts per microsecond (V/μs). Without the complete value, it is not possible to determine the correct answer.

    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 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

      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



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

        Jay Nambiar answered
        Introduction:
        A bipolar junction transistor (BJT) is a three-layer semiconductor device that consists of two PN junctions. It is widely used in electronic circuits for amplification, switching, and signal processing. Understanding the number of PN junctions in a BJT is crucial for analyzing its behavior and characteristics.

        Explanation:
        A BJT is composed of three semiconductor regions, namely the emitter, base, and collector. Each region is doped differently to create the necessary PN junctions. Let's examine the PN junctions in a BJT in detail:

        1. Emitter-Base Junction:
        The emitter-base junction is formed between the heavily doped emitter region and the lightly doped base region. This junction is a PN junction, where the P-region is the base and the N-region is the emitter. The emitter-base junction is forward-biased to allow the flow of majority carriers (electrons in an NPN transistor) from the emitter to the base.

        2. Base-Collector Junction:
        The base-collector junction is formed between the lightly doped base region and the moderately doped collector region. This junction is also a PN junction, where the P-region is the base and the N-region is the collector. The base-collector junction is reverse-biased to prevent the majority carriers (electrons) injected from the emitter from flowing into the collector.

        Summary:
        In a bipolar junction transistor, there are two PN junctions:
        1. Emitter-Base Junction
        2. Base-Collector Junction

        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

        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: 

        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

        If a certain Op-Amp has a closed loop gain of 20 and an upper critical frequency of 10 MHz, the gain bandwidth product is :
        • a)
          50 MHz
        • b)
          200 MHz 
        • c)
          10 MHz
        • d)
          40 MHz
        Correct answer is option 'B'. Can you explain this answer?

        Pie Academy answered
         
        To determine the gain bandwidth product of an op-amp:
        • The closed-loop gain is 20.
        • The upper critical frequency is 10 MHz.
        Using the relationship:
        • Gain-bandwidth product = Closed-loop gain x Upper critical frequency
        Thus, the calculation becomes:
        • Gain-bandwidth product = 20 x 10 MHz = 200 MHz
         

        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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