Monday, December 8, 2014

Previous GATE Questions on Multistage Amplifiers (1987 - Till Date)


1.       The configuration of cascode amplifier is
                  a.       CE – CE
                  b.      CE – CB
                  c.       CC – CB
                  d.      CC – CC
Answer:     B
            Solution :             


1.       Each transistor in the Darlington pair shown below has hFE = 100. The overall hFE of the composite transistor, neglecting the leakage currents is
                  a.       10000
                  b.      10001
                  c.       10100
                  d.      10200
Answer:    C
            Solution :           

2.       The amplifier circuit shown below uses a composite transistor of a MOSFET and BIPOLAR in cascade. All capacitances are large. gm of the MOSFET is 2 mA/V, and hfe of the BIPOLAR is 99. The overall Transconductance gm of the composite transistor is
                  a.       198 mA/V
                  b.      9.9 mA/V
                  c.       4.95 mA/V
                  d.      1.98 mA/V
Answer:    D
            Solution :           


1.       The Two Port Darlington impedance booster of figure uses identical transistors (hie = 1 K, hfe = 100, hre = hoe = 0).

      Calculate the Z – parameters of the network. Use relevant approximations.

Answer:     Z11 = Z21 = 10.3 MΩ and Z12 = Z22 = 1 KΩ


1.   In the amplifier circuit shown, determine the value of R such that Q2 is biased at VCE2 = 7.5 volts. Assume Q1 and Q2 to be identical with VBE = 0.7 volts.
      Also determine the small signal input impedance of Q1 and Q2, if both of them have β = 200. Use VT = 26 mV.

Answer:     R= 2.72 kΩ, Ri1 < 1 kΩ, Ri2 = 5.65 kΩ


1.       A Darlington stage is shown in the figure. If the Transconductance of Q1 is gm1 and Q2 is gm2, then the overall Transconductance gmc [icc/vbec] is given by 
                  a.       gm1
                  b.      0.5 gm1
                  c.       gm2
                  d.      0.5 gm2
Answer:    C
             Solution :


1.       A cascode amplifier stage is equivalent to
                  a.       A common emitter stage followed by a common base stage
                  b.      A common base stage followed by an emitter follower
                  c.       An emitter follower stage followed by a common base stage
                  d.      A common base stage followed by a common emitter stage
Answer:    A


1.       A multistage amplifier has a low pass response with three real poles at s = - ω1, - ω2 and -ω3. The approximate overall bandwidth B of the amplifier will be given by

Answer:    B


1.       Three identical RC coupled transistor amplifiers are cascaded. If each of the amplifiers has a  frequency response as shown in the figure, the overall frequency response is as given in

  Answer:    A

2.   Below figure shown is a 2 – stage amplifier.
      The transistors Q1 and Q2 are identical with current gain β = 100, further βDC = βac = β. The zener diode Dz has a break down region and its dynamic resistance rz is zero. The capacitors C1 and C2 are large and provide negligible impedance at signal frequencies.
      a.       Identify the configuration in each of the amplifier stages. (i.e. whether CE, CC or CB)
      b.      Determine the quiescent quantities of IC1 and VC1.
      c.       Determine an expression for the voltage gain Vo/Vs and determine its value. (Assume VBE = 0.7 volts, ro = ∞ and thermal voltage VT = 25mV)



1.   Three identical amplifiers with each one having a voltage gain of 50, input resistance of 1 K and output resistance of 250 are cascaded. The open circuit voltage gain of the combined amplifier is
                  a.       49 dB
                  b.      51 dB
                  c.       98 dB
                  d.      102 dB
Answer:    C


1.       The cascode amplifier is a multistage configuration of
                  a.       CC – CB
                  b.      CE – CB
                  c.       CB – CC
                  d.      CE – CC
Answer:    B


1.       In the silicon BJT circuit shown below, assume that the emitter area of transistor Q1 is half that of transistor Q2. The value of current Io is approximately
                  a.       0.5 mA
                  b.      2 mA
                  c.       9.3 mA
                  d.      15 mA
Answer:    B


1.       A cascade connection of two voltage amplifiers A1 and A2 is shown in the figure. The open loop gain Avo, input resistance Rin, and output resistance Ro for A1 and A2 are as follows:

A1 : Avo = 10,   Rin = 10 K,   Ro = 1 K

A2 : Avo = 5,     Rin = 5 K,      Ro = 200

The approximate overall voltage gain Vout / Vin is …………….

Answer:    35
            Solution :

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