GATE Questions on "Bipolar Junction Transistors (BJT)" (1987 to Till Date)


1.  Which of the following effects can be caused by a rise in the temperature ?
       a. Increase in MOSFET current
       b. Increase in BJT current
       c. Decrease in MOSFET current
       d. Decrease in BJT current
Answer:    B & C
Solution :


   1.       Discrete transistors T1 and T2 having maximum collector current rating of 0.75 Amp are connected in parallel as shown in figure. This combination is treated s a single transistor to carry a total current of 1 Amp, when biased with self bias circuit. When the circuit is switched ON, T1 draws 0.55 Amp and T2 draws 0.45 Amp.  If the supply is kept on continuously, ultimately it is very likely that………….
      a. T1 gets damaged and T2 will be safe
b. Both T1 and T2 get damaged
c. Both T1 and T2 will be safe
d. T2 gets damaged and T1 will be safe
Answer:   A


   2.       In the figure shown, all transistors are identical and have a high value of beta (β). The voltage VDC is equal to ……………


Answer:    5 Volts
Solution :

1.       In a transistor having finite β, the forward bias across the base emitter junction is kept constant and the reverse bias across the collector base junction is increased. Neglecting the leakage across the collector base junction and the depletion region generating current, the base current will………….. (Increase / Decrease / Remains constant).

Answer:    Decreases
                Solution :

2.       If the transistors in figure, have high values of β and a VBE of 0.65 volts, the current I flowing through the 2 K resistance will be…………

                Answer:    A

3.       A PNP transistor shown in figure has uniform doping in the emitter, base and collector regions, where in the doping concentrations are 1019 cm-3, 1017 cm-3 and 1015 cm-3 respectively. The minority carrier diffusion lengths in the emitter and the base regions are 5 µm and 100µm respectively.
Assuming low level injection conditions and using law of the junction, calculate the collector current density and the base current density due to base recombination.
[Suitable approximations may be made if required.]

 In all the regions of transistors, Dp = 8 cm2/sec, Dn = 16 cm2/sec, ni = 1.5 X 1010 cm-3, KT/q = 26 mV and q = 1.6  X 10-19 C.

Solution :  

1.       The reverse saturation current of the collector base junction (ICBO) of a BJT is found to be 10 nA at low collector voltages. The low current amplification factor (α) is 0.98. Find out the change in collector current with its base open (ICEO) when the collector voltage is increased such that α increases by the 1 %.

Answer:    96%
       Solution :


1.       Match the following :

Answer:    a-2, b-3, c-1
                 Solution :

2.  In the common emitter amplifier shown, the transistor has a forward current gain of 100, and a base to emitter voltage of 0.6 volts. Assume ICO to be negligible. Choose value for R1 and R3 such that the transistor has a collector current of 1 mA and a collector to emitter voltage of 2.5 volts.

Answer:    41 kΩ and 2.2 kΩ

1.       The breakdown voltage of a transistor with its base open is BVCEO, that with emitter open is BVCBO then
              a.       BVCEO = BVCBO
              b.      BVCEO > BVCBO
              c.       BVCEO < BVCBO
              d.      BVCEO not related to  BVCBO
Answer:    C
                 Solution :

2.       A BJT is said to be operating in the saturation region, if
              a.       Both the junctions are reverse biased
              b.      Base emitter junction is in reverse biased, and base collector junction is forward biased
              c.       Base emitter junction is in forward biased, and base collector junction is reverse biased
              d.      Both the junctions are forward biased
Answer:    D
                 Solution :

3.       The Ebers – Moll model is applicable to
              a.       Bipolar junction transistors
              b.      nMOS transistors
              c.       Unipolar Junction transistors
              d.      Junction field effect transistors
Answer:    A
                 Solution :

4.   A transistor having α = 0.99 and VBE = 0.7 volts, in the circuit shown, then the value of the collector current will be…………

Answer:    3.725 mA
                Solution :

5.   The circuit shown in figure, supplies power to an 8 loud speaker. The values of IC and VCE for this circuit will be

3.725 mA and 16.4 Volts
               Solution :

6.   In a Bipolar junction transistor, if 

Answer:    a-3, b-5, c-3
               Solution :


1.       In bipolar junction transistor(BJT), at room temperature, if the emitter current is doubled, then the voltage across its base emitter junction
              a.       Doubles
              b.      Halves
              c.       Increases by about 20 mV
              d.      Decreases by about 20 mV
Answer:    C
                  Solution :

2.       If a transistor is operating with both of its junctions forward biased, but with the collector base forward bias greater than the emitter base forward bias, then it is operating in the
              a.       Forward active mode
              b.      Reverse active mode
              c.       Reverse saturation mode
              d.      Forward saturation mode
Answer:    C
                  Solution :

3.       The common emitter short circuit current gain β of a transistor
        a.       Is a monotonically increasing function of the collector current IC
        b.      Is a monotonically decreasing function of IC
        c.       Increases with IC, for low IC, reaches maximum and then decreases with further increase in IC.
        d.      Is not a function of IC.
Answer:    C
                  Solution :


1.       In a common emitter BJT amplifier, the maximum usable supply voltage is limited by
              a.       Avalanche breakdown of base emitter junction
              b.      Collector base breakdown voltage with emitter open (BVCBO)
              c.       Collector emitter breakdown voltage with base open (BVCEO)
              d.      Zener breakdown voltage of the emitter base junction
Answer:    B
                  Solution :

2.     In the cascade amplifier circuit shown, determine the values of R1, R2 and RL such that the quiescent current through the transistors is 1 mA and the collector voltages are VC1 = 3 volts, and VC2 = 6 volts. Take VBE = 0.7 volts and assume β of the transistors is very high and base currents to be negligible. 

Answer:    59 kΩ, 31 kΩ, 3 kΩ
                  Solution :

3.       Find static noise margins for a BJT inverter shown in figure. Transistor used is an NPN type with the specifications as follows.

Answer:    (NM)L = 1.3 volts, (NM)H = 3.5 volts
                  Solution :

4.       For a typical NPN transistor, the following data are available :
WC = 20 µm and collector doping = 5 X 1018 cm-3
WE = 1 µm and emitter doping = 10 X 1019 cm-3
Base doping = 5 X 1015 cm-3
Minority carrier life time in the base region is τn = 5 µsec.

Answer:    τt = WB2/2DB, β = τnt and α = β/β+1
                  Solution :

   1.       The early effect in a bipolar junction transistor is caused by
              a.       Fast turn ON
              b.      Fast turn OFF
              c.       Large Collector – Base reverse bias
              d.      Large Emitter – Base forward bias
Answer:    C

   2.       In the cascode amplifier shown in figure, if the common emitter stage (Q1) has a Transconductance of gm1 and the common base stage (Q2) has Transconductance of gm2, then the overall Transconductance g = Io / Vi of the cascode amplifier is 

              a.       gm1
              b.      gm2
              c.       gm1/2
              d.      gm2/2
Answer:    A


1.       In circuit shown, assume that the transistor is in active region. It has a large β and its base emitter voltage is 0.7 volts. The value of IC is 

              a.       Indeterminate since RC is not given
              b.      1 mA
              c.       5 mA
              d.      10 mA
Answer:    B
               Solution :


1.       The transistor shunt regulator shown in figure has a regulated output voltage of 10 volts, when input varies from 20 volts to 30 volts. The relevant parameters for the zener diode and the transistor are: VZ = 9.5 volts, VBE = 0.3 votls, β = 99. Neglect the current throught RB. Then the maximum power dissipated in the zener diode (PZ) and the transistor (PT) are

Answer:    C
                Solution :


1.       If the transistor in figure is in saturation, then

Answer:    D
             Solution :

2.       Each transistor in figure, has a dc current gain βdc = 50, cut in voltage Vγ = 0.65 volts and VBEsat = 0.75 volts. The output voltage Vo for T2 in saturation can be as high as 0.2 volts. Assume 0.7 volts drop across a conducting PN junction. 

              (a)  The minimum value IB2 necessary to keep T2 saturation.
              (b)  The maximum permissible value for the resistance RB1.
              (c)  The worst case high input (logic 1)  and the worst case  low input  (logic 0) 
                    for which T2  will be  either in saturation or in cut off.
Answer:   (a) 96 µA (b) 14.5 kΩ (c) 2.1 volts and 1.35 volts

             Solution :


1.       The impurity commonly used for realizing the base region of a silicon NPN transistor is
              a.       Gallium
              b.      Indium
              c.       Boron
              d.      Phosphorous
Answer:    C
               Solution :

2.       If for a silicon NPN transistor, the base to emitter voltage (VBE) is 0.7 volts and collector to base voltage (VCB) is 0.2 volts, then the transistor is operating in the
              a.       Normal active mode
              b.      Saturation mode
              c.       Inverse active mode
              d.      Cutoff mode
Answer:    A
               Solution :

3.       Consider the following statements S1 and S2.
S1: the β of a BJT reduces if the base width is increased
S2: the β of a BJT increases if the doping concentration in the base is increased
Which of the following is correct?

              a.       S1 is FALSE and S2 is TRUE
              b.      Both S1 and S2 are TRUE
              c.       Both S1 and S2 are FALSE
              d.      S1 is TRUE and S2 is FALSE
Answer:    D
                Solution :

4.       Assuming VCEsat = 0.2 volts and β = 50, the minimum base current (IBmin) required to drive the transistor in figure to saturation is

Answer:    A
                Solution :

5.       The given figure is the voltage transfer characteristic of

Answer:    C
                Solution :

6.   The neutral base width of a bipolar transistor biased in the active region is 0.5 µm. The maximum electron concentration and the diffusion constant in the base are 1014 cm-3 and Dn = 25 cm2/sec respectively. Assuming negligible recombination in the base, the collector current density is 

a.       800  A/cm2 
b.      8        A/cm2 
c.       200  A/cm2 
d.      2        A/cm2 

Answer:    B
                Solution :

7.   Assuming that the β of the transistor is extremely large and VBE = 0.7 volts, IC and VCE in the circuit shown are

Answer:    C
                Solution :


1.       For NPN transistor connected as shown in figure, VBE = 0.7 volts. Given that the reverse saturation current of the junction at room temperature is 10-13 Amp, then the emitter current is
Answer:    C

                 Solution :


1.       The phenomenon known as “early effect” in a BJT refers to a reduction of the effective base width caused by
              a.       Electron – hole recombination at the base
              b.      The reverse biasing of the base collector junction
              c.       The forward biasing of emitter base junction
              d.      The early removal of stored base charge during saturation to cutoff switching
Answer:    B
                Solution :


1.       The DC current gain (β) of a BJT is 50. Assuming that the emitter injection efficiency is 0.995, the base transport factor is:
              a.       0.980
              b.      0.985
              c.       0.990
              d.      0.995
Answer:    B
               Solution :

2.       Group I lists four different semiconductor devices. Match each device in Group I with its characteristic property in Group II.
Answer:    C

3.       For the BJT circuit shown, assume that the β of the transistor is very large and VBE is 0.7 volts. The mode of operation of the BJT is 

              a.       Cut off
              b.      Saturation
              c.       Normal active
              d.      Inverse active
Answer:    B
               Solution :


1.       In a uniformly doped BJT, assume that NE, NB and NC are the emitter, base and collector doping in atoms/cm3 respectively. If the emitter injection efficiency of the BJT is close to unity, which of the following conditions is TRUE?

Answer:    B
                Solution :


1.       For the BJT Q1 in the circuit shown,  β = ∞, VBEon = 0.7 volts, VCEsat = 0.7 volts. The switch is initially closed. At time t=0, the switch is opened. The time at which Q1 leaves the active region is

              a.       10 ms
              b.      25 ms
              c.       50 ms
              d.      100 ms
Answer:    C
               Solution :

2.       For a BJT, the common base current gain α = 0.98 and the collector base junction reverse bias saturation current, ICO = 0.6 µA. This BJT is connected in the common emitter mode and operated in the active region with a base current (IB) of 20 µA. The collector current IC for this mode of operation is
              a.       0.98 mA
              b.      0.99 mA
              c.       1.0 mA
              d.      1.01 mA
Answer:    D
               Solution :


1.       In the circuit shown below, the silicon NPN transistor Q has a very high value of β. The required value of R2 in K to produce IC = 1 mA is 

              a.       20
              b.      30
              c.       40
              d.      50
Answer:    C
                  Solution :

Set – 1 (15th February 2014 (Forenoon))

1.       A BJT is biased in forward active mode. Assume VBE = 0.7 volts, KT/q = 25 mV and reverse saturation current IS = 10-13 Amp. The Transconductance of the BJT (in mA/volt) is…..
Answer:    5785
                  Solution :

SET – 2 (15th February 2014 (Afternoon))

1.       An increase in the base recombination of a BJT will increase
              a.       the common emitter DC current gain, β
              b.      the breakdown voltage BVCEO
              c.       the unity gain cutoff frequency, fτ
              d.      the Transconductance gm
Answer:    B
                  Solution :

SET – 3  (16th February 2014 (Forenoon))

1.       In the circuit shown, the PNP transistor has |VBE| = 0.7 volts and β = 50. Assume that RB = 100 K. For Vo to be 5 volts, the value of RC (in K) is ……..

Answer:    1.07
                  Solution :

2.       In the circuit shown, the silicon BJT has β = 50. Assume VBE = 0.7 volts and VCEsat = 0.2 volts. Which one of the following statements is correct?

              a.       For RC = 1 kΩ, the BJT operates in the saturation region
              b.      For RC = 3 kΩ, the BJT operates in the saturation region
              c.       For RC = 20 kΩ, the BJT operates in the cutoff region
              d.      For RC = 20 kΩ, the BJT operates in the linear region
Answer:    B
                  Solution :

SET  - 4  (16th February 2014 (Afternoon))

1.       If the emitter resistance in a common emitter voltage amplifier is not bypassed, it will
              a.       Reduce both the voltage gain and the input impedance
              b.      Reduce the voltage gain and increase the input impedance
              c.       Increase the voltage gain and reduce the input impedance
              d.      Increase both the voltage gain and the input impedance
Answer:    B
                  Solution :

2.       Consider two BJTs biased at the same collector current with area A1 = 0.2 µm x 0.2 µm and A2 = 300 µm x 300 µm. assuming that all other device parameters are identical, KT/q = 26 mV, the intrinsic carrier concentration is 1 x 1010 cm-3, and q = 1.6 x 10-19C, the difference between the base emitter voltages (in mV) of the two BJTs (i.e. VBE1 – VBE2) is …………….
Answer:    380
                  Solution :


3.       A BJT in a common base configuration is used to amplify a signal received by a 50 antenna. Assume KT/q = 25 mV, the value of collector bias current (in mA) required to match the input impedance of the amplifier to the impedance of the antenna is ………..
Answer:    0.5
                 Solution :


1.       In the circuit shown, the BJT has a current gain (β) of 50. For an emitter base voltage VEB = 600 mV, the emitter collector voltage VEC (in volts) is_______________

Answer:    2

 Solution :

2.       If the base width in a bipolar junction transistor is doubled, which one of the following statements will be TRUE?
a.       Current gain will increase
b.      Unity gain frequency will increase
c.       Emitter base junction capacitance will increase
d.      Early voltage will increase
Answer:    C
      Solution :

3.       An NPN BJT having reverse saturation current Is = 10-15 A is biased in the forward active region with VBE =700 mV. The thermal voltage (VT) is 25 mV and the current gain (β) may vary from 50 to 150 due to manufacturing variations. The maximum emitter current (in µA) is ____________________________
Answer:    1446

        Solution :


  1. A base current of 1mA is to be given in order to drive the BJT into saturation. The approximate ratio of the resistances R1:R2 such that the current through R2 is also 1mA, is


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