86. The amplifier output resistance is:

1. 1. 4.76 kΩ

   
   
   

   2. 100 kΩ

   
   
   

   3. 5 kΩ

   
   
   

   4. 0

   
   
   

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1. View Hint View Answer Discuss in Forum

   From small-signal equivalent circuit shown in fig.
   R₀ = r₀ || R_D = 100 || 5
   

   =	100 × 5	=	500	= 4.76 kΩ
   	100 + 5		105

   
   

   Correct Option: A

   From small-signal equivalent circuit shown in fig.
   R₀ = r₀ || R_D = 100 || 5
   

   =	100 × 5	=	500	= 4.76 kΩ
   	100 + 5		105

   
   

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87. The small-signal voltage gain A_ν is:

1. 1. – 8.2

   
   
   

   2. – 5.6

   
   
   

   3. – 3.1

   
   
   

   4. – 1.9

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: B

   NA

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88. In the common-base stage of figure below I_CQ = 1 mA and β = 75. The input resistance is:
    
    
    

1. 1. 45 kΩ

   
   
   

   2. 45.26 Ω

   
   
   

   3. 90 kΩ

   
   
   

   4. 90 Ω

   
   
   

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1. View Hint View Answer Discuss in Forum

   By making small-signal hybrid π-model for given fig.
   

   rπ =	βVT	=	75 × 26 × 10–3	= 1.94 kΩ
   	ICQ		1 × 10–3

   
   v_i = I_b (r_π + 1.5 × 10³)
   I_in = I_e = I_b + I_c = I_b + β I_b = (1 + β) I_b
   The input resistance,
   

   Rin =	Vin	=	Ib(rπ + 1·5 × 103)
   	Iin		(1 + β)Ib

   
   

   =	1·94 × 103 + 1·5 × 103
   	1 + 75

   
   

   =	3440	= 45.26 Ω
   	76

   
   Hence alternative (B) is the correct choice.
   

   
   

   Correct Option: B

   By making small-signal hybrid π-model for given fig.
   

   rπ =	βVT	=	75 × 26 × 10–3	= 1.94 kΩ
   	ICQ		1 × 10–3

   
   v_i = I_b (r_π + 1.5 × 10³)
   I_in = I_e = I_b + I_c = I_b + β I_b = (1 + β) I_b
   The input resistance,
   

   Rin =	Vin	=	Ib(rπ + 1·5 × 103)
   	Iin		(1 + β)Ib

   
   

   =	1·94 × 103 + 1·5 × 103
   	1 + 75

   
   

   =	3440	= 45.26 Ω
   	76

   
   Hence alternative (B) is the correct choice.
   

   
   

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89. The early voltage of a BJT is V_A = 75 V. The minimum required collector current, such that the output resistance is at least r₀ = 200 kΩ, is:

1. 1. 1.67 mA

   
   
   

   2. 5 mA

   
   
   

   3. 0.375 mA

   
   
   

   4. 0.75 mA

   
   
   

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1. View Hint View Answer Discuss in Forum

   Given that V_A = 75 V
   r₀ = 200 kΩ
   

   r0 =	VA
   	ICQ

   
   

   or ICQ =	VA	=	75	= 0.375 mA
   	r0		200 × 103

   Correct Option: C

   Given that V_A = 75 V
   r₀ = 200 kΩ
   

   r0 =	VA
   	ICQ

   
   

   or ICQ =	VA	=	75	= 0.375 mA
   	r0		200 × 103

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90. The circuit of fig. below β =100 and V_A = ∞
    

    The small signal voltage gain Aν =	ν0	is:
    	νs

    
    
    

1. 1. – 1.96

   
   
   

   2. – 3.18

   
   
   

   3. – 4.73

   
   
   

   4. – 10.43

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: A

   NA

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