51. If an npn transistor has a beta cut-off frequency f_B of 1MHz, and emitter short circuit low frequency current gain β₀ of 200, then unity gain frequency f_T and alpha cut off frequency f_α respectively are
    

1. 1. 200 MHz, 201 MHz

   
   
   

   2. 200 MHz, 199 MHz

   
   
   

   3. 199 MHz, 200 MHz

   
   
   

   4. 201 MHz, 200 MHz

   
   
   

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1. View Hint View Answer Discuss in Forum

   f _T = β₀ × f_β = (200) × 1 MHz = 200 MHz
   

   and fα =	fβ	=	fβ
   	1 - α		1 -	β
   				1 + β

   
   = f _β (β+ 1) = 10⁶ (200 + 1) 10⁶ × 201 = 201 MHz
   

   Correct Option: A

   f _T = β₀ × f_β = (200) × 1 MHz = 200 MHz
   

   and fα =	fβ	=	fβ
   	1 - α		1 -	β
   				1 + β

   
   = f _β (β+ 1) = 10⁶ (200 + 1) 10⁶ × 201 = 201 MHz
   

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52. If an npn transistor (with C = 0.3 pF) has a unity gain cut-off frequency f_T of 400 MHz at a d.c. bias current I_c = 1 mA, then value of its C is approximately (V_T = 26 mV)
    

1. 1. 15

   
   
   

   2. 30

   
   
   

   3. 50

   
   
   

   4. 96

   
   
   

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1. View Hint View Answer Discuss in Forum

   Given : C = 0.3 pF
   

   ∴ gm =	|IC|(mA)	=	1	A/V
   	26 mV		26

   
   

   ⇒ C =	gm	=	1 / 26	= 15 pF
   	2π fT		2π × 4 × 108

   
   

   Correct Option: A

   Given : C = 0.3 pF
   

   ∴ gm =	|IC|(mA)	=	1	A/V
   	26 mV		26

   
   

   ⇒ C =	gm	=	1 / 26	= 15 pF
   	2π fT		2π × 4 × 108

   
   

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53. An amplifier has an open loop gain of 100, an input impedance of 1kΩ, and an out put impedance of 100 Ω. If a feedback network with a feedback factor of 0.99 is connected in a voltage series feedback mode then new input and output impedances are
    (a)
    (b)
    (c)

1. 1. 10 Ω and 1 Ω

   
   
   

   2. 10 Ω and 10 k Ω

   
   
   

   3. 100 k Ω and 1 Ω
      

   
   
   

   4. 100 kΩ 10 k Ω

   
   
   

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1. View Hint View Answer Discuss in Forum

   Z_if = Z_i (1 + βA) = 1 k (1 + 100 × 0.99) = 100 kΩ
   

   Zof =	Z0	=	100	= 1 Ω
   	1 + βA		100

   
   

   Correct Option: C

   Z_if = Z_i (1 + βA) = 1 k (1 + 100 × 0.99) = 100 kΩ
   

   Zof =	Z0	=	100	= 1 Ω
   	1 + βA		100

   
   

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54. RC network shown in the given figure can provide maximum theoretical phase shift of
    
    

1. 1. 90°

   
   
   

   2. 180°

   
   
   

   3. 270°

   
   
   

   4. 360°

   
   
   

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1. View Hint View Answer Discuss in Forum

   Each section having phase shift of 60°, adding up to 180°

   Correct Option: B

   Each section having phase shift of 60°, adding up to 180°

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55. The resonant frequency of the Wein bridge oscillator of given in figure, is
    
    

1. 1. 19597 Hz

   
   
   

   2. 1592.35 Hz

   
   
   

   3. 3120.7 Hz

   
   
   

   4. 4777 Hz

   
   
   

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1. View Hint View Answer Discuss in Forum

   f =	1	=	1	= 3120.7 Hz
   	2πRC		2π(51 × 103)(0.001 × 10-6)

   
   

   Correct Option: C

   f =	1	=	1	= 3120.7 Hz
   	2πRC		2π(51 × 103)(0.001 × 10-6)

   
   

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