161. If an amplifier with gain of 1000 and feedback of β = – 0·1 had a gain change of 20% due to temperature, the change in gain of the feedback amplifier would be—

1. 1. 10%

   
   
   

   2. 5%

   
   
   

   3. 0·2%

   
   
   

   4. 0·01%

   
   
   

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1. View Hint View Answer Discuss in Forum

   Given,
   A = 1000
   β = – 0·1
   

   
   

   dA	= 20% = 0·20
   A

   dAf	= ?
   Af

   
   

   We know that Af =	A 1	…(i)
   	1 + Aβ

   
   Where, A_f = Gain after feedback
   A = Gain
   β = Feedback factor
   

   Now,		dAf	=	1	…(ii)
   		dA		(1 + Aβ)2

   
   From equation (i) & (ii)
   

   		dAf	=	dA/A
   		Af		1 + Aβ

   
   

   or		dAf	=	0.20
   		Af		1 + 1000 × (0·1)

   
   

   =	0.20	≈ 0·2%
   	101

   
   Hence alternative (C) is the correct choice.

   Correct Option: C

   Given,
   A = 1000
   β = – 0·1
   

   
   

   dA	= 20% = 0·20
   A

   dAf	= ?
   Af

   
   

   We know that Af =	A 1	…(i)
   	1 + Aβ

   
   Where, A_f = Gain after feedback
   A = Gain
   β = Feedback factor
   

   Now,		dAf	=	1	…(ii)
   		dA		(1 + Aβ)2

   
   From equation (i) & (ii)
   

   		dAf	=	dA/A
   		Af		1 + Aβ

   
   

   or		dAf	=	0.20
   		Af		1 + 1000 × (0·1)

   
   

   =	0.20	≈ 0·2%
   	101

   
   Hence alternative (C) is the correct choice.

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162. If α = 0·995, I_E = 10 mA and I_CO = 0·5 µA, then I_CEO will be—

1. 1. 25 µA

   
   
   

   2. 100 µA

   
   
   

   3. 10·1 µA

   
   
   

   4. 10·5 µA

   
   
   

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1. View Hint View Answer Discuss in Forum

   Given, α = 0·995, I_E = 10 mA, I_CO = 0·5 µA, I_CEO =?
   

   We know that ICEO =		1 +	α		ICO
   			1 – α

   
   

   or ICEO =		1 +	0.995		x 0.5 µA
   			1 - 0.995

   
   

   =	0·5 µA	= 100 µA
   	0·005

   Correct Option: B

   Given, α = 0·995, I_E = 10 mA, I_CO = 0·5 µA, I_CEO =?
   

   We know that ICEO =		1 +	α		ICO
   			1 – α

   
   

   or ICEO =		1 +	0.995		x 0.5 µA
   			1 - 0.995

   
   

   =	0·5 µA	= 100 µA
   	0·005

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163. For a PNP transistor, α = α_R = 0·9 and I_CO = 10 µA the value of I_CEO will be—

1. 1. 15·2 µA

   
   
   

   2. 52·6 µA

   
   
   

   3. 72·5 µA

   
   
   

   4. 1·00 µA

   
   
   

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1. View Hint View Answer Discuss in Forum

   Given, α = α_R = 0·9
   I_CO = 10 µA
   ICEO =?
   

   We know that, ICEO = (1 + β) ICO =		1 +	α		ICO
   			1 – α

   
   

   or ICEO =		1 +	0.9		ICO
   			1 – 0.9

   
   = 10 × 10 µA = 100 µA
   Hence alternative (D) is the correct choice.

   Correct Option: D

   Given, α = α_R = 0·9
   I_CO = 10 µA
   ICEO =?
   

   We know that, ICEO = (1 + β) ICO =		1 +	α		ICO
   			1 – α

   
   

   or ICEO =		1 +	0.9		ICO
   			1 – 0.9

   
   = 10 × 10 µA = 100 µA
   Hence alternative (D) is the correct choice.

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164. If the transistor in figure is in saturation then—
     
     
     

1. 1. I_C is always equal to β_dc I_B

   
   
   

   2. I_C is always equal to – β_dc I_B

   
   
   

   3. I_C is more than or equal to β_dc I_B

   
   
   

   4. I_C is less than or equal to β_dc I_B

   
   
   

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1. View Hint View Answer Discuss in Forum

   The given figure (i.e., fig)
   For saturation
   I_B ≥ I_B(min)
   

   or IB ≥	Ic
   	hFE

   
   

   or IB ≥	Ic
   	βdc

   
   or I_c ≤ β_dcI_B
   i.e. I_c is less than or equal to b_dc·I_B
   Hence alternative (D) is the correct choice.

   
   

   Correct Option: D

   The given figure (i.e., fig)
   For saturation
   I_B ≥ I_B(min)
   

   or IB ≥	Ic
   	hFE

   
   

   or IB ≥	Ic
   	βdc

   
   or I_c ≤ β_dcI_B
   i.e. I_c is less than or equal to b_dc·I_B
   Hence alternative (D) is the correct choice.

   
   

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165. In the cascode amplifier shown in figure, if the commonemitter stage (Q₁) has a transconductance g_m1, and the common base stage (Q₂) has a transconductance of the cascade amplifier is—
     
     
     

1. 1. g_m1

   
   
   

   2. g_m2

   
   
   

   3. g_m1 / 2

   
   
   

   4. g_m / 2

   
   
   

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1. View Hint View Answer Discuss in Forum

   The given figure (i.e., fig)
   From figure, for transistor Q₁
   

   gm1 =	ic1
   	Vi

   
   and for transistor Q₂
   

   or gm2 =	i0
   	Vi

   
   

   or gm2 =	ie1
   	Vi

   (Since for common base transistor current gain ≈ 1 i.e. i ₀ = i _e1)
   or g_m2 = g_m1
   Therefore the transconductance of the cascode amplifier is g_m1.

   
   

   Correct Option: A

   The given figure (i.e., fig)
   From figure, for transistor Q₁
   

   gm1 =	ic1
   	Vi

   
   and for transistor Q₂
   

   or gm2 =	i0
   	Vi

   
   

   or gm2 =	ie1
   	Vi

   (Since for common base transistor current gain ≈ 1 i.e. i ₀ = i _e1)
   or g_m2 = g_m1
   Therefore the transconductance of the cascode amplifier is g_m1.

   
   

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