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            Exam Questions Papers - Engineering
Q1: For a npn BJT transistor fβ is 1.64 x 108 Hz. Cμ = 10-14 F; Cp = 4 x 10-13 F and DC current gain is 90. Find fT and gm (fβ = cut off frequency, Cμ = capacitance, Cp = parasitic capacitance, gm = transconductance, fT = gain BW product)A 
fT = 1.47 x 1010 Hz ; gm = 38 milli mho
  B 
fT = 1.64 x 108 Hz ; gm = 30 milli mho
  C 
fT = 1.47 x 109 Hz ; gm = 38 mho
  D 
fT = 1.33 x 1012 Hz; gm = 0.37 m-mh
  ANS:A - fT = 1.47 x 1010 Hz ; gm = 38 milli mho  
∴ fT = 90 x 1.64 x 108 = 1.47 x 1010 Hz
 
gm = 2pfT(Cμ + Cp) = 2 x p x 1.47 x 1010
= (10-14 + 4 x 10-13)
gm= 38 mμ.
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∴ fT = 90 x 1.64 x 108 = 1.47 x 1010 Hz
gm = 2