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Exam Questions Papers - Engineering

Q1:

It is desired to design a phase shift oscillator using an FET having a gm = 5000 μSiemen, rd = 40 kΩ and feedback circuit value of R = 10 K. Then value of C for oscillator operation at 1 kHz and that of RD for A = 40 to ensure oscillator action are

A 6.4 nF, 80 K

B 6.5 nF, 10 K

C 6.5 pF, 10 K

D 65 pF, 80 K

ANS:B - 6.5 nF, 10 K

For a FET oscillator, the oscillator frequency is ∴ C = 6.4 nF For FET Oscillator, |A| = gmRL = 5000 x 10-6 RL i.e., 40 = 5000 x 10-6 x RL ∴ = 8 KΩ Now, Using RL = 8 K and rd = 40 K (given) We find RD as follows: ∴ ∴ = 10 K RD = 10 K.