Electronics and Communication Engineering - Exam Questions Papers - Discussion

1. 

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

Answer: Option B

Explanation:

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.


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