The JFET in a circuit shown in figure has an IDSS = 10 mA

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The JFET in a circuit shown in figure has an I_DSS = 10 mA and V_P = – 5V. The value of the resistance R_S for a drain current I_DI = 6·4 mA is (select the nearest value)—

1. 150 Ω
2. 470 Ω
3. 560 Ω
4. 1 k Ω

Correct Option: A

The given circuit

Given, I_DSS = 10 mA, V_p = – 5V, I_D = 6·4 mA, R_S =?
From given circuit

I_D = I_DSS		1 -	V_GS		2
			V_p

Or		I_D	=		1 -	V_GS			2
		I_DSS				V_p

Or		√6.4	= 1 -	V_GS
		10		V_p

or 0·8 = 1 –	V_GS
	V_p

Or	V_GS		= 0.2
	V_p

or V_GS = 0·2 V_p = 0·2 (– 5)
or V_GS = – 1 V
Since V_GS = – I_D R_S
or – 1 = – 6·4 mA × R_S

or R_S ≈ 150 Ω
Hence alternative (A) is the correct choice.

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or R_S =	1		= 156·25 Ω
	6·4 × 10^–3