JFET Volt Ampere Characteristics Questions and Answers

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “The JFET Volt-Ampere Characteristics”.

1. For low value of V_DS, the JFET behaves like a __________
a) Voltage Variable Resistor
b) Constant Voltage Device
c) Amplifier
d) Switch
View Answer

Answer: a
Explanation: When V_DS voltage is very less, there will not be much current flow, since as the V_DS changes, the current value changes very little. Hence we can say that FET works as a voltage controlled Resistor.

2. If a JFET with length L=10µm, a=2µm, W=8µm, V_p=-4V.What is the value of r_ds at V_gs = 0V?
a) 2KΩ
b) 5.2KΩ
c) 10KΩ
d) 9.8KΩ
View Answer

Answer: d
Explanation: r_ds=L/(2aqN_D µ_nW) = Nd = 2V/qa²=1.33×10²¹ atoms/m³
µn=0.15m²/v-sec
On substituting the values, we get r_ds=9.8KΩ.

3. A JFET has I_D=10mA, I_DSS=1A, V_p=-1v, what is the value of V_gs?
a) -0.9V
b) 1V
c) -1V
d) 0.5V
View Answer

Answer: a
Explanation:

The value of Vgs is -0.9V for JFET has ID=10mA, IDSS=1A, Vp=-1v

4. Where does the transfer curve lie for a p- channel FET?
a) First quadrant
b) Second quadrant
c) Third quadrant
d) Fourth quadrant
View Answer

Answer: a
Explanation: For a P- channel FET, since the pinch off lies on right side of the origin, the current Id will increase from V_P and rise to its left until it reaches I_DSS. Usually I_DSS lies on positive Y axis, therefore we can say transfer curve for the p-channel FET lies on first quadrant.

5. For an n-channel FET, the current v at V_gs=-2V and V_ds=5V was found to be 2mA, what will be the value of I_D at V_gs=0V and V_ds=5V?
a) 0A
b) 2mA
c) Lesser than 0A
d) Greater than 2mA
View Answer

Answer: d
Explanation: We know that for a n channel FET, the current will flow only if V_gs is greater than V_p, since in this case current is flowing, which indicates V_gs is greater than pinch off. But as V_gs increases the flow of current also increases, hence it will be greater than 2mA.

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6. For a p-channel FET, the current I_D at V_gs = 2V and V_ds = 5V and V_gs = 3V was found to be 2mA, what will be the value of I_D at V_gs = 0V and V_ds = 5V?
a) 0A
b) 2mA
c) Lesser than 0A
d) Greater than 2mA
View Answer

Answer: a
Explanation: We know that for a p channel FET, the current will flow only if V_gs is lesser than V_gs, but here pinch off voltage is 3V, when V_gs=5V which is greater than pinch off voltage the channel will be destroyed resulting in no conventional current flow.

7. A FET is biased at I_D = I_DSS/4, at this point the value of trans conductance is __________________ (I_D=10mA, V_p=-5V).
a) 1mA/V
b) 2mA/V
c) 3mA/V
d) 4mA/V
View Answer

Answer: b
Explanation:

8. What is the value of resultant g_m if two non identical FETs are connected in parallel?
a) (µ₁ r_d1 + µ₂ r_d2)/ (r_d1+r_d2)
b) (µ₁ r_d2 + µ₂ r_d1)/ (r_d1+r_d2)
c) 0
d) µ₁ r_d1+ µ₂ r_d2
View Answer

Answer: b
Explanation: we know that µ=g_m r_d
But g_m’=g_m1+g_m2
And r_d’=r_d1 ×r_d2/r_d1+ r_d2
From the above two equations, g_m’=(µ₁ r_d2 + µ₂ r_d1)/ (r_d1+r_d2).

9. What will be the value of r_d, if two identical FETs are connected in parallel?
a) Doubles
b) Reduces to half
c) 0
d) Infinite
View Answer

Answer: b
Explanation: r_d’=r_d1 ×/r_d1+ r_d2
If r_d1=r_d1=r_d => then r_d’=r_d²/2r_d =r_d/2.

10. What will be the value of trans conductance if two Identical FETs are connected in parallel?
a) Doubles
b) Reduces to half
c) 0
d) Infinite
View Answer