Test: Basic FET Amplifiers- 2

QUESTION: 1

In the common drain amplifier shown below, if R_S= 4 KΩ, R_G= 10 MΩ, μ = 50 and r_d= 35 KΩ then the voltage gain A_v of the amplifier is approximately equal to

A.
0.66
B.
0.84
C.
0.77
D.
0.92

Solution:

The small signal equivalent circuit of common-drain amplifier is shown below.

Using Thevenin’s theorem, the output voltage is

(where, V_gd= V_i = input voltage)
∴ Voltage gain is

QUESTION: 2

The value of V₀/V_in for the circuit shown below is (Assume M₁ in saturation)

Solution:

M₂ is also saturated because its gate and drain are shorted together. The small signal circuit is shown below:

Here,
For most MOSFET transistor, r_d1 and d₂ are much greater than 1/g_m2.
Hence,
Now,
∴
= Required voltage gain

QUESTION: 3

Consider the following statements:
1. In most linear applications of field-effect transistors the device is operated in the. constant-current portion of its output characteristics.
2. In the region before pinch-off, where V_DS is small, the FET is useful as a voltage variable resistors (VVR).
3. The VVR can be used in AGC amplifier to vary the voltage gain of multistage amplifier.

Q. Which of the above statements is/are correct?

A.
2 only
B.
1 and 2 only
C.
1, 2 and 3
D.
2 and 3 only

Solution:

QUESTION: 4

In the saturation region, the I_D- V_GScharacteristics of a MOSFET are

A.
linear
B.
quadratic
C.
exponential
D.
hyperbolic

Solution:

For amplifier operation, MOSFET is always operated in saturation region.
Since, in saturation region,therefore the I_D- V_GS characteristics of a MOSFET are quadratic as evident from the above equation.

QUESTION: 5

The source-bias circuit provides DC biasing for the FET. The resistance from gate to ground (R_S) in that circuit is necessary because, without it,

A.
the input time-constant will become infinite.
B.
the input AC signal will have no path to ground.
C.
there will be no DC potential established at the gate terminal.
D.
the circuit will actually work fine; R_g is not really necessary.

Solution:

QUESTION: 6

In an FET common-source high frequency amplifier, which one of the following is the correct expression for input capacitance C_i ?

A.
C_i = C_gs+ (1 - A_V)C_gd
B.
C_i = C_gs+ (1 - 1/A_V)C_gd
C.
C_i = C_gd+ (1 - A_V)C_gs
D.
C_i = C_gd+ (1 - A_V)C_gs

Solution:

QUESTION: 7

A JFET is set up as a source follower. Given, μ = 200, r_d = 100 kΩ and source load resistance R_L = 1 kΩ. The output resistance R₀ is given approximately by