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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

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 V_{0}/V_{in} for the circuit shown below is (Assume M_{1} in saturation)

Solution:

M_{2} 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 d_{2} 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?

Solution:

QUESTION: 4

In the saturation region, the I_{D }- V_{GS }characteristics of a MOSFET are

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,

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} ?

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 R_{0} is given approximately by

Solution:

We know that,

μ = g_{m}r_{d}

or,

Output resistance,

QUESTION: 8

A FET tuned amplifier with g_{m} = 5 mA/V, r_{d} = 20 kΩ has a resonant impedance of 20 kΩ. The gain at resonance is given by

Solution:

A_{V} = Gain at resonance = g_{m}R

Here, R = r_{d} ║R_{max}

∴ A_{V} = 5x 10^{-3} x 10 x 10^{3} = 50

QUESTION: 9

The unity gain bandwidth f_{T} of a JFET is given by

Solution:

QUESTION: 10

The transconductance for a JFET having I_{DSS }= 8 mA, V_{P} = -5 V and biased to operate at V_{GS} = - 1.8 volt will be given by

Solution:

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