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
The small signal equivalent circuit of commondrain amplifier is shown below.
Using Thevenin’s theorem, the output voltage is
(where, V_{gd }= V_{i} = input voltage)
∴ Voltage gain is
The value of V_{0}/V_{in} for the circuit shown below is (Assume M_{1} in saturation)
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
Consider the following statements:
1. In most linear applications of fieldeffect transistors the device is operated in the. constantcurrent portion of its output characteristics.
2. In the region before pinchoff, 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?
In the saturation region, the I_{D } V_{GS }characteristics of a MOSFET are
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.
The sourcebias circuit provides DC biasing for the FET. The resistance from gate to ground (R_{S}) in that circuit is necessary because, without it,
In an FET commonsource high frequency amplifier, which one of the following is the correct expression for input capacitance C_{i} ?
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
We know that,
μ = g_{m}r_{d}
or,
Output resistance,
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
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
The unity gain bandwidth f_{T} of a JFET is given by
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
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