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The voltage gain for the given common source FET circuit is:


The voltage gain for the given common source FET circuit is:

Common source FET amplifier circuit diagram

FET Parameters

  • Yos = 10 μS
  • Yfs = 3000 μS

Options

  • A. -3
  • B. -7.9
  • C. 2.54
  • D. -5

Solution

Given:


Yfs = gm = 3000 μS = 0.003 S
Yos = gds = 10 μS

Drain resistance of the FET:


ro = 1 / gds
ro = 1 / (10 × 10^-6)
ro = 100 kΩ

Given circuit resistances:

  • RD = 2.7 kΩ
  • RS = 2.7 kΩ

Since capacitor C2 bypasses RS, the source is AC grounded. Therefore the voltage gain for a common-source amplifier is: 


Av = - gm (RD || ro)

Calculate parallel resistance:


RD || ro = (2.7k × 100k) / (2.7k + 100k)
≈ 2630 Ω

Voltage gain:


Av = -0.003 × 2630
Av ≈ -7.9

Correct Answer: B. -7.9

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