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I am currently designing a MOSFET common source amplifier for my electronics lab class. My issues is that currently I am unable to obtain a gain out of my current circuit design.

I am using a 2N7002 MOSFET in my simulated design My input signal is 1v/100hz with a 12v power supply.

What could be the issue here?

My current desgin Circuit design

Red is my input signal

Green is my output Input and Output

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    \$\begingroup\$ where are you measuring the output? \$\endgroup\$
    – jsotola
    Commented Jul 17, 2020 at 3:39
  • \$\begingroup\$ How did you derive the biasing conditions? What DC voltage do you see on gate, drain and source? Is that within theFET's linear region? \$\endgroup\$
    – user16324
    Commented Jul 17, 2020 at 9:56

1 Answer 1

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With the 2 MΩ and 3.9 MΩ input resistors, the time taken for the bias to set-up correctly on the gate will be approximately the parallel combination of those resistors multiplied by the input capacitance: -

$$\text{(2 MΩ || 3.9 MΩ) x 10 μF} = \text{12.9 seconds}$$

You are only running your simulation for 100 ms so you will never reach the point when the transistor starts to operate correctly. I believe LTSpice can be initialized with the capacitor pre-charged to the appropriate value (7.93 volts). You might want to do that.

Alternatively, try reducing the input capacitor to around 33 nF and watch the MOSFET spring into action a lot quicker.

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