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I have a small question. I was doing a lab with MOSFET 2N7000. The circuit is something like the one below:enter image description here

There was a question asking if the gain should change if the resistor R1 was changed to 10K? The teacher said the gain should change (slightly) but I did not see any change on the Oscilloscope.

Can someone please advice whether the gain would change if R1 is increased?

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    \$\begingroup\$ Are you sure the gain change was not predicated on R1 being before R2? In that arrangement, the pair of resistors acts as a voltage divider, so gate voltage would drop on increasing R1, hence leading to lower gain. \$\endgroup\$ – Anindo Ghosh May 21 '14 at 4:30
  • \$\begingroup\$ @AnindoGhosh, R2 connects after R1 which is used for complete switching off of the fet. But why would the gain decrease when increasing R1? \$\endgroup\$ – David Norman May 21 '14 at 4:34
  • \$\begingroup\$ Is this the complete circuit? or you have any biasing part? Is there anything connected to drain other than your Oscilloscope?? \$\endgroup\$ – nidhin May 21 '14 at 5:30
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    \$\begingroup\$ @DavidNorman ideally mosfet gate current is zero and hence whatever be value of \$R_1\$, you will get Vgs =5V. And hence Vds/Vgs will also remain the same \$\endgroup\$ – nidhin May 21 '14 at 6:46
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    \$\begingroup\$ @David: I suppose you mean "decreased a bit" - and that would be correct. However, I doubt if such a small change can be observed on the scope. \$\endgroup\$ – LvW May 21 '14 at 6:58
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Of course the gain will change when R1 is increased but this will only be more obvious at higher frequencies. This is due, in main the to gate-source capacitance - it will form a low pass filter and reduce the signal seen at the gate as the inputted frequency increases.

Looking at the data sheet for the 2N7000, it has an input capacitance of typically 20pF and with R1 at 10kohm, the 3dB point will be about 800kHz.

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  • \$\begingroup\$ @DavidNorman are you done with this question now? If so please consider formal acceptance of my answer or, if there is still something that you need clarity on, leave a comment and I'll get back to you. \$\endgroup\$ – Andy aka Jun 18 '20 at 9:09

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