# Is it possible to use 2n7000 to build a common-gate amplifier?

Discussion in 'General Electronics Chat' started by simpsonss, Dec 11, 2010.

1. ### simpsonss Thread Starter Active Member

Jul 8, 2008
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hi,

I'm wondering is it possible to build a basic common-gate by using E-mosfet 2n7000? because as what i know, to bias an E-mosfet we need to meet the threshold voltage which is 0.8v- 3.0v for 2N7000. Since for common-gate design i have grounded the gate. So how am i gonna get a positive value of Vgs?

thank you.

Feb 5, 2010
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3. ### Jony130 AAC Fanatic!

Feb 17, 2009
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In CG amplifier we need to short gate to GND, but this "short" is needed only for AC signal. This is not a necessary DC short.
So we can bias gate with DC voltage by using voltage divider and then use capacitor to short the gate for AC signal.

Last edited: Dec 11, 2010
4. ### beenthere Retired Moderator

Apr 20, 2004
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If the gate is at ground, you can pull the source voltage down to a negative value to produce the needed voltage difference. The gate will be relatively positive with respect to the source.
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5. ### Ron H AAC Fanatic!

Apr 14, 2005
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The lecturer states on p.8 that the 4.7Meg gate resistor will have no other effect on the circuit other than to force the gate to ground.
This is true for the biasing, but the 4.7Meg resistor will have a huge effect on the frequency response, due to Cgs and Cgd, which are not shown in the small-signal equivalent circuits, but will always be present in real-world devices.

Last edited: Dec 11, 2010
6. ### simpsonss Thread Starter Active Member

Jul 8, 2008
173
0
ok.from the link given by kermit2, the source resistor is connected to -Vss. Before that i tried with my own circuit which is similar to the one, just that i connect the source resistor to GND. and i found that it is not working because Vgs =0. Then, i tried to connect my source resistor to -5V.which is exactly the same with the link and then it works. i manage to get a very high gain. But not sure whether correct onot because i haven done my gain calculation yet. Anyway, thanks for the replies.