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

#### simpsonss

Joined Jul 8, 2008
173
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.

#### Kermit2

Joined Feb 5, 2010
4,162

#### Jony130

Joined Feb 17, 2009
5,539
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.

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#### beenthere

Joined Apr 20, 2004
15,819
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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#### Ron H

Joined Apr 14, 2005
7,063
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.

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