# question about common-drain E-mos amplifier

Discussion in 'General Electronics Chat' started by simpsonss, Dec 12, 2010.

1. ### simpsonss Thread Starter Active Member

Jul 8, 2008
173
0
hi,

I build my common-drain amplifier by referring to common-source amplifier which using a voltage divider to bias the E-mosfet. The CD circuit is as below.

R1=100k R2=19K RS=100 and RL=10k Vdd=+10V

When i input in 40mVp-p sine wave, i cant get any result at the output part. Vgs=0 which means the 2n7000 is not biasing. Anyone can help me on this? Isnt it the voltage divider should bias the mosfet ?thanks.

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
290
Look at your divider values. The gate is only 1.59 volts above the source. The data sheet for a 2N7000 says the threshold VGS is 2.1 volts. The device is not conducting.

when you say:
that means something in your setup is incorrect. Assuming 10 volts is applied to R1, R1 & R2 are connected, and R2 is tied to ground, the the voltage at the junction of R1 & 2 should be 1.5966 volts, by calculation. If the measured voltage is 0, then the hexfet is probably bad. Try a new one.

Do you understand that a hexfet is not at all like a transistor?

3. ### Wendy Moderator

Mar 24, 2008
20,993
2,731
And the correct term for these types of transistor is a MOSFET. HexFET being a subset.

Last edited: Dec 15, 2010
4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
659
A common drain amplifier is also known as a source follower. The drain is at AC ground, either directly or through a capacitor, and the output is taken from the source.
In your circuit, it looks like you are trying to take the output from Vdd, which is the positive supply rail. This is at AC ground, so will have no signal on it.