# A little confirmation need on MOSFET circuit

Discussion in 'Homework Help' started by Hanck, May 14, 2009.

Nov 25, 2008
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2. ### Audioguru Expert

Dec 20, 2007
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Most Mosfets will not completely turn on when the gate voltage is only 6V. They need 10V. Logic-level Mosfets turn on very well when the gate is only 4.5V or more.

Your 50k gate resistor limits the current that charges the very high capacitance of the gate of the Mosfet so it will turn on and turn off very slowly.

3. ### mik3 Senior Member

Feb 4, 2008
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Replace the 50K resistor with a 50R one.

4. ### Hanck Thread Starter Member

Nov 25, 2008
18
0
Thanks for the information, Audioguru and mik3.

But its not like i can modified the circuit ^^ its a pass year question.

Actually its given a transconductance curve of the E mosfet, and the threshold voltage for this EMOSFET is 2V.

so i need to know the value of Vgs and so that i can get its IDQ . So... is the Vgs of the circuit is 6V?

Because of what i know is that FET has no current on Gate, so i assume 50k Res will not causes drop. Please help me to point out if I were wrong. Thanks

5. ### mik3 Senior Member

Feb 4, 2008
4,846
69
For this circuit Vgs=6-Id*5K

The 50K resistor is not a big problem when Vgs settles to its final value. The problem with the 50K is when you switch the MOS on/off. It limits the current needed to charge/discharge the gate and delays the MOS to turn on/off. This increases the switching losses.

6. ### Audioguru Expert

Dec 20, 2007
10,794
1,216
The threshold voltage is the max Vgs that causes the Mosfet to barely conduct only 0.25mA. It is almost turned off.
You need to know the minimum voltage that it is turned on so that it can conduct many Amps. It might be 10V.