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mosfet as a switch
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madhu.wesly01:

I'm implementing mosfet switching circuitry using the P channel enchancement mode mosfet DMP2305U.Where Vs = 3V and Vg is using as control voltage and toggleing between 0 and 3V . When Vg is zero volts , I'm getting output at Vd as high , when Vg is 3Volts I'm getting output at Vd as low as desired functionality, but i'm not getting the sufficient current through the mosfet . It seems like Vth of this mosfet is -0.9V and as a ground rule , Vgs should be greater than Vth to supply the max current , but here in my case Vgs is -3V (When the mosfet is on Vg=0v and Vs=3) .Here, do we need to consider the signs ?(negative , positive) of Vgs and Vth ?

David Hess:
As shown in your schematic, you are using the MOSFET backwards.
photomankc:
Yep, You have the symbol backwards to your text labels.




In your example, assuming it's the right way around, then a 0V signal applied at the gate would give you about -3.0V Vgs and that should be sufficient to turn on and conduct significant current based on the datasheet curves.  If you actually have it the right way around then we may need to see more of the real circuit to guess why it's not working fully. 
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