Friday 2 May 2014

Low threshold N-channel MOSFET


I have a circuit which does something not important, but it results in an output being normally low, and then on activation climbs to a little below Vcc. At the moment, it appears my Vcc is 1.8 V and the signal gets up to between 1.2 V and 1.4 V. I want to use this signal to control the gate of an n-channel MOSFET which is grounded on the source and has the drain tied to Vcc through a large resistive load.


The problem is the threshold voltage of the standard MOSFETs I have appears to be too high, and I have to raise Vcc to about 2.2 V in order to get the circuit to trigger the MOSFET correctly. Are there better MOSFETs for this situation or some other design idea that would work well and solve my problem?



BTW, the switching speed is not really a consideration... we're talking 1-200 Hz for frequency.


Application Note, see page 3. I was hoping I could just use their circuit exactly and just mess with the tuning component, but it doesn't seem to work at 1.8 V.



Answer



Go to eg Digikey
(1) CETSEMI make some superb parts. Available in NZ via agents. May be hard to find in US.


Example CETSEMI CEM8208
20V, 7A dual MOSFET. See graph below. At Vgs = 1V and 25C it will handle about 500 mA at OK Vds. Warmer is better :-).


enter image description here


(2) Search favorite sources for MOSFET N Channel logic
eg Digikeys MOSFET N Channel logic search results.

Select single MOSFETS.
Select Vgth range desired - say up to 400 mV.


The following are all under $1/1 at Digikey (last getting a bit dearer).


The BSH103 may do although a bit unhappy at low low Vgs


NTJS3157 - better


Better - Si4836Dy http://www.onsemi.com/pub_link/Collateral/NTJS3157N-D.PDF




(3) Bipolar NPN will do this with ease. For super low Vsat drive base at much higher current than collector. eg if you have 10 uA collector current then eg 100 uA or 1 mA base current will give much lower than usual Vce_sat.


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