MOSFET Question

Discussion in 'General Electronics Chat' started by Mazaag, Nov 20, 2006.

  1. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    Hey guys,

    I have a question with regards to NMOS's.. I am aware that the NMOS switches from Cut-off to Triode/Saturation once the Vgs is larger than the Threshold Voltage(which is a process parameter).

    Now my question is as follows. How is it possible to alter the switching voltage such that the NNOS can be configured to switch on at a set voltage level? Basically, I want the NMOS to switch On at a voltage of say 2 volts (rather than Vt).

    Thanks guys
  2. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    The FET will have Vgs threshold stated as some min/max value such as 1V to 3V, and this will not change (unless you select a different FET, which will still have a min/max spread) -- due to the process as you noted. Therefore in using it as a switch, you would want to make sure to apply more than 3V (using above numbers) to the gate to make sure it is fully on -- 2V would not really suffice.

    One way to overcome this is to use a +5V comparator with the minus (-) side referenced at your desired voltage (say +2.0V), with the input signal on the plus (+) side. While the input signal is less than +2V the comparator outpu will remain low and the FET off. Whenever your input is above +2.0V reference, the comparator out would drive the FET gate sufficiently to saturation (and, for the most part, indiscriminately of which FET you choose).

    There are many references available for comparator circuits.
  3. jonel

    New Member

    Nov 16, 2006
    And also other way is to put a zener diode of more or less having a zener voltage of 2V connected to gate in series to a resistor value across the Gate and source