Enhancement MOSFET are usually used as switch to turn on and off power supply or for digital circuit. When E-MOSFET is used as a switch it is biased in the Ohmic region. In ohmic region the enhancement MOSFET acts a resistor. Here it is shown how to bias Enhancement MOSFET in the ohmic region to in order to use it as a switch. We will use the 2N7000 N- channel enhancement MOSFET as an example.
The circuit diagram of E-MOSFET as a switch is shown below.
In the above circuit, the digital logic level signal(ON or OFF) shown as Vin is applied to the gate of the E-MOSFET. The drain is connected to the power supply VDD via the drain resistor RD. The output shown as Vout is taken from the drain.
In order to bias the enhancement MOSFET in the ohmic region we must ensure that the Q-point lies in the ohmic region. For 2N7000 MOSFET the ohmic region in the drain curve is shown below.
Step 1
Let suppose that the input signal is ON and OFF signal with 5V when ON and 0V when OFF. This means the gate to source voltage \(V_{GS}\) is either 5V or 0V.
Now from the drain curve above (or from the E-MOSFET datasheet) we have to obtain \(V_{DS(on)}\) and \(I_{DS(on)}\) corresponding to \(V_{GS}=V_{GS(on)}\). We assume that \(V_{DS(on)}=V_{th}\) and thus obtain \(I_{DS(on)}\) for which \(V_{GS}=V_{GS(on)}\) and \(V_{DS(on)}=V_{th}\).
For example, from the drain curve, when \(V_{GS}=V_{GS(on)}=5V\) and when \(V_{DS(on)}=V_{th}=2.1V\) we have,
\(I_{DS(on)}=264mA\)
Step 2To ensure that the E-MOSFET is biased in the ohmic region we must have the following,
\(I_{DQ} < I_{D(on)}\)
When we select \(V_{DSQ}=V_{DS(on)}/2=V_{th}/2\) it is ensured that \(I_{DQ} < I_{D(on)}\).
So let select a Q-point with \(V_{DSQ}=V_{DS(on)}/2=V_{th}/2=frac{2.1}{2}=1.05V\) and find out the \(I_{DQ}=171mA\) which is less than \(I_{D(on)}=264mA\). The Q-point on the drain curve is shown below.
So here when \(V_{GS}=V_{GS(on)}=5V\) is applied to the gate, \(I_{D(sat)}=171mA\) and the enhancement MOSFET is biased in ohmic region.
Step 3
Calculate the drain resistor \(R_D\) at which the saturation drain current \(I_{DQ}=171mA\),
\(R_D= \frac{V_{DD}-V_{DSQ}}{I_{DQ}}=\frac{5V-1.05V}{171mA}=23.1\Omega\)
Step 4
Determine the MOSFET drain to source resistance,
\(R_{DS} = \frac{V_{DS(on)}}{I_{D(on)}}= \frac{2.1V}{264mA}=7.95 \Omega\)
We can also use the online E-MOSFET biasing and amplifier calculator. The following shows the calculated values using the calculator.
The final circuit diagram is shown below.
Below shows the measured gate to source voltage\(V_{GS}\), the drain current\(I_{DQ}\) and the drain to source voltage\(V_{DSQ}\) values in circuit simulator.
So in this way we can bias a enhancement MOSFET in ohmic region to operate as a switch. Some N-channel Enhancement MOSFET application as a switch are shown in the video.
