In this project, we will go over how to connect an N-Channel JFET to a circuit for it to function as an electronic switch to power on a load.
Testing a JFET with a multimeter might seem to be a relatively easy task, seeing as how it has only one PN junction to test: either measured between gate and source, or between gate and drain. Testing Continuity of an N-channel JFET. Testing continuity through the drain-source channel is another matter, though. N-channel JFET is normally on device. It means at zero gate voltage, it turns on. Moreover, it does not require any biasing voltage for the current to flow from drain to source. It is in its saturation mode.
N-CHANNEL JFET DESCRIPTION: The CENTRAL SEMICONDUCTOR 2N4391 series types are N-Channel silicon JFETs designed for analog switching and chopper applications. MARKING: FULL PART NUMBER MAXIMUM RATINGS: (TA=25°C) SYMBOL UNITS Gate-Drain Voltage VGD 40 V Gate-Source Voltage VGS 40 V Gate Current IG 50 mA Power Dissipation (TC=25°C) PD 1.8 W. 2N3819 is a general-purpose N-channel Junction Field Effect Transistor. It operates in depletion-mode and requires reverse biasing to turn off. It is designed for medium to high range frequencies. Furthermore, it has a high gain for wideband frequencies and has an importance in VHF/UHF based systems.
A JFET is a transistor which is normally on. This means that the JFET will conduct current across from the drain to the source without any voltage input into the gate terminal. JFETs don't need any biasing voltage at the base to turn on. It will conduct across without gate voltage. To turn off an N-Channel JFET, this is when we need to apply voltage- we apply sufficient negative voltage to the gate of the transistor to turn the JFET. This is why JFETs are referred to as being normally on.
The type of JFET we will use specifically in this circuit is a MPF102. This is a pretty popularN-Channel JFET.
The pinout for the MPF102 is shown below:
If you deal with BJTs often, it's easy to get into the habit of thinking that the middle pin is the gate, like for a BJT, the middle pin is normally the base, but many times for FETs, this is not the case. The gate can be in the middle or be the 3rd pin, so it's always best to check the datasheet for the pinouts of all JFETs. Otherwise, the circuit will be wiredwrong and it won't work.
So that you know which pin, we can go over all components needed for us to build this circuit.
Components Needed for JFET Switch Circuit
- MPF102 JFET
- LED
- 270Ω resistor
- DC Power Supply or 4 'AA' batteries
The MPF102 can be obtained from a number of different electronics online retailers, such as Tayda Electronics.
The datasheet for the MPF102 can be found at the following link: MPF102 N Channel JFET Datasheet.
In this circuit, we power on an LED. We use a 270Ω resistor to act as a current-limiting resistor to the LED.
You can really use any other output device you want such as a buzzer. It doesn't have to be an LED.
N-Channel JFET Switch Circuit Schematic
The schematic diagram of the N-Channel JFET Switch Circuit we will build is shown below.
This circuit is very basic in setup and in operation.
When there is sufficient voltage supplied to the LED and no voltage at the gate terminal, the JFET is fully operational and conducts across from drain to source, turning on and lighting the LED.
To stop current flow, all that is needed is sufficient negative voltage supplied to the gate terminal, about 3-4V. With 3 to 4 volts of negative voltage, the JFET will no longer conduct from drain to source and the LED will turn off.
If using batteries, 2 'AA' batteries (3V) should be enough to turn off the JFET. However, 3 'AA' will give 4.5Vwhich will definitely shut off the JFET, impeding all current flow. So you have the choice of using 2 or 3 batteries at the gate terminal to stop current flow.
And this is how an N-Channel JFET works.
To see how this circuit works in real life, see the video below.
N Channel Jfet Characteristics
Related Resources
How to Connect a Transistor as a Switch in a Circuit
How to Connect a (NPN) Transistor in a Circuit
Types of Transistors
Bipolar Junction Transistors (BJTs)
Junction Field Effect Transistors (JFETs)
Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)
Unijunction Transistors (UJTs)
What is Transistor Biasing?
How to Test a Transistor
A N-Channel JFET is a JFET whose channel is composed of primarily electrons as the charge carrier. This means that when the transistor is turned on, it is primarily the movement of electrons which constitutesthe current flow.
This is in contrast to P-Channel JFETs, whose channel is composed primarily of holes, which constitute the current flow.
A N-Channel JFET is composed of a gate, a source and a drain terminal.
It is made with an N-type silicon channel that contains 2 P-type silicon terminals placed on either side. The gate lead is connected to the p-type terminals, while the drain and source leads are connected to either ends of the N-type channel.
When no voltage is applied to the gate of a N-Channel JFET, current flows freely through the central N-channel. This is why JFETs are referred to as 'normally on' devices. Without any applied to the gate terminal of the transistor,they conduct current across from drain-source region.
How a N-Channel JFET Works
This is a typical diagram you would see of voltage biasing of a N-channel JFET. This diagram also servesto show you all the parts of a N-channel JFET.
How to Turn on an N-Channel JFET
To turn on an N-channel JFET, apply a positive voltage +VDD to the drain terminal of the transistor with no voltage applied to the gate terminal of thetransistor. This will allow a current to flow through the drain-source channel. If the gate voltage, VG, is 0V, the drain current is at its largest value for safe operation, and the JFET is in the ON active region.
So with a sufficient positive voltage, VDD, and no voltage (0V) applied to the base, the N-channel JFET is in maximum operation and has the largest current.
How to Turn Off an N-Channel JFET
To turn off the N-channel JFET, there are 2 steps you can take. You can either cut off the bias positivevoltage, VDD, that powers the drain. Or you can apply a negative voltage to the gate. When a negative voltage is applied to the gate, the drain current is reduced. As the gate voltage, VG, becomes more negative, the current lessens until cutoff, which is when then JFET is in the OFF region.
Characteristics Curve of a N-Channel JFET
The characteristics curve of an N channel JFET transistor shown below is the the graph of the drain current, ID versusthe gate-source voltage, VGS.
This curve represents the transconductance, or simply the gain, of the transistor.
N Jfet Symbol
The transconductance of a transistor really means the gain of the transistor.
So this transconductance characteristics below shows the gain of the transistor, how much current the transistor outputs based on the voltage input into the gate terminal. Remember that gain is the the output over the input. The input is how much voltage is fed to the gate terminal. The output is how much current the transistor outputs.
You can see based on this N channel JFET transconductance curve that as the negative voltage to the gate increases, the gain decreases. You can see that the gain, the current ID output by the transistor, is highest when the voltage fed to the gate terminal is 0V. As we increase this voltage (negatively), again, as stated, the gain decreases.
This transconductance curve is important because it shows the operation of a N channel JFET.
You can also see that the transconductance curve, as for all semiconductor devices, is nonlinear, for most of the curve,meaning changes to VGSdo not directly (linearly) increase or decrease drain current, ID, even though this is a lesser issue.
N Fet
The big point is that, an N-Channel JFET turns on by having a positive voltage applied to the drain terminal of the transistor and ideally no voltage applied to the gate terminal. The transistor circuitshuts off by taking in a negative gate voltage, VGS, greater than about -4V or so. The transistor is in its fully conductive state and is in maximum operation when the voltage at the gate terminal is 0V. As we increase the amount of negative voltage the gate terminal receives, the transistor becomes less conductive. Once the negative voltage reachesa certain threshold, the N channel JFET circuit stops conducting altogether across the drain-source terminal.
The Regions that make up a transconductance curve are the following:
Cutoff Region- This is the region where the JFET transistor is off, meaning no drain current, ID flows from drain to source.
Ohmic Region- This is the region where the JFET transistor begins to show some resistance to the drain current, Id that is beginning to flow from drain to source. This is the only region in the curvewhere the response is linear.
Saturation Region- This is the region where the JFET transistor is fully operation and maximum current, for the voltage, VGS, that is supplied is flowing. During this region, the JFET is On and active.
Breakdown Region- This is the region where the voltage, VDD that is supplied to the drain of the transistor exceeds the necessary maximum. At this point, the JFET loses its ability to resist current because too much voltage is applied across its drain-source terminals. The transistor breaks down and current flowsfrom drain to source.
Related Resources
How to Build an N Channel JFET Switch Circuit
P-Channel JFET Basics
N-Channel MOSFET Basics
P-Channel MOSFET Basics