In this tutorial it is illustrated how quickly and easily we can design a common gate JFET(Junction Field-effect Transistors) amplifier with the help of the free online JFET Biasing and Amplifier Design Calculator. The calculator calculates the biasing resistors value and the coupling capacitor values. Inputs to the calculator are the gate shorted drain current, the gate to source cutoff voltage, the power supply used, the frequency of the input signal to the amplifier, the drain voltage and the load resistor value. This calculator helps to save time when you want to build JFET amplifier. Although here it is shown how to design common gate JFET amplifier with the help of that calculator, you can also use the calculator to design common source based JFET amplifier with different biasing method(self bias, voltage divider bias) and common drain JFET source follower circuit.
The circuit diagram of a common gate JFET amplifier is shown below.
In common gate configuration, the gate of the JFET is grounded. The input signal is applied to the source via coupling capacitor CC1 and the output is coupled via capacitor CC2 to the load RL. The common gate JFET configuration has low input impedance and high output impedance. It is used as amplifier, impedance matching circuit and as buffer circuit.
To design JFET amplifier with common gate configuration we need input parameters. This includes the power supply voltage, the input signal frequency, the drain voltage expected, the load resistor value. Also we need JFET parameters which are the shorted gate drain current and the gate to source cutoff voltage. After having the knowledge of the input parameters we can then use the online calculator to calculate the biasing resistor values which are the drain resistor value and the source resistor value and the input and output coupling capacitor values.
Here we will use 2N5459 N-channel JFET with the following parameters:
Shorted gate Drain Current, \(I_{DSS}=10mA\)
Gate to Source Cutoff Voltage, \(V_{GS(off}=-1V\)
The other inputs are:
\(V_{DD}=5V\)
\(f=1KHz\)
\(V_D=2.5V\)
With the knowledge of these input parameters we can use the online JFET amplifier design calculator to design the common gate JFET amplifier. The following shows the screenshot of the inputs to the calculator and the outputs from the calculator.
As you can see with just one click all the components values required to build the JFET amplifier are calculated. Also the various current and voltages are also calculated.
The calculated drain current is 2.5mA and the drain to source voltage is 0.5V. So the quiescent point or the Q-point or the biased point is (0.5V,2.5mA). This Q-point lies in the active region. The drain resistor and the source resistor which are the biasing resistor are calculated to be 1KOhm and 200Ohm respectively. The calculated voltage gain is 10. The input and output coupling capacitor calculated ara 135uF and 1.59uF respectively.
The following shows the completed circuit schematic of the common gate JFET amplifier.
We can use circuit simulator like Proteus to measure the current and voltages in the above circuit to compare with the calculated values. This is shown below.
Using oscilloscope in the Proteus circuit simulator software we can view the input and output signal into and out of the above designed common gate JFET amplifier. This is shown below.
As can been seen from the oscilloscope the output signal is amplified version of the input and that they are in phase.
We can verify that the input impedance of a common gate amplifier is low and the output impedance of a common gate JFET amplifier is high from the calculated values from the calculator. The calculated input impedance is 40Ω while the calculated output impedance is 1KΩ.
The following video illustrates the process of using the JFET amplifier design calculator to obtain the component values.
So here we showed how one can use the online common gate JFET amplifier design calculator to quickly design common gate JFET amplifier. Similarly we can use the calculator to design common drain JFET amplifier and common source JFET amplifier with voltage divider biasing method or JFET self biasing method.The online JFET amplifier design calculator not only is helpful to design JFET amplifier circuit but also for designing JFET based oscillator as illustrated in the tutorial Self Biased JFET Colpitts Oscillator.
