Step 4: Designing a JFET amplifier using the load line technique
In this section, I’m going to tell you how to design a JFET amplifier using the load line technique. This was used many years ago to design original valve equipment and the same techniques can be applied to JFETs provided you have access to the drain characteristics chart.
How this works is you must be able to edit the chart in some way (for example using Microsoft paint or printing off the chart and doing it with a pencil.) and you must draw on the load line of your chosen values.
Since my chart only shows up to 1.2mA of drain current, I’m going to use a current value of 0.8mA at 9v so I can utilize my chart.
So the first step in using the load line technique is to believe it or not, draw a load line! A load line is literally a line that is drawn from the Vcc (drain to source voltage on this chart) to how much current will be flowing, so in my case from 9v to 0.8mA. This line will be used to calculate the bias point and the resistor value used for Rd and consequently Rs.
Calculating Rd is very easy in this case, it literally consists of:
Rd = 9/0.0008 = 11250 Ohm
To calculate Rs, we must now look at the load line and decide where we want our bias point. To bias the amplifier so it can have a full swing, we must choose the bias point in the middle of the load line. As you can see in my picture, I have put a little blue dot where the bias point lies. If you look at the chart, you can see the thick black lines, they are the characteristics of the JFET. If you see to the right of the chart, there are voltages (VGS values). As you can also see, my bias point lies just below the -0.4v point (between -0.4v and -0.6v) it is about -0.44v. All that this means is that if the gate is 0.44v LOWER than the source, it will follow this curve. Since we are pulling the gate to ground, we can achieve the same thing by putting the source to +0.44v. Therefore the gate is -0.44v relative to the source.
We can now calculate Rs:
Rs = V(Bias) / Id = 0.44 / 0.0008 = 550 Ohm
Now, the only problem with this method is if you look at the curves of the JFET, once you get past the linear region and into the saturation region, not much happens in terms of transconductance, you increase Vds and the Id hardly increases. This has a profound effect on the output waveform by squashing one end of the waveform. This can be very musical on guitar and adds a fair amount of second harmonic content (something lots of guitarists crave!). This can be changed by moving the bias point much more towards or away form the linear region, all of which can be done by replacing the source resistor for a potentiometer or variable resistor.
Once again, since these aren't standard value resistors, the values:
Rd = 10k Ohm
Rs = 560 Ohm
Could be used.