Since I first started properly doing electronics, I always liked the idea of an electronic component as essentially simple as the JFET.
The JFET was first theorized in 1925 by Julius Lillenfield but wasn’t built until much later. As much as the JFET was theorized many years before many bipolar junction transistors that you see today, it wasn’t built until much later. In amateur, they don’t have a very large amount of uses (unlike MOSFET’s and BJT’s).
If you observe the transfer curves of a JFET before saturation, it is very much like a triode’s transfer curve. This is a very linear region and will provide clean and uncoloured amplification. I have included a comparison picture of the common 12ax7 triode compared to a 2n5457 JFET. Another advantage of a JFET is that the gate has a very high input impedance, which is very useful to instruments like guitar in preserving the high frequencies. The downside to JFET’s is that they can at times have too low of a drain current to drive low impedance sources. Luckily, in this day and age, guitar amplifiers and post guitar effects generally have quite a high input impedance, for example, the humble Ibanez Tubescreamer features an input impedance of around 500k.
In JFET Based amplifiers, the input impedance is pretty much determined by the gate resistor which comes after the input capacitor to ground.
Step 1: Choosing the design parameters
The parameters we need to design ourselves a JFET amplifier are:
• Vcc (Positive supply voltage)
• Minimum Rds On of the JFET (resistance of the JFET when fully on biased)
• Ids (Current flowing through the JFET from drain to source)
• Cut off frequency of the JFET preamplifier
• Vgs Cut off voltage
From these parameters, we can calculate the values for Cin (Input capacitor), Rg (Gate resistor), Rs (Source resistor), Rd (Drain resistor) and Cout (Output capacitor).
When designing the circuit, you must ensure you don’t exceed the voltage and current rating of the JFET. With the JFET I am using (2N5457), the maximum VDS (maximum voltage across the drain and source) is 25v, the Vgs (off) is -2.5v and the maximum Ids (maximum current across the drain and source) is 3mA. On the data sheet, the Ids is called Idss. All that Idss means is the current flowing through the drain of the JFET with the gate grounded. This value should not be exceeded and as a general rule, you should not exceed about 60% of the absolute maximum rating, this will prevent any broken devices and failures.
Since the majority of guitar circuits run off 9v, I will choose 9v as my Vcc. After comparing this to my data sheet value, my JFET will still be within the voltage range. For the current, unless you purposely need a low current circuit for battery saving or what not, I will just run my JFET at 60% of the maximum Ids. In this case, this is:
0.6 x 0.005A = 0.003, 3mA.
So far, we have chosen the vital parameters for designing a JFET amplifier.