I've been studying and building JFET circuits to serve as a high-impedance input to a DIY guitar amp. In my research on voltage divider biasing, I've come across a circuit in Nuts & Volts Magazine that has me perplexed. It's shown on the left in the diagram below.
What I've seen in textbooks and other examples on the internet has always looked more like the circuit on the right.
I'm trying to figure out what the advantage of a gate resistor (R3) is. Resistor values were not shown in the original circuit, but I got the feeling R3 would be on the order of 1M or more with R1 and R2 being lower. This would keep the input impedance high, while allowing more significant current to flow through R1 & R2.
My question is this: What is the advantage of having R3 as opposed to using large value biasing resistors for R5 & R6 in the textbook circuit? Couldn't I get a 1M input impedance using 2.2M resistors for R5 & R6 just as easily as using a 1M R3 and lower value R1 & R2? It seems the only difference is more current flowing through R1 & R2. How is this an advantage?
The only thing I can think of is that such an arrangement would allow a person to replace R1 and R2 with a potentiometer and adjust the bias without adversely affecting input impedance. But beyond that, I'm struggling to see a reason.