The "triode" emulator revisited

After reading the excellent page at http://www.runoffgroove.com about the Fetzer Valve circuit and the paper that supports it (Dimitri Danyuk, "Triode Emulator", 116th Audio Engineering Society Convention, May 2004 in Berlin, Germany), I wonder how the results in that paper could be obtained analytically. This page contains my impressions on it. I've found that the exact value of the magic constant is $latex2png equation$ .

The basic circuit of a class A JFET amplifier is given in the following figure.

Plain circuit of a class A JFET amplifier.

The basic equations that governs the behavior of a JFET, in the saturated regime, are given by $latex2png equation$ Where $latex2png equation$ is the threshold voltage (pin-off voltage) and $latex2png equation$ is the drain current with $latex2png equation$ . Then we can write the drain current as a function of the input voltage as $latex2png equation$ In order to simplify the analysis let us define the variables (normalized current and voltage, respectively) $latex2png equation$ and the constant, which determine the value of the source resistor, $latex2png equation$ We also need the normalized input voltage $latex2png equation$ The relation between the normalized current $latex2png equation$ and voltage $latex2png equation$ can be put into the form $latex2png equation$

We want to obtain a power law relation between normalized current $latex2png equation$ and the normalized input voltage $latex2png equation$ and, in particular, the exponent 3/2 for the triode emulation. So, in order to get that, let us assume that one has $latex2png equation$ that is $latex2png equation$ Taking the logarithm derivative of the last expression and solving it in order of $latex2png equation$ one gets $latex2png equation$ or $latex2png equation$

The next figure shows the value of the exponent $latex2png equation$ as a function of the normalized input voltage $latex2png equation$

The exponent n as a function of the normalized input voltage z.

The value of the constant $latex2png equation$ that determines the value of the source resistor is given by $latex2png equation$ For $latex2png equation$ and $latex2png equation$ one gets $latex2png equation$ which is closed to the value obtained by Dimitri, 0.83, but not exactly the same. This is the exact value of the constant that determines the value of the source resistor.

One can also see that the expression $latex2png equation$ permits to obtain for any working voltage $latex2png equation$ and exponent $latex2png equation$ the value of the parameter $latex2png equation$ that emulates a super-triode, a "triode" with a exponent between one and two.

Who says that math doesn't pay off?

Fetzer Valve - A mu-amp variation

You could also make a mu-amp variation

by biasing the upper jfet in a different way!

If you have any comments about this page, please fell free to send me an email:

Created: NaN

Last updated: 23-01-2025 [00:04]

For attribution, please cite this page as:

Charters, T., "The "triode" emulator revisited": https://nexp.pt/triode.html (23-01-2025 [00:04])

(cc-by-sa) Tiago Charters - tiagocharters@nexp.pt