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 .
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 Where is the threshold voltage (pin-off voltage) and is the drain current with . Then we can write the drain current as a function of the input voltage as In order to simplify the analysis let us define the variables (normalized current and voltage, respectively) and the constant, which determine the value of the source resistor, We also need the normalized input voltage The relation between the normalized current and voltage can be put into the form
We want to obtain a power law relation between normalized current and the normalized input voltage and, in particular, the exponent 3/2 for the triode emulation. So, in order to get that, let us assume that one has that is Taking the logarithm derivative of the last expression and solving it in order of one gets or
The next figure shows the value of the exponent as a function of the normalized input voltage
|The exponent n as a function of the normalized input voltage z.|
The value of the constant that determines the value of the source resistor is given by For and one gets 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 permits to obtain for any working voltage and exponent the value of the parameter 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!
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çPalavras chave/keywords: DIY, triode valve, fet, electronics
Última actualização/Last updated: 23-01-2017 [09:20]
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