Hawes Amplifier Archive |
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Plate Resistance & Amplification Factor for JFETS & D-MOSFETs |
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Are FETs Really Like Tubes?Are FETs really like tubes? The short answer is YES! As preamplifiers, JFETs and depletion MOSFETs behave a lot like pentodes. There are depletion power FETs that act the like tubes, too. For example, the DN2540 by Microchip. How do triodes compare? For those who love triode tubes because of their lovely, even harmonics: Every JFET comes with a built-in Triode Mode. So does every depletion MOSFET. We'll go into these ideas later on this page. For now though, the bottom line is this: Nothing is exactly like a tube, except another tube. But JFETs and depletion MOSFETs are as close as semiconductors come. And that truth makes them attractive. Plus, they have their own advantages...
Further questions. Still, if you're technical, you have further questions. That's why we're here. Please read on... Why don't FETs have an RP or a Mu (µ) rating? Datasheets often do offer some of this data. But unfortunately, manufacturers bury the data. Here, we reveal how to recover this obscure information. You can then apply Hendrik van der Bijl's famous tube formula to FETs. (1.) This is the formula...
When we apply the formula to FETs, it becomes...
Of course, FET datasheets don't include RD. Yet Malvino points out that drain resistance RD is the reciprocal of output transconductance, the variable GOS. And variable GOS does appear on many datasheets. (2.) (Note that some datasheets name this same variable YOS. Nothing like standardization!) Amplification factor, µ doesn't appear. But now, we can calculate it! First, we adapt the formula to what we find on datasheets...
Example...
Compare that example to a vacuum tube...
Pentode Mode vs. Triode ModeFair play. Actually, the “JFET-tube comparison” above isn't exactly fair. Sure, a JFET has three electrodes, just like a triode does. But the JFET normally behaves like a pentode. That statement assumes that the drain potential is above the pinch-off voltage. Then the device is a voltage-controlled current source, a “pentode.” (With the drain below the pinch-off voltage, the device is a voltage-controlled resistor, a “triode.” The same rules apply to depletion MOSFETs.) Beyond pinch-off, the JFET drain (plate) voltage ceases to affect drain current. Instead, the drain current remains constant. This behavior is the way a pentode acts, but unlike triode behavior. And the 12AX7 that we used in our example is a triode. So let's rethink the 12AX7 comparison, and check out the performance of a pentode, the 6J7...
Maybe a bit more like our JFET. The 6J7 has a high plate resistance and high transconductance, just as our JFET has. Possible conclusion: The JFET is “just like” a pentode. But not so fast! Unlike a pentode, our JFET has two modes of operation. So far, we've only looked at one of these. Here are the two...
Coping with incomplete data. In a minute, we'll have more about Triode Mode. But first, we need to solve another problem that might have occurred to you: What if your datasheet doesn't include a GOS rating? How can you obtain the drain resistance value? Answer: There's a simple trick with Ohm's Law. Find the AC value of RD
RD Calculation, Pentode Mode Example
How do engineers harness the Triode Mode? How can I build a Triode-Mode preamp? Visit our next page! Footnotes1. Alfred A. Ghirardi, Radio Physics Course, 2nd ed. New York: Technical Division, Murray Hill Books Inc.: 1932, 419. ▶Re: Vacuum tube formula relates transconductance, amplification factor, and plate resistance. 2. Albert Paul Malvino, PhD, Electronic Principles, Second Edition. (New York: McGraw-Hill Book Company, 1979), 330. ▶Re: Parameter GOS is the reciprocal of drain resistance. 3.
George J. Christ, Radio Tube Fundamentals,
Bradley, IL: Lindsay Publications, 2008, 24-27. 4.
Vishay Siliconix, 2N3819, document 70238, S-04028, Rev. D
(Sunnyvale, CA: Vishay Siliconix LLC, 2001).
5.
Christ, Radio Tube Fundamentals, 24-27.
6.
Vishay Siliconix, 2N3819, document 70238, S-04028, Rev. D.
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