The previous page introduces Pentode Mode and Triode Mode for JFETs and depletion
MOSFETs. That page also describes how to find plate resistance and amplification
factor for these solid-state devices. This page continues the discussion, including
schematics for practical circuits.
In Triode Mode, a FET behaves in a linear fashion. That is, as drain voltage
RD increases, so does drain current ID. Result: The delightful,
euphonious tone that only a triode can produce. This linear behavior lasts
until the device reaches its pinch-off voltage. To the right of pinch-off is a
“knee” in the drain curve graph. At the knee, the FET channel saturates.
Then the maximum safe current flows. During saturation, drain current ceases to effect
drain voltage. The device becomes a voltage-controlled current source.
Fig. 1. Triode Mode, left side of the graph (red outline).
(1.) The green line marks the edge of the usable section.
Triode Region Shape. See Figure 1. Note that pinch-off and the knee
occur at a different point in each curve. As gate voltage VGS becomes
more negative, the Triode Region narrows. The region as a whole is roughly the
shape of a right triangle: The apex is at the bottom-left, and the
hypotenuse on the right. The real estate at the top-left section is
outside the curves and unusable. My experiments show that the bottom few
curves are also unusable.
Pentode Mode: A Review. As the previous page said: In the
Constant-Current Region, a FET acts like a pentode. This fact is the
source of the terms “Pentode Region” and “Pentode
Mode.” Pentode-Mode behavior occurs in the Pentode Region, on
the right side of the drain curves. Similarly, Triode Mode occurs
in the Triode Region, on the left side of the drain curves. The
drain voltage selects the region of operation. In both regions,
the gate bias selects the drain current, ID. Uniquely in
the Triode Region, the drain voltage also affects drain
current. Engineers design most amplifier circuits to operate in
the Pentode Mode. Perhaps this fact is why the Triode Region is
vestigial. As we'll see, the Triode Region is most useful
Engineers use the Triode Mode (TM) to implement voltage-controlled
resistors. But there's a catch: Most FETs only operate in Triode Mode within the
first few volts of drain potential. That is, when the drain voltage is left of the
Broad Triode Mode. On the other
hand, some FETs have a high pinch-off voltage. (Examples: Up to 10 volts for the •2N4304 by
A lovely, 8-volt maximum for the •2N3819 and the •MPF102.) With a
high pinch-off voltage, the triode region is fairly broad. A circuit designer
might reckon that this broad region is a frontier to develop.
Figure 3 expands an example of the Triode Mode so that we can
study it carefully. These curves reveal triode-like behavior:
Drain voltage affects drain current. This situation is most
unlike the behavior within the Pentode Mode (Constant-Current
Mode). There, drain voltage has little effect on drain
Fig. 3. Triode Mode for 2N5484, expanded curves (3.)
Could a Triode-Mode audio amp be possible? I wondered.
I couldn't find any useful applications online. But might such applications
be achievable? Maybe so! My TM circuit is a conventional, Class-A
amplifier that operates in a unusual region: Triode Mode.
(See Figure 3.) My approach avoids gimmickry: No “magic
components.” I abstained from bludgeoning a JFET into an improvised
device with tube-like manners.
First one? Could this preamp be the first of its kind?
Possibly, at least with the device that I used. Better yet,
my TM preamp seems to work well, and doesn't cost much to try.
Design process. I started with my Pentode Mode FET preamp circuit. That circuit uses an MPF102
JFET. A 2N3819 is similar, and what I chose for this project.
According to device curves, the Triode-Mode operation region for either
device is broad, a requirement. (See Fig. 8). Here in three
steps is how I proceeded...
1. Move VD left. The drain voltage
controls the left-right, or X-axis. By reducing VD, I
moved the drain voltage leftward, from 9 to 3 VDC. Now, my device would
operate smack in the middle of the Triode Mode.
2. Move ID up. The first test
circuit didn't work. I needed to increase the drain current. On the Y-axis,
I moved the operating point upward. I did that by reducing RS
Fig. 4. Above is the method that I used to move the operation zone
into the Triode Mode. (4.)
Fig. 5. Triode-mode preamp. Would like to hear a
guitar through it!
3. Tweak RS. A further
RS reduction shifted the
drain voltage to exactly where it
should be, 1.5 volts.
I assembled the Triode-Mode circuit on
August 28, 2020. My new design biases properly:
VD(average) measures 1.5 volts,
about ideal. See Figure 5. (The test circuit
omitted coupling capacitors and protection devices,
such as gate zeners.)
Fig. 6. Perfboard layout
Success. There were two successful
builds, one on a solderless breadboard, and one
on a perfboard. In the final circuit, two
separate devices each produced 1.5 volts at the
drain. The average current draw was five
milliamps. Let's locate the operation zone
using Figure 8: The Triode-Mode circuit
operates on the “-0.2V” curve.
(RS provides source bias of
[47Ω x 0.0047A]= 0.22V.)
Fig. 7. Circuit under power, with VD
reading 1.42VDC, 95% of goal.
Triode-Mode Preamp (3 Volts)
Changed RG to 3.3M.
♦ CAUTION: Top Drain Curve. The highest drain curve in Figures
1, 4, and 8 tops out at just under 7 mA. The flat part of this curve is at
IDSS, or 0V bias. Yet the manufacturer's spec for IDSS
covers a range between 2 mA and 20 mA. That is, the top curve could be
higher or lower than where it appears on Figure 1. On this figure, my
Triode-Mode circuits should operate within the Triode Region. Still, due to
spec variations, not all devices would operate in the Triode Region.
School of Hard Knocks. Experiments also
included the attempt to design a more current-stingy
amp. This amp would have had a two-milliamp average
draw. Yet the attempt failed. The 2N3819 remained
off. Reducing the source resistor value to zero
ohms delivered no change. (At that point, I had a
three milliamp, nonfunctional circuit.) The drain
remained at VDD(the supply
potential), indicating ID pinch-off.
The lack of drain voltage change suggested the need
to increase ID.
Fig. 8. Expanded Triode Mode for 2N3819. (Actually extends
to 8 V. I trimmed it off where the curves are still partly diagonal.)
A few more milliamps of current moved the
operation zone onto a more diagonal curve, promising
enhanced linearity. The higher curves also enjoy a
wider Triode Region than the lower curves offer.
♦ CAUTION: FIGURE 8. I stretched this figure
graphically, not mathematically. As a result, the slope of
the curves is steeper than it appears. The stretching
process adds other inaccuracies. For instance, the
horizontal span between data points is wider than in
the vertical dimension.
Penlight power. The new circuit only draws 7.5 mW. That's 15 mW less
than my Pentode-Mode JFET preamp requires. Two penlight cells should keep me
in Triode Mode for a long time. I'll use two cells in series, either AA or AAA.
Mods. You may change resistor RG to any value from 100K to 10M. For
larger values, I suggest using metal-film resistors for their low-noise characteristic.
To dial in the proper, 1.5V bias VD, adjustment of RS might be
necessary. Otherwise, I don't recommend altering the RD and RS
values. You may use a gate-stop resistor if you wish. I suggest 68KΩ. For
Musicians: If you plan to play loud, a 1M (or larger) gate-stop resistor might be
necessary, or the preamp might clip. Capacitor values are suggestions. Substitute FETs:
See Other Triode-Mode Devices, toward
the bottom of this page.
Stages. This preamplifier is suitable for use in guitar amps. You
may cascade two or more such preamplifiers.
Inputs. For use with piezo pickups, the input capacitor is
optional. Some amps use input caps. They isolate the amp from DC in
effects circuits that could upset the bias on the FET input. Other
amps don't have the capacitor. Your choice. If you go without
the capacitor, please use a 68K grid-stop resistor.
Outputs. Low-impedance headphones (32Ω) will load the preamp
down, as will typical PM speakers. For use with medium-impedance outputs, use
a larger-value output capacitor, at least 4.7 µF. For use with
high-impedance pickups or cascade amps, the schematic capacitor value will
be fine. I suggest polypropylene capacitors.
For more gain, you may add a source-resistor bypass capacitor. See
the two-stage cascade amplifier below. I haven't built this version, but
it will probably work. Since the source resistor is small though, the
bypass capacitor must be large. This capacitor should be an
aluminum electrolytic type. Use the best quality cap that you can
afford. If the amp howls, you'll have to do without the bypass
cap. You know the maxim: “Gain is great. Instability
Fig. 9. Cascaded, two-stage version of my Triode-Mode
amp. Includes grid-stop resistors at the front end. For
guitar or mike. With a third stage, might drive a power
MOSFET output. Experimental! I haven't built it.
What type power amp (PA)? If you need a power amp to drive a
speaker, you may add the PA after the second JFET stage. The three-volt power
supply will be insufficient for the power amplifier. To produce a reasonable
volume at the speaker, drive the MOSFET with 30, 50, or more volts. Pick
a MOSFET that can produce about 60 mA at the chosen power voltage. Then you
can use a tube output transformer with your PA.
Antique Electronic Supply
sells such transformers (Hammond, and other brands). Another vendor who
can help you is Triode Electronics.
Fig. 10. Block diagram: Triode-Mode amp with level
Level-shifter. Using the Triode-Mode preamp with a MOSFET
output might require a level-shifter stage. This stage would
stand between the preamp and the power amp. A JFET or BJT transistor
operating at the power amp's drain voltage would be appropriate.
The level shifter would increase the preamp output voltage to the
threshold voltage of the MOSFET. Otherwise, a MOSFET power amplifier
with a low threshold voltage might be necessary.
How low can you go? How about a 1.5V preamp? On September 30, 2020, I built
a 1.5-volt Triode-Mode preamp. This experiment produced a working amplifier with a
measured output voltage of 0.76V. The schematic appears at right. Think of operating
on just one penlight cell! Below are some other specs for the preamp...
Legacy device. Triode-Mode devices require a high pinch-off voltage, so
that the Triode Region is broad. FETs with these desirable characteristics tend to
be legacy devices. Fortunately, there are foundries that still make these devices.
More about that topic in a moment. First, we should focus on concerns about device
replacement in our Triode-Mode circuits. The CAUTION below discusses those
♦ CAUTION: DEVICE TYPE. In the circuits on this page, very few
devices can substitute for the 2N3819 or MPF102.
FETs aren't interchangeable to the extent that bipolar transistors are.
Bias requirements, current-handling ability, and other specs vary
widely between FETs.
Most devices don't have a high enough pinch-off voltage to allow
these circuits to work properly.
Some high pinch-off devices only operate at a high current, and
would be unsuitable for portable applications. (Example: 2N4856, with a
desirable, 10-volt pinch-off voltage. But IDSS is 50 mA,
To find a substitute FET, you may consult InterFET, a foundry in Texas.
(6.) InterFET claims to be the world's largest source of JFETs. Its
stock is impressive. But certainly it isn't the only source.
(7.) stocks FETs by and many foundries. The names of
some of these foundries appear in the table below.
(To contact companies in the foundry table, please click the footnotes.)
FETs from these sources might help with the projects on this page,
or with similar projects. Mouser's competitive pricing among
different brands is an important advantage over buying factory-direct.
I culled the part numbers below from part specs for 500 devices.
Each part on my list has a pinch-off voltage of 7.8 to 10 volts.
For my research, I used InterFET's JFET Master Table, and manufacturer
datasheets. The InterFET table is an excellent reference.
JFETs with High Pinch-Off Voltages
(May be compatible
with the project on this page)
NOTE: I haven't tested these parts. I offer no guarantee of compatibility in the circuit on this page.
1. Vishay Siliconix, 2N3819, document 70238, S-04028, Rev. D
(Sunnyvale, CA: Vishay Siliconix LLC, 2001). ▶Re: My datasheet overlay indicates the boundaries
of the Linear Mode (“Triode Mode”). See Figure 1.
2. Rufus P. Turner, FET Circuits, Second Edition. (Indianapolis, IN:
Howard W. Sams & Co., Inc., 1977), 108-109. ▶Re: Schematic diagram of a
voltage-variable resistor. The circuit uses a medium power JFET that operates in its
Triode Mode. The book also provides a graph of the variable
output voltage, which varies from 1mA to 30 mA. The variation doesn't
remain linear over the entire range of operation. This is still a very
3. Fairchild, 2N5484/5485/5486, MMBF5484/5485/5486, Rev. 1.0.0
(San Jose, CA: Fairchild Semiconductor Corporation, 2007). ▶Re: Triode-Mode
graph from the 2N5484 datasheet. This closeup of the Triode Mode clearly portrays
how these curves resemble triode tube curves. (The Datasheet is for an N-channel
JFET.) The original manufacturer was Fairchild Semiconductor. Fairchild obsoleted
the part, but other semiconductor foundries still manufacture the 2N5484. Examples:
InterFET and On Semiconductor. On Semiconductor acquired Fairchild Semiconductor
in 2016. (Central Semiconductor discontinued the 2N5484 and issued an end-of-life
4. Vishay Siliconix, 2N3819. ▶Re: Figure 5, How I moved
the Operation Zone for a JFET into the Triode Mode. I started with my
Pentode Mode preamp and modified it. The info graphic shows my three-step
5. Ibid. ▶Re: Figure 8, Extended Triode
Mode graph for the 2N3819 JFET. I graphically stretched and extensively retouched
the Siliconix plot. The data in Figure 8 isn't new. Probably some plotting errors
resulted from the stretching. For a more accurate plot, please refer to Figure
6. InterFET Corporation, “World's Largest Supplier of JFET Products,”
Access on 09-15-2020. ▶Re: Home page for InterFET corporation,
a semiconductor foundry that specializes in manufacturing JFETs. These
products include legacy devices that large foundries such as
Fairchild and Siliconix have discontinued. (Examples: 2N3819, MPF102,
and J201.) You may buy factory-direct from this site. Or buy from
InterFET's distributors, including Mouser Electronics. InterFET
address: 715 North Glenville Drive # 400, Richardson, TX 75081.
Phone: (972) 238-9700.
7. Mouser Electronics, Inc., “Mouser makes buying easy,”
Access on 09-15-2020. ▶Re: Home page for Mouser Electronics.
Mouser markets electronic products, including JFETs and MOSFETs. Warehouse
address: 1000 North Main Street, Mansfield, TX 76063. Phone: (800) 346-6873.
8. InterFET, https://www.interfet.com/jfet-master-table/
Access on 09-15-2020. ▶Re: InterFET corporation,
JFETs. See Footnote 6, above.
9. Central Semiconductor Corp., “Products/Solutions,”
Access on 09-15-2020. ▶Re: Home page for Central Semiconductor.
Central is a foundry that manufactures JFETs, transistors, diodes, etc.,
including legacy devices. Central address: 145 Adams Avenue,
Hauppauge, NY 11788. Phone: (631) 435-1110
10. On Semiconductor Corporation, “ON Semiconductor -
Energy Efficient Solutions,” https://www.onsemi.com/
Access on 09-15-2020. ▶Re: Home page for On Semiconductor.
The On Semi foundry manufactures JFETs and many other
devices, including legacy devices. On Semiconductor Address:
5005 E McDowell Road, Phoenix, AZ 85008-4229. Phone: (602)
244-6600. Product Inquiries: (888)-743-7826.
11. Microchip Technology Inc., “LND150
N-Channel Depletion-Mode DMOS FET,” https://www.microchip.com/wwwproducts/en/LND150
Access on 09-15-2020. ▶Re: LND150 page for Microchip Technology.
The Microchip foundry manufactures D-MOSFETs and many other
devices, but no legacy devices. The LND150 has a 5-volt pinch-off
voltage (approximately). The device can operate at a VDD
of several hundred volts. (Microchip acquired Supertex, which formerly
manufactured these devices.) Microchip Address: 2355 W Chandler Blvd
Chandler, AZ, 85224-6199. Phone: (480) 792-7200. Tech support:
12. Littelfuse, Inc., “Littelfuse: Expertise Applied | Answers
Access on 09-23-2020. ▶Re: Home page for
Littelfuse, Inc. The Littelfuse foundry manufactures D-MOSFETs
and many other devices, but no legacy devices. Products include
impressive depletion MOSFETs. For example, the IXTY01N100D is an amazing
power device with respectable gain. It also has a high pinch-off voltage,
some 11 volts on the top curves. The maximum VDD for this device
is 1,000 volts. (Littelfuse's subsidiary IXYX makes this device.)
Littelfuse Corporate Headquarters: 8755 West Higgins Road, Suite 500,
Chicago, Illinois 60631. Phone: +1 (773) 628-1000.
13. NXP Semiconductors NV, “NXP Semiconductors | Automotive,
Security, IoT,” https://www.nxp.com/
Access on 10-19-2020. ▶Re: Home page for
NXP Semiconductors NV. NXP manufactures D-MOSFETs and JFETs, both
surface-mount and through-hole devices. NXP Corporate Headquarters: High
Tech Campus 60 Eindhoven, 5656 AG Netherlands. Phone: 31-40-272-9999.
14. InterFET Corporation, “JFET Master Table,”
Access on 09-15-2020. ▶Re: Operating specifications for about 1,000
JFETs that InterFET markets.
▲ WARNING. This is your project. Your achievement is entirely yours.
I assume no responsibility for your success in using methods on these pages. If you
fail, the same is true. I neither make nor imply any warranty. I don't guarantee
the accuracy or effectiveness of these methods. Parts, skill and assembly methods
vary. So will your results. Proceed at your own risk.
▲ WARNING. Electronic projects can pose hazards. Soldering irons
can burn you. Chassis paint and solder are poisons. Even with battery projects,
wiring mistakes can start fires. If this page baffles you, this project is too
advanced. Try something else. Again, damages, injuries and errors are your
responsibility. — The Webmaster
♦ CAUTION: SPEC SPREAD. Between devices, JFET and depletion
MOS specs vary considerably. New builds of the Triode-Mode circuits
in this article might not produce the same output voltages. Also: Whether
these Triode-Mode amps are linear across the audio band is an unknown.