Elliott Sound Products

Using HEXFETs in High Fidelity
Audio

Using HEXFETs in High Fidelity Audio
Mitch
Hodges
Edited and Updated by Rod Elliott (ESP)
Last Update:
March 2014

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Articles
Index
Main Index

Contents

1 -
Introduction

2 -
Description

3 -
Conclusion

4 -
References

5 -
Footnote

5.1 - Update
(March 2006)

Introduction

When we build linear power amplifiers, we always need to choose some device
for the output stage. This could be any power device including valves, BJTs,
IGBTs, and MOSFETs. Each has its own strengths and weaknesses which forces us to
choose between them. If, perchance, we wanted to build a very simple and
accurate amplifier, we can safely ignore valves, since they all need heating
circuitry and are not simple for a true hi-fi amplifier. That it is possible to
build a valve amp to a high specification is not in doubt, but they tend to be
complex and expensive.

BJTs are often used, but they do not respond well to even momentary
overloads. This is because they suffer from second-breakdown - an instantaneous
and catastrophic failure mode. IGBTs (Insulated Gate Bipolar Transistors) are
seldom used, and will be very similar to a BJT, only with an insulated gate.
They still need thermal compensation and a suitable gate drive design, and can
suffer from a 'latch-up' condition in some cases. Lastly there is the MOSFET,
which does not suffer any second-breakdown effects (although this is not
strictly true - see below for more
info). MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) come in two
primary types - vertical and lateral.

These devices are extremely rugged, yet they do have a large nonlinear gate
capacitance to deal with. If driven incorrectly, they show high distortion
levels, especially vertical types - most commonly these days, HEXFETs. This is
why I wrote this article - to show how to use HEXFETs properly in audio
applications.

The update
below has some important information that I recommend you read thoroughly and
make sure you understand before settling on the use of HEXFETs in your next amp
project. While there appear to be many advantages to their use over BJTs,
HEXFETs may often suffer from exactly the same problems - thermal runaway and a
failure mode that is suspiciously similar to second breakdown. On top of this,
there is a much larger voltage loss ... 2-4V is needed to bias the HEXFETs to
the on condition, vs. 0.65V (nominal) for BJTs. This voltage is usually taken
from the main supplies, so for a given supply voltage, expect a little less
output power.

2 - Description

Many people say (including IR) that HEXFETs are not suited for linear audio
circuits and should be avoided. Well, that is the easy route to take for
designing an output stage. Any device can be used for audio and give
great performance if a proper design is found. It is just easier to use more
linear devices. Lateral MOSFETs are usually specified for audio, but there are
relatively few different devices on the market. When found, they tend to be
quite expensive. On the other hand, HEXFETs are very common, reasonably priced,
and only need a good design to do well.

This article is intended for Class-AB designs. HEXFETs will run Class-A with
barely any problems besides driving the gate. If you are designing a class A
amplifier, the first trick (see below) should be used (the second is not needed
since the bias is already quite high).

Alright, now for some explanations. Comparatively, HEXFETs usually have a
low gate capacitance than other vertical MOSFETs, yet have a higher gate
capacitance than their lateral counterparts. There is not only one capacitance
to deal with, but two (one from the gate to source and the other from the gate
to drain). This is the main problem: to find a way to drive the gate
capacitance of the HEXFETs. Through a lot of time and molten breadboards, I
found the best two things to design for are the following:

1

Fix:

Drive the gates with as much current as possible. This may include
adding a class AB driver stage.

Why:

HEXFETs have a nonlinear transfer curve up to about an amp or two,
depending on the device(s) used. In a class AB amplifier, this
characteristic is the cause for a majority of the THD. When driven with
enough current, the device will follow the 'new' linear curve, since it
is balancing out the nonlinear gate capacitance. The lower impedance
of the driver stage the better.

2

Fix:

HEXFETs like to run hot. This does not mean use an inadequate
heatsink, but the bias between devices should be a bit more than many
are used to. 250mA of idle current is not a bad bias figure for these
devices.

Why:

To balance out the nonlinear curve, we can simply cut if off where
it seems too bad by using bias. This will increase dissipation, though.

For the design of the amplifier, I will assume a single LTP input stage.
Better performance can be seen by using multiple LTPs, but this will not be a
simple design (in fact it will be quite complex with high frequency stability
issues needing attention).

When choosing the complementary output components, one can obviously choose
the IRFPxxx and its IRFP9xxx complement. If we look at these complementary
device data sheets, we will see very different figures for current capability,
on resistance, and, most importantly, gain (or forward transconductance). But
if we use a matching tool, we will find that the gain varies considerably from
the actual devices vs. the data sheet. That is why we need to buy a few extra
and match them together. Since the gain varies a bit from batch to batch, it is
quite easy to find a IRFPxxx and IRFP9xxx that are very similar, at least with
gain factors.

Also take note that HEXFETs will require a Vbe multiplier for thermal
compensation, since the negative temperature coefficient does not come into play
until the device has about 10 amps through it (at least for the IRFP240). The
exact values around the Vbe multiplier (also known as a bias servo) are critical
to ensure that the thermal performance is matched as closely as possible.

In every practical design I have tried I had to use a class AB driver stage.
A class-A driver will work fine if you really want an electric heater, as you
will see in the next calculation. Now, in order to size-up the proper driver for
the FETs, we need to do a little maths. I promise it is not hard. An example
would work nicely here ... if we wanted to design a class AB driver stage with
five IRFP240 and five IRFP9240 devices, how much current will we need at
minimum for full functionality up to 50kHz? For a better understanding,
a simplified output stage circuit is shown below.

Figure 1 - MOSFETs and Driver Circuit

We will do calculations using the gate charge method, which IR recommends
(AN-944). Looking at the data-sheets, we find the IRFP240 has a total gate
charge (Qg) of 70nC and the IRFP9240 has a Qg of 44nC. Don't add these yet! We
will find each device's needs individually. The general formula to determine
gate current is ...

I = 100Qgf   where I is current
needed, Qg is the total gate charge in Coulombs, and
f is frequency of operation.

The multiplication factor of 100 gives the headroom needed for accurately
reproducing a square wave (or high frequency sinewave), since the gate driver
needs a lot of current to quickly switch the MOSFET from OFF to ON. Although the
requirement for this is minimal (the CD format is incapable of anything even
approaching a square wave above a couple of kHz), it has become an expectation
that power amps should be able to reproduce perfect square waves at 10kHz as a
minimum.

When we plug our figures in for the IRFP240, we get I = 100 *
(70E-9 * 50,000) = 350mA per device

For the IRFP9240 we get I = 100 * (40E-9 * 50,000) = 200mA
per device

Multiplying each figure by five (because there are five devices of each
polarity) gives us 1.75 amps for the upper driver and 1 amp for the lower
driver. So a Class-A driver would need bias set to 3.5 amps to get the job done
with a reasonable safety margin.

The value for R7 will depend on the linearity of the driver transistors. I
had to guess and check with my ammeter to get a good value. This can range
anywhere from 100 Ohms up to perhaps 5k. Make sure you check the idle current
before calling the design done! These drivers (Q7 and Q8) may need a heatsink.
Also note the capacitor in parallel with R7. This should be of a high value
(i.e. 100uF or more), and 470uF works fine for my 10 MOSFET stage shown here. It
helps with discharging the MOSFET gates by providing a path for the gate
current.

These current figures seem quite high, but keep in mind this current will
only last a very short time compared to the signal, and virtually no current is
needed to keep the devices either in the OFF or ON state. The current to
reproduce a sinewave will be a bit lower, since it is a smooth curve, but this
much headroom will drastically lower distortion. This is why we cannot
practically use a class A driver, unless, of course we use one pair of output
devices.

For some comparison, below is a HEXFET setup driven by a class A driver at
13mA bias:

Figure 2 - Spectrum of HEXFET with 13mA Class-A driver

The large notch is at the second harmonic, and the small bump to the right is
the fourth harmonic. Barely any third harmonic is seen. This shows 0.25%
distortion at the second harmonic at ¾ power, and 250mA amp bias. Not very good
for a true hi fi, unless we are making a valve-like amplifier. Even this will
not show the same effects as a true valve amp - the nature of the distortion
components will almost certainly be different.

Adjusting the bias to 1 amp removes nearly all distortion, yet now we are
approaching a heater... I mean class A.

After fixing the problem by adding a class AB driver, distortion was greatly
decreased ...

Figure 3 - Spectrum of HEXFET with Class-AB driver

As the picture shows, the second harmonic was reduced considerably, while the
fourth harmonic is below the noise floor. This shows 0.04% distortion solely on
the second harmonic at ¾ power and still with 250mA bias. This greatly improved
the amplifier. At one watt, the distortion is not measurable at all, unlike
with the class A driver. Reducing the gate resistors to 4.7 Ohms to get more
current through does nothing noticeable, so the use of 10 Ohm resistors is fine.
There is no evidence of 'notch' distortion or any other nasty odd harmonic,
only a 'nice' second harmonic added in. Also note that this amp was built on a
breadboard. A compact and nicely wired PCB should decrease distortion even
further.

Below is the final simplified schematic of the entire amplifier ...

Figure 4 - Simplified Schematic of Complete HEXFET Amplifier

It looks very simple, and includes the class AB driving stage to improve gate
driving. It's very simple compared to amps with multiple LTP stages. The minimum
stability network (Zobel) shown is always needed, and a series inductor (with
parallel resistor) may also be required. The values of these components will be
found by experiment.

For some further reductions in distortion, the following work quite well:

Design a compact PCB layout with feedback taken from as close as possible
to the load terminal. We want to measure what the load is doing, not
what it should be doing.

Don't run any signal wire close to a power supply wire or the power
transformer.

Make big traces on the PCB with decent sized spacing between other
traces.

Make sure your measuring equipment is decently calibrated!

For any output device, use some sort of emitter/source resistors (as
shown) to help balance the load when parallelling.

Use a current mirror with the LTP input to get as much gain as possible to
allow for negative feedback, but do not solely rely on the feedback to make
the amplifier better.

Match the output devices as close as possible, especially for
transconductance at low values of VGS.

3 - Conclusion

HEXFETs are decent devices once the gates are driven correctly. They are
much more rugged than BJTs as my burned parts pile shows, and sound very good
when a class AB driver is added. I hope this short article with aid others in
using these 'switching' and 'not linear enough for audio' devices to get
distortion figures below many good amplifiers with 'very linear' devices. And
remember one thing - any output device can be precise if a proper design
is found. Finding the correct design parameters becomes more complex with
non-linear devices.

4 - References

The following are all PDF files, and are direct links to the International
Rectifier web site ...

IRFP240 data sheet
IRFP9240 data sheet
AN-936 application note - The Do's and Don'ts of Using
MOS-Gated Transistors
AN-937 application note - Gate Drive Characteristics and
Requirements for HEXFET® power MOSFETs
AN-941 application note - Paralleling HEXFET® power
MOSFETs
AN-944 application note - Use Gate Charge to Design the
Gate Drive Circuit for Power MOSFETs and IGBTs
AN-948 application note - Linear Power Amplifier Using
Complementary HEXFET Power MOSFETs

... And many pages from ESP

5 - Footnote

The above article is a contribution from Mitch Hodges, and ESP has not
verified all aspects of the design process described. While the circuit can be
(and has been) simulated quite readily with good results, this is no guarantee
that everything will work as expected. I added diodes and zeners to protect the
MOSFET gates from excessive voltage. It may be possible to select the zeners to
achieve basic current limiting, giving the amp some protection from overload
conditions. Because of the high gain of HEXFETs, this simple protection scheme
will not be particularly effective. Also, remember that a series inductor may
also be required.

It will be noted that there are no component values supplied - this is quite
deliberate, and is not an omission. The article is intended to describe the
design process and how to work around the inherent non-linearity of vertical
MOSFETs, and is not intended to be a construction project. Requests for
component values will not be fulfilled.

It must be understood that at the suggested current (250mA per MOSFET pair),
the total quiescent current will be in the order of 1.25A - at a typical supply
voltage of perhaps ±50V, this represents a total quiescent dissipation of 125W!
This is a formidable amount of heat to dispose of, and will require very large
heatsinks (and/ or forced air cooling). It is probable that the constructor will
be forced to compromise, using a significantly lower quiescent current than
suggested just to maintain a sensible heatsink size and temperature. Reducing
your expectations of the maximum frequency that needs to be passed at full power
will reduce the loading on the Class-AB drivers, but does nothing for the MOSFET
low current linearity. Compromise will be almost essential (IMHO).

Finally, I'd like to thank Mitch for his contribution, since it describes the
issues and how to solve them in an easy to follow manner, keeping complexity to
the absolute minimum in the final design example.

Rod Elliott

5.1 - Updates (March 2006 & October 2014, ESP)

I have had occasion to build a HEXFET based power amp as a test for a client.
While it worked well enough, giving the expected power output and with fairly
low distortion, as noted above the required bias current is quite high to reduce
crossover distortion to an acceptable figure. While the circuit I built was
actually quasi-complementary (using only N-Channel MOSFETs), the basic
principles apply regardless.

As it transpires, the design I was looking at was unsuitable for the intended
purpose, because the quiescent current needed to remove crossover distortion was
too high to be practical. In many cases, the lowest heat output possible is
highly desirable, and HEXFETs are simply unsuited to any application where very
low Iq is desirable or necessary. Lateral MOSFETs would be fine, but are too
expensive for the application (in case anyone was wondering).

Bias stability is definitely an issue as discussed above. It is commonly (and
erroneously) stated that MOSFETs are 'safe' because they have a positive
temperature coefficient, so as a device gets hotter, its drain-source
(RDS(on)) resistance increases. This much is true, but this
alleged 'benefit' is actually completely useless in a linear circuit. (It can
also cause major problems in switching circuits, but that's another topic
altogether and will not be covered here.)

What is not commonly noted is that all MOSFET devices have a fairly
high negative temperature coefficient for the gate-source threshold
voltage (Vth). At the gate-source voltages needed to obtain
typical bias currents, even a small temperature increase causes a large
drain-source current increase, so the use of a carefully designed bias servo
(Q5, R5 and R6 in the Figure 4 schematic) is absolutely essential. This
point is made above, but is sufficiently important that repetition will not go
astray.

To illustrate this, Figure 5 shows the graph from the IRF540 data sheet, and
although it does not continue down to the levels we are interested in, the trend
is clearly visible. At a VGS of 4.5V, we see ID of
around 12A at Tj = 25°C, rising to a little over 20A at
Tj = 175°C. While the graph might seem to indicate that the
effect will be greatly exaggerated at lower gate voltages and drain-source
current, the initial tests that I did indicate that the effect is roughly
similar. The graph was taken from the IRF540 data sheet, but has been colour
coded to make identification of each graph easier. The use of source resistors
to help force current sharing is essential, and these should be as high as
practicable. While 0.1 ohms is common for BJT amplifiers, I would recommend
values not less than 0.47 ohms for HEXFETs. Higher values provide more stable
quiescent current with temperature variations. For example, with 1 ohm source
resistors, the current can increase at a maximum of 1mA/mV or 1A/V. Should
Vth fall by 100mV, Iq can only increase by 100mA. This eases the
design of the bias servo.

Figure 5 - Temperature Coefficient, VGS (IRF540)

The test I ran was very simple. Apply a suitable voltage to the drain, then
carefully adjust the gate voltage until a suitable measurement current was
drawn. The current started at a relatively low value (around 1A in my test), and
as the device heated up this was seen to rise. It stabilised fairly quickly
because the heatsink prevented further (possibly damaging) heat levels, but with
two MOSFETs in parallel, the current between them was different, and (more
importantly) it remained different, even as they became hotter. The claims for
better (and 'automatic') current sharing apply only to devices operated as
switches, or where the two curves shown in Figure 5 cross over each other. Only
lateral MOSFETs provide a crossover point on their transfer characteristics that
is low enough for linear operation.

Something that is missing from nearly all MOSFET data sheets I have looked at
is gate threshold voltage vs. temperature, although it is shown in the
data I have for the IRF840. This will show that the threshold voltage falls as
Tj increases - a negative temperature coefficient. The
positive coefficient of RDS(on) is insignificant at the current
levels needed for setting quiescent current accurately. Note that the two curves
cross over, but the point where the temperature curves cross is when
VGS is at a current of over 40A and the gate-source voltage is
5.5V - this is not useful. Lateral MOSFETs (as used in P101) have exactly the
same issue, but the curve changes from negative to positive at a much lower
current (around 100mA), and this is visible on the transfer characteristic graph
(but you will need to look for it carefully - it is not specified in a useful
manner IMO).

In an application note [
1 ], OnSemi describe this transition as the 'inflection' point, and it is
determined by VGS, although it appears to be related more to the
drain current than gate voltage. However, the two are directly related, so the
point is moot. Lateral MOSFETs are usually quite safe here, because the
inflection point is at such a low voltage and current, but vertical MOSFETs
(HEXFETs and similar) are prone to thermal runaway. There is also the
possibility of a failure mode very similar to second breakdown when HEXFETs used
in linear circuits, where VGS is usually (well) below the
inflection point. This must remain a very good reason to stay clear of these
devices for audio, unless you are fully aware of the potential risks, and how to
avoid them. Note that the article above does not address this potential failure
mode, (nor do many others), and your only choice is to find MOSFETs where the
thermal 'changeover' occurs at the lowest possible drain current. Lateral
devices are almost unbeatable in this respect.

A careful examination shows that the 'inflection' point is actually the
region where the negative temperature coefficient of VGS exactly
compensates for the positive temperature coefficient of RDS(on).
That's the reason it's at such a high current for HEXFETs (because of the
usually low RDS(on) value), and it is known that the inflection
point is inversely proportional to RDS(on). Note that this
only applies if the device is used in linear mode. When switching,
Vth and its temperature coefficient is not relevant because the
gate voltage is invariably selected to provide 'hard' switching to minimise
losses.

In contrast, a typical lateral MOSFET (such as the 2SK1058) has an
RDS(on) of around 1.5 to 1.7 ohms at ~10A, compared to an IRF540N
with 0.044 ohms at 16A. When choosing HEXFETs for use in linear circuits, I
suggest that you use devices with the highest value of
RDS(on) that you can find. This is entirely counter-intuitive,
and is almost certainly the exact opposite of what you would expect.
RDS(on) is actually meaningless in a linear application until the
amp starts to clip, and even then only reduces the maximum output power
slightly. As a figure of merit, it only has meaning for switching
applications.

Figure 6 - Normalised Vth Vs. Junction Temperature

From Advanced Power Devices, their application note [
2 ] provides the graph shown in Figure 6. Normalising simply means that
everything is taken back to a reference of unity, so simply multiply the claimed
Vth by the figure shown along the left side, for the temperature
at which your device will operate. If your MOSFET will undergo a temperature
change from 25° to 100°, then Vth will fall to 0.83 of the
ambient temperature value at 100°C. This application note also mentions the
possibility of a failure mode similar to second breakdown when operating
switching MOSFETs as linear amplifiers. The App. Note refers to this failure
mode as 'hot-spotting' or 'current tunnelling', but it's very similar to
traditional second breakdown. Indeed, two of the articles listed below refer to
the fact that using a HEXFET much bigger than needed (to provide a safety
margin) has exactly the opposite effect. Rather than increasing the
safety margin, the larger device is more likely to fail if it is working
well below the 'inflection' current in a linear circuit.

Do not be mislead by claims that MOSFETs are immune from thermal runaway,
although lateral MOSFETs are much better in this respect than vertical MOSFETs
(HEXFETs and similar high-gain switching devices). Based on the above, it is
quite apparent that vertical MOSFETs can easily get into thermal runaway if the
bias servo is not set up correctly. Using just a pot (as shown in P101) is
absolutely forbidden with vertical FETs - a matched bias servo thermally coupled
to the MOSFET heatsink is essential to prevent both thermal runaway and
crossover distortion.

Further searching revealed a document from Solid State Optronics [
3 ], where the temperature coefficient for VGS is said to be
-1.5mV/°C (the above chart shows it as 1mV/°C for VGS of 4.5V at
25°C). It is claimed to be 'insignificant', and for switching applications this
is true. It is definitely not insignificant for a linear circuit (as shown in
Figure 4), and especially so because of the relatively high transconductance of
vertical MOSFETs. A few (tens of) millivolts of gate voltage is the difference
between acceptable quiescent current and overheating.

What of the second breakdown effect that the manufacturers deny even happens
other than in the (very) fine print? HEXFETs, and indeed most other MOSFET
devices, are made using a multiplicity of individual small MOSFETs called cells.
If the device as a whole exhibits a negative temperature coefficient for
VGS, so must each internal cell. If one cell has a slightly lower
VGS than the others (perhaps due to microscopic variations in the
silicon) it will take more of the total current. This will cause it to get
hotter, so the threshold voltage will fall further and it will then draw more
current, causing it to get still hotter. This process continues until the cell
fails due to over temperature, at which point the MOSFET suffers catastrophic
failure.

While this scenario is not common in switching application, if the MOSFET is
used linearly it is very real, and has caused problems in the past. It will
continue to cause problems if designers are unaware that this failure mode even
exists - after all, most comments seen describe MOSFETs as almost
indestructible. While this is true to an extent, it is obvious that it is not a
general rule upon which one should rely in all circumstances. HEXFETs operated
in linear mode need to be derated from the claimed maximum dissipation, and my
suggestion is that a maximum of half the rated power dissipation is
reasonable. Likewise, the peak current should also be much lower than the rated
maximum.

Now you know why International Rectifier and other vertical MOSFET
manufacturers don't recommend HEXFETs or their equivalent for linear
applications - they are simply not designed for the purpose. Yes, they most
certainly will work, but you must be aware of the limitations. I suggest that
high voltage, relatively low current devices are preferable to the reverse, as
they will have an inherently higher RDS(on), and therefore a
lower inflection point. Alternatively, you can look for 'vertical' MOSFETs that
are specifically designed for linear operation. They exist, but are probably
considered 'exotic' by most suppliers. An example is the IXYS IXTK22N100l (with
'extended' FBSOA), but
at over AU$70 each (one off price) when I looked most people will think that's a
bit rich.

Toshiba used to make vertical MOSFETs that were designated as being suitable
for audio amplifiers. The 2SK1530 and 2SJ201 were basically switching devices,
but with a comparatively high (but unspecified) RDS(on) of
between 0.3 and 0.6 ohms. I've tested an amplifier that used them, and they
perform well enough, but it's doubtful that there was really any advantage over
using bipolar output transistors (other than cost). These devices are now
obsolete, but they were classified for VGS(off), with '0' classed
devices having a threshold voltage of 0.8-1.6V, and 'Y' class with 1.4-2.8V.
This was unique amongst MOSFETs, and I don't know of any other MOSFET type that
has provided a degree of 'pre-matching' when you buy them.

If you intend to use vertical MOSFETs for any linear application, you need to
be aware that the published SOA curves do not apply to linear operation.
Feel free to read that again to make sure that you understand the ramifications.

There's little or nothing in the datasheets to warn you, and many data sheets
even show the SOA for DC. A few careful calculations will show that there is no
way that the MOSFET can be operated at full rated power while keeping the die
temperature below the absolute maximum (typically 175°C). The only way that you
can be assured of safety is to keep the peak dissipation well below the claimed
maximum, thus minimising the die temperature. This is a minefield, and I suggest
that you tread very carefully. See the IR application note for more detailed
information [ 4
].

Interestingly, some of the earliest MOSFET fabrication processes are less
likely to fail if used in linear mode, because they have a comparatively high
RDS(on). However, most of the early types are obsolete, and their
nominal replacements are 'better' in that RDS(on) is lower then
the previous version(s). While this is good for switching (reduced losses), it
also ensures that they are less suited to linear operation.

It's worth looking at RDS(on) for lateral MOSFETs as a point
of comparison. The figure isn't quoted, but it can be calculated using the
Drain-Source saturation voltage figure provided. VDS(sat) will
normally be in the order of around 12V (gate shorted to drain) at a current of
7-8 amps, so RDS(on) is in the order of 1.5-1.7 ohms! Compare
that with the figures you see for HEXFETs - the IRFP240 has an
RDS(on) of 0.18 ohm, but it's also important to understand that
completely different test methods are used so a direct comparison isn't as easy
as it seems.

One thing that is a dead giveaway as to whether a MOSFET is lateral or
vertical is to compare the pinouts. Lateral MOSFETs have the source pin
in the centre, while vertical types invariably have the centre pin as the
drain.

Figure 7 - Vertical Vs. Lateral MOSFET Pinouts

As far as I'm aware, there are no exceptions to the above. There are very few
lateral MOSFETs in the TO-220 case, and all true lateral high-power MOSFETs use
the TO-247 case or a small variation thereof. It's worth noting that the TO-220
package is useless (regardless of what's inside) for getting rid of any more
than about 20W worth of heat, unless extraordinary care is taken with mounting.

As should now be quite obvious, it's very hard to recommend using HEXFETs in
an audio amplifier. Mitch has made a compelling case, but much has changed since
the article was written, and there is also more information available. This
won't stop people from trying, because HEXFETs are cheap compared to lateral
MOSFETs and high power bipolar transistors, and seem to offer many
advantages. As is now clear (I hope), most of the 'benefits' are an illusion,
and can easily lead to tears if the warnings here aren't heeded.

References

1   On Semiconductor - AND8199
2   Advanced Power Technology - AN0002 - no longer found at the source, but now available
from the ESP site
3   Solid State Optronics - Application Note 50
4   International Rectifier -
AN1155

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Copyright Notice. This
article, including but not limited to all text and diagrams, is the
intellectual property of Rod Elliott and Mitch Hodges, and is Copyright ©
2006. Reproduction or re-publication by any means whatsoever, whether
electronic, mechanical or electro- mechanical, is strictly prohibited
under International Copyright laws. The author (Mitch Hodges) and editor
(Rod Elliott) grant the reader the right to use this information for
personal use only, and further allow that one (1) copy may be made for
reference. Commercial use is prohibited without express written
authorisation from Mitch Hodges and Rod Elliott.

Page created and copyright © 15 Jan 2006./ Updated 13 March
2006 - Added update with thermal characteristic data and additional caveats./
Mar & Oct 2014 - further updates on HEXFETs used in linear
mode.