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Build a MOSFET Citation 12
6
Build a MOSFET Citation 12
N
ELSON
P
A S S
by
K
IRK
R
A D E R
Audio Amateur
has published a number of
projects modifying popular kits, usually
Dynacos. Obviously it is much less ex-
pensive to use the chassis and power
supply, pots, connectors, switches, PC
boards, and heat sinks that can be had
for the price of a kit than to buy the com-
ponents individually or have them
made. Creating a one-off copy commer-
cially typically costs as much as making
10 copies of the same item, because the
dominant costs are design and set-up
time which do not increase with quanti-
ty. Small wonder then that most techni-
cians and designers who write for this
and similar magazines base their pro-
jects on yesterdays readily available
kits, which, often as not, languish unap-
preciated in closets and basements. This
is one such project.
We will, however, do two things here
you may not have seen before. Well
modify the Harmon Kardon Citation
12, a popular device which has somehow
escaped the kit modifiers attention; and
well do so with power mosfet circuitry
instead of the traditional bipolar power
transistors. This project is for experienc-
ed builders only, and Harmon Kardon
want me to remind you that it will void
any warranty coverage on the Citation
12:
PLEASE NOTE
After this article was prepared In-
ternational Rectifier withdrew the
IRF-100 from production and dis-
tribution and offered an alter-
native, the IRF-130 as a replace-
ment.
Author Pass has tested these new
devices in the prototype and re-
ports that performance is as good
or better than that using the
IRF-100s. The change came too
late to alter the diagrams so the text
and drawings have been left in
their original form. In all cases
where IRF-100 is indicated, the
IRF-130 should be
THE AUDIO AMATEUR
THE HARMON KARDON
CITATION 12
The Citation 12, introduced about 10
years ago, is based on the circuitry
found in the RCA transistor manuals
(for example, see RCA data sheet file
This circuitry was the basis for
nearly all the quasi-complementary
1
MOSFE T TUBE BIPOLAR
I I
1.
tube, and bipolar drawing.


BIPOLAR
M O S F E T
GRID 2
3oov
2 v 3 v 4 v
6V 7 v
B I P O L A R MOSFET ON VOLTAGE VS CURRENT
-35v -30v
-20v
-5v ov
GRID C O N T R O L V O L T A G E V S C U R R E N T
2 Characteristics of
tubes, and designs in existence, as typified by the
large power amplifiers of the early 70s.
In its time the Citation 12 was a truly
excellent
amplifier,
employing a
number of concepts which have only
recently become popular: dual split
power supplies, DC output coupling,
and no active current limiting. These
characteristics allowed it to sonically
outperform the Dynaco 120 which was
its popular competitor. It could drive
anything with high reliability, and in the
time I spent as a repair technician I
a few Dynaco, Phase Linear,
McIntosh,
Pioneer,
a n d S a n s u i
amplifiers, but Ive never seen a broken
Citation 12.
32 0 0
--
100 --
7 0 - -
5 0 - -
3 0 - -
2 0 - -
I have another, nostalgic, reason for
choosing the Citation 12: it was the
amplifier I built myself. About eight
years ago, when I was an impecunious
student working part-time for ESS Inc.
(no, I did not design their electronics),
the home-brew power amplifier built by
a friend broke down, leaving me without
music. Like most of you in a similar
position, I decided to do without some
other necessity, and shortly my kitchen
table was covered with solder blobs and
the parts to the Citation 12.
10

2
3
7
2 0
D C C O L L E C T O R C U R R E N T
( A M P S )
Fig. 3. Bipolar current gain

7 V
I decided to fire up the completed unit
on a variac instead of merely plugging it
into the wall: and was dismayed to
discover that both channels exhibited ex-
tremely large DC offset when I applied
AC power. After some weeks of recheck-
ing my work and finding no errors, I
finally decided to connect it up and plug
it into the wall anyway?* When I did,
the woofers on my speakers plunged ful-
ly forward as if attempting to escape the
amplifier; but after a few seconds they
returned to normal and music appeared.
Thus I discovered one of the Citations
few faults: an enormous subsonic
on thump.
P U L S E T E S T
- -
4 v
I kept the amplifier for several years
and ultimately sold it to a friend, only to
end up buying it back. It sat in my closet
for a long time, a candidate for that
perfect project, until one day at
Threshold we discovered a vendor had
accidentally shipped us several power
mosfets instead of diodes, and this arti-
cle began falling into place. The power
mosfets,
reasonable analogs of the
hornetaxial npn devices in the Citation
12, fit quite neatly into the original
layout and provide for a state-of-the-art
super-position upon a classic piece of
circuitry,
improving the speed and
distortion characteristics in an even
more simple topology than in the
original.
3 v
0
I
0
2 o v
3 o v
4 o v
D R A I N
V S D R A I N T O S O U R C E
V
O
L
T
A G E
Fig. 4.
IRF
will treat the mosfet as a black box
possessing certain simple characteristics.
In the power mosfet we have a gain
device which combines the best proper-
ties of tubes and bipolar transistors.
Seen in Fig. 1, the gate is analogous to
the grid and the base, the source cor-
responds to the cathode and emitter,
and the drain corresponds to the plate
and collector.
POWER MOSFETS
As is the case with most electronic com-
ponents, an exact understanding of
mosfet operation is a formidable goal,
and I wont address it here, rather we
In all cases the current through the
device (terminal 1 to terminal 3) is a
function primarily of the voltage be-
tween terminals 1 and 3. As the voltage
increases between 2 and 3 and/or be-
tween 1 and 3, so does the current
through terminals 1 and 3. Figure 2
shows a typical relationship of these
parameters for mosfets, tubes, and
bipolar transistors.
change in output current versus input
voltage is greatest in the bipolar tran-
sistor, a difference resulting from its be-
ing a current gain device, where the in-
put current is to be multiplied linearly
by the gain. Figure 3 gives a better pic-
ture of the bipolar devices gain and its
comparative linearity. As Fig. 2 shows,
power mosfets and tubes have similar
characteristics, but the mosfet has much
higher transconductance and operates at
bipolar voltages. Considered overall, the
power mosfet is like a tube but retains
the high transconductance and current
of bipolar technology. Power mosfets
can also be made complementary, with
the p and n
comparable
to pnp and npn bipolar transistors,
where the devices polarity makes them
mirror
each other.
a feature not


In Fig. 2 we note that the rate of
available in tubes.
THE AUDIO AMATEUR
198 1 ADVANTAGES OF
POWER MOSFETS
The power mosfets used in this project
are International Rectifier IRF
a
150 watt, 100 volt, 12 amp device in a
TO-3 style case. They are part of a
relatively new line of transistors using
proprietary hexfet process, which
combines a particularly high voltage
capability with very fast switching times
and low saturation losses. Except for the
steep price tags, this type of device has a
number of advantages over regular bi-
polar transistors in
audio power
amplifier use.
First, their simple low current cir-
cuitry eliminates the need for driver
transistors. Second, they are immune to
the second breakdown phenomenon
which robs bipolar transistors of their
power rating at higher voltages. This
breakdown results from the positive
temperature
coefficient which en-
courages local current hogging within a
area on the chip, so that at higher
voltages one small part of the transistor
tends to do most of the work, resulting
in more probable failure. In a power
mosfet the temperature coefficient is
negative, and energy dissipates more
evenly across the surface of the chip,
allowing full power application at the
highest rated voltage. This characteristic
also avoids the bipolar design problem
of thermal bias runaway, and eliminates
costly compensations to maintain reli-
able operation over a range of condi-
tions.
Third, because mosfets are majority
carrier devices (bipolars are minority
carriers) their intrinsic speed is much
higher. Rise and fall times are about
while similarly rugged bipolar
devices have rise and fall times several
factors larger.
DISADVANTAGES OF
POWER MOSFETS
Basically, there arent any. Some ad-
vantages popularly attributed to them
are however, not
true. For ex-
ample, the negative temperature coeffi-
cient does not in itself guarantee the
device is indestructible. While this cha-
racteristic eliminates the second break-
down mechanism, it does not offer bet-
ter reliability at lower voltage levels, so
that in many well design