This photo shows the Auto-Zero circuit installed in the 120W PA Amplifier. It reduces the offset voltage
to less than 100JLV.
Null your amplifier's
DC output to zero
The problem with DC coupled power
amplifiers is that they all have a residual DC
voltage at the output. If this voltage is too
high it can cause problems. This simple
circuit can be used to null the DC offset
down to 100 microvolts or less.
Design by JOHN CLARKE
All modern power amplifiers are
DC coupled and as a result, they all
have a small DC voltage at the output. Typically, this is around ± 20
to ± 30mV but it can be a lot more,
depending on how well the input
differential pair of transistors is
matched and what the amplifier's
operating temperature is.
If your loudspeakers have a
nominal impedance of 80 and the
80
SILICON CHIP
amplifier has a DC output voltage of
20 to 30mV then you don't really
have a problem. On the other hand,
if your amplifier has to drive a 70V
or 100V line transformer (as in a
PA system) or your loudspeakers
contain transformers (such as in
electrostatics) that is a problem of
an entirely different complexion.
Transformers present a real problem to power amplifiers because
they have a very low primary DC
resistance. This is usually around
100 milliohms (0.10) or less. Now if
your amplifier has the typical offset
voltage quoted above, around
± 30mV or thereabouts, the DC current through the transformer
primary winding will be more than
300 milliamps.
Clearly, this is a big problem, not
only because it can cause saturation of the transformer (which
causes severe distortion) but it can
also cause severe heating and
perhaps even thermal runaway and
ultimate destruction of the amplifier.
Our Auto-Zero circuit brings the
DC output offset of any amplifier
down to a small fraction of a
millivolt. It uses a new integrated
circuit from National Semiconductor, the LMC669, which has been
+50V
0.47
PARTS LIST
2.2k
o-=t'•+~Wr....-----11----t
LINE
TRANSFORMER
22k
..,.
-50V
22k
1k
120W PA AMPLIFIER CONFIGURATION
Fig.la: basic configuration of the 120W PA Amplifier (December
1988). The power amplifier drives a 100V line transformer.
0.47
<>-=I
+
2 2k
•
LINE
TRANSFORMER
INPUT
-50V
1 PC board, code
SC01108891, 69 x 75mm
1 LMC669CCN auto-zero IC
(20-pin version)
2 1 5V 1W zener diodes
7 PC pins
Capacitors
2 1 0µF 1 6VW PC electrolytic
capacitors
3 0.1 µF metallised polyester
capacitors
1 .0022µF metallised polyester
capacitor
Resistors (¼W, 5%)
2 100k!l
2 1 0k!l
1 5.6k!l
1 4.7k!l
2 1.8k!l 5W wirewound
1 330
22k
1k
No effect on amplifier
performance
+
22:r
+50V
-50V
CORRECTING
VOLTAGE
OUTPUT
AMPLIRER SIGNAL
GROUND
OUTPUT
OFFSET
INPUT
AUTO ZERO MODULE
-.
-.
AMPLIFIER POWER
GROUND
AUTO ZERO MODULE CONNECTED TO 120W PA AMPLIFIER
Fig.lb: how the Auto-Zero module is connected into circuit. It
samples the amplifier's output and applies a correction signal
back to the input via the 22k0 resistor.
designed for just this sort of
application.
While our circuit may be simple,
the LMC669 is not a simple device.
It contains a four stage comparator
for sampling, a clock and divider
circuit, an internal reference
voltage and an output amplifier.
In effect, the LMC669 can be
regarded as a very precise DC
operational amplifier with an extremely low DC offset error. When
connected in the Auto-Zero circuit,
it works to reduce the output offset
of the external amplifier by comparing it to an internal reference
voltage. When the DC output of the
external amplifier swings above or
below ground by more than 10µ V
(the typical offset of the LMC669's
comparator), the Auto-Zero circuit
applies a correction signal to the input of the amplifier and so brings it
back to zero.
The correction process is not immediate but takes place progressively in a large number of
ever-decreasing steps as the
LMC669 continually samples the
amplifier output voltage, generates
another smaller correction and so
on. Ultimately though, depending on
how large the initial offset error
voltage is, the LMC669 will settle at
a final minimum offset of less than
100µV.
While the Auto-Zero effectively
nulls out any DC errors in the output of a DC-coupled amplifier, it
does not limit the performance in
any way. It has no effect on the
amplifier's residual noise, frequency response, slew rate, harmonic
distortion or any other parameter.
The proviso here, of course, is
that the Auto-Zero circuit needs to
be connected in a particular way,
otherwise performance could be
prejudiced.
Fig.1 shows a typical connection
of the Auto-Zero circuit module to a
power amplifier such as our 120W
power amplifier described in the
December 1988 issue of SILICON
CHIP. This amplifier was designed
to have a particularly low offset
voltage but we are using it as an example of how any amplifier could
be corrected.
The existing 22k!l input bias
resistor in the 120W amplifier must
be disconnected from the 0V line
and connected instead to the correction output of the Auto-Zero
module. The only other connections
required from the power amplifier
are from the amplifier output, the
input reference line and the
positive and negative supply lines.
Fig.2 shows the circuit of the
SEPTEMBER 1989
81
*R1
1.Bk
5W
100k
P O S I T I V E O - ~ f r l r - - + - - - - - - - - - - -....w.A.-e---.
SUPPLY
10
16VW
0.1
+
_
AMPLIFIER
POWER
GROUNO
u----------+-----i,-----+-------+-----------
AMPLIFIER
u---------------~
POWER
GROUNO
v+
1
13
15
12
•RESET
OUTPUT
OFFSET
100k
INPUT 0-'YW,~>---.........,..-t
(FROM
AMPLIFIER
10
+
OUTPUT)
16VW
10k
CORRECTING
--+--'4'A.7~k,+--t--ov8~JtJ/
OUTFe----11...--.is'N.&111,k
IN1
IC1
LMC669
IN OUT
REF REF
2
6
, - AGND
3
14
0.1
19
9
(TO GROUND END OF
NON-INVERTING
AMPLIFIER
BIAS RESISTOR)
10
AMPLIFIER
SIGNAL
GROUNO
v----------+-------------------------~
0.1,
*SEE TEXT
N~~m~EO-..,*'NR\1112- - - - - - _ _ .
1.Bk
5W
AUTO ZERO
Fig.2: the Auto-Zero circuit is based on National Semiconductor's LMC669 IC. It reduces the output offset
of an external amplifier by comparing any offset to an internal reference voltage. A correction signal is
then applied to the amplifier's input.
The parts for the Auto-Zero circuit are all accommodated on a small PC board.
This should fit easily into virtually any amplifier.
82
SILICON CHIP
Auto-Zero module itself. The connection from the amplifier output is
via a voltage divider consisting of
two resistors, lO0kD and lOkD. This
divider protects the input of the
LMC669 in case a large DC fault appears at the output of the amplifier.
A lOµF capacitor filters the
signal from the divider so the circuit only responds to DC and frequencies below about 1.8Hz. The
resultant slowly varying signal is
fed to INl and IN2 (pins 1 and 20)
on the LMC669.
At the output of the LMC669 is a
.0022µF capacitor which smoothes
out the voltage steps produced by
the circuit. The output is then further filtered by a series 5.6kD
resistor and O. lµF capacitor after
which it is fed to a voltage divider
consisting of a 4. 7kD and 330
resistor.
Note that the LMC669 includes
three analog signal ground pins (3,
14 & 19) and a separate data
ground, pin 18. The analog grounds
allow the INl, IN2 and INREF inputs (pins 1, 20 & 2) to have a
grounded "guard" electrode incorporated into the printed circuit pattern. The guard electrode helps
OUTPUT OFFSET
INPUT
____.... ~t~Jf!
AMPu:wM~NAL
---1.111
I '""'...::---•AMPLIFIER
POWER
GROUND
CORRECTING VOLTAGE-----41,l
OUTPUT
NEGATIVE
SUPPLY
Fig.3: here's how to install the parts on the PC board. Use PC
stakes to terminate the external wiring connections and take care
with the orientation of polarised components.
package for the LMC669. This is
denoted as LMC669CCN.
As noted previously, we installed
the Auto-Zero circuit into the 120W
public address amplifier. We made
up a couple of small brackets to
mount the Auto-Zero board above
the amplifier board. After that, the
various leads need to be run to the
amplifier board and the positive
and negative supply capacitors.
Connect the offset input to the
output of the power amplifier. To
connect the correction voltage output from the Auto-Zero board to the
power amplifier, the earthy end of
the 22k0 resistor at the input of the
power amplifier needs to be lifted
from the PCB. The resistor is then
stood on end and the correction
voltage lead soldered to the free
end.
When all wiring is complete,
check it very carefully. The
amplifier operation can then be
checked, without any load connected. Turn on the power and
measure the output voltage at the
amplifier with a digital multimeter.
The voltage should quickly settle to
below ± 0.1 millivolts.
~
D.DAUNER
ELECTRONIC
COMPONENTS
WE STOCK A WIDE RANGE
OF ELECTRONIC PARTS
Fig.4: this is an actual size reproduction of the PC artwork.
•
shield the inputs against spurious
signals which may cause errors in
the circuit operation. The separate
grounds also help prevent the noise
from the digital switching within
the IC reaching the analog ground
and thence into the amplifier.
Our circuit shows a number of
link options (LKl, LK2, LK3 and
LK4) which enable the internal
clock speed of the LMC669 to be
selected. However, links LK 1 and
LK2 are the ones to be used, as
shown on the PC component overlay
diagram of Fig.3.
Power for the Auto-Zero circuit
is derived from the amplifier supply
lines using dropping 1.8k0 5W
resistors Rl and R2 and regulating
with 15V zener diodes. The 1.8k0
dropping resistors will be suitable
for powering the circuit from
amplifiers with supply voltages
ranging from ± 40V to ± 90V.
Construction
The Auto-Zero is assembled onto
a small printed circuit board
measuring 69 x 74mm (code SC
01108891). The main things to
watch out for when assembling the
board is the polarity of the zener
diodes and the electrolytic capacitors, and the orientation of the
LMC669. Note that we used the
more readily available 20-pin
for
Development • Repair
• Radio Amateur
• Industrial Electronic
• Analog and Digital
WHILE STOCKS LAST
* THE FAMOUS TR-DIP METER
*
(as featured May '89) $98.00
CRO-VALVE 3ACP2A $45.00
* PHILIPS IR-IMAGE CONVERTER
VALVE XX1080 $45.00
(For use in infrared sight experiment systems)
Come and see.
Showroom:
51 Georges Crescent,
Georges Hall, NSW 2198
(Behind Caltex Service Station In Blrdwood Road)
Phone 724 6982
TRADING HOURS:
Monday to Friday 9.00 a.m. to 4.00 p.m.
Saturday from 9.00 a.m. to 12.00 noon.
SEPTEMBER1989
83
