CIRCUIT
DESCRIPTION
Current
,-------~C
Difference
Circuit
A B
Electronic Wattmeter
CHAPTER
3
input
o---r-~:::::J~----<1
Shunt
Resistor
3.1
Multiplying Circuit
Voltage
~----~====~~====~~i~nput
~
21
Fig 3.1
~
2[
Series
Resistor
The EW604 is designed around a four-quadrant analogue
multiplier circuit which makes use of the logarithmic relationship
between the emitter base voltage and the emitter current
of
a
transistor,
to
effect the multiplication process.
The circuit is designed to accept current inputs directly
so
that
interfacing with the wattmeter input terminals needs
only a
series resistor for the voltage input and a pair of current-dividing
resistors for the current input.
The basic multiplying circuit is shown
in
fig
3.1
while the full
circuit
is
shown
in
the Maintenance Chapter as fig 4.3.
Transistors
TR
1
to
TR
5
are a matched array.
All the emitters
are
at
virtual earth points and all the base currents are supplied
by
operational amplifiers
AC
or AA.
The output product is the difference between collector currents
at
A and
B.
These points
are
again held
at
'virtual earths'
at
about +0.7V
by
operational amplifiers
in
the difference circuit (AD and
AE
in
fig
4.3) which are connected
to
provide
an
output voltage
at
C
(fig
3.1) proportional to the difference
in
current of A and
B.
The
gain of the difference circuit (mutual resistance) is adjusted
by
a
potentiometer (R42
in
fig 4.3). Another operational amplifier
(AF
in
fig
4.3) is connected
as
a voltage follower to drive the
moving coil meter.
2 1
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