Datasheet

AD834 Data Sheet
Rev. F | Page 12 of 20
BIASING THE OUTPUT
The AD834 has two open collector outputs as shown in Figure 13.
The +V
S
pin, Pin 6, is tied to the base of the output NPN
transistors. The following general guidelines maximize
performance of the AD834.
+V
S
+V
S
MULTIPLIER
CORE
–V
S
W1 W2
+5V +5V +5V
RW
49.9Ω
RW
49.9Ω
R
CC
75Ω
BIAS
–5V
OUTPUT OF AD834
AD834
5
4
6
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Figure 13. Output Stage Block Diagram
12.5mA
RW
W COLLECTOR
HEADROOM
–
+
W BASE
R
CC
+5V +5V
+V
S
NEGATIVE OUTPUT
VOTLAGE SWING
SITUATION 1
I
POS
SUPPLY
8.0mA TO 14mA
(GENERALLY 10.5mA)
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Figure 14. Negative Swing
Figure 14 shows the currents at the input when the AD834
swings negative. Generally, +V
S
should be biased at +4 V or
higher. For best performance, use resistor values that do not
saturate the output transistors. Allowing for adequate transistor
headroom reduces distortion.
Headroom = Voltage at W
COLLECTOR
− Voltage at W
BASE
When either output swings negative, the maximum current
flows through the RW resistors. It is in this situation that
headroom is at a minimum.
Headroom
NEGATIVE SWING
= (I
POS
SUPPLY × R
CC
) − (12.5 mA × RW)
Try to keep headroom at or above 200 mV to maintain adequate
range. Headroom ≥ 200 m V.
This recommendation addresses the positive swing of the
output as shown in Figure 15. It is sometimes difficult to meet
this for negative output swing.
4.5mA
RW
W COLLECTOR
HEADROOM
–
+
W BASE
R
CC
+5V +5V
+V
S
POSITIVE OUTPUT
VOTLAGE SWING
I
POS
SUPPLY
8.0mA TO 14mA
(GENERALLY 10.5mA)
00894-115
SITUATION 2
Figure 15. Positive Output Swing
The current through RW is smaller for positive output swings.
Headroom
POSITIVE SWING
= (I
POS
SUPPLY × R
CC
) − (4.5 mA × RW)
For dc applications or applications where distortion is not a
concern, the headroom may be zero or as low as −200 m V.
However, for most cases, size the resistors to give the output
adequate headroom.
TRANSFORMER COUPLING
In many high frequency applications where baseband operation
is not required at either inputs or the output, transformer coupling
can be used. Figure 16 shows the use of a center-tapped output
transformer, which provides the necessary dc load condition
at the outputs, W1 and W2, and is designed to match into the
desired load impedance by appropriate choice of turns ratio.
The specific choice of the transformer design depends entirely
on the application. Transformers can also be used at the inputs.
Center-tapped transformers can reduce high frequency distortion
and lower HF feedthrough by driving the inputs with balanced
signals.
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8 7 6 5
1 2 3 4
X2
X1 +V
S
W1
Y1 Y2
–V
S
W2
AD834
X-INPUT
±1V FS
Y-INPUT
±1V FS
TERMINATION
RESISTOR
TERMINATION
RESISTOR
49.9Ω
+5V
4.7Ω
–5V
LOAD
1µF
CERAMIC
1µF
CERAMIC
Figure 16. Transformer-Coupled Output
A particularly effective type of transformer is the balun
1
, which
is a short length of transmission line wound onto a toroidal
ferrite core. Figure 17 shows this arrangement used to convert
the bal(anced) output to an un(balanced) one (therefore, the
use of the term). Although the symbol used is identical to that
for a transformer, the mode of operation is quite different. First,
the load should now be equal to the characteristic impedance of
the line (although this is usually not critical for short line lengths).
The collector load resistors, R
W
, can also be chosen to reverse-
terminate the line, but again this is only necessary when an
electrically long line is used. In most cases, R
W
is made as large
as the dc conditions allow to minimize power loss to the load.
The line can be a miniature coaxial cable or a twisted pair.
1
For a good treatment of baluns, see Transmission Line Transformers by Jerry
Sevick; American Radio Relay League publication.