7
assures that the characteristics of
the two diodes are more highly
matched than would be possible
through individual testing and
hand matching.
bias
detector diode
PA
V
bias
bias
differential
amplifier
HSMS-282K
reference diode
to differential amplifier
differential
amplifier
matching
network
HSMS-282P
Figure 15. High Power Differen-
tial Detector.
Figure 17. Voltage Doubler
Differential Detector.
matching
network
HSMS-2825
Figure 13. Differential Detector.
The concept of the voltage
doubler can be applied to the
differential detector, permitting
twice the output voltage for a
given input power (as well as
improving input impedance and
suppressing second harmonics).
However, care must be taken to
assure that the two reference
diodes closely match the two
detector diodes. One possible
configuration is given in Fig-
ure 16, using two HSMS-2825.
Board space can be saved
through the use of the HSMS-282P
open bridge quad, as shown in
Figure 17.
bias
While the differential detector
works well over temperature,
another design approach
[3]
works
well for large signal detectors.
See Figure 18 for the schematic
and a physical layout of the
circuit. In this design, the two
4.7 KΩ resistors and diode D2 act
as a variable power divider,
assuring constant output voltage
over temperature and improving
output linearity.
RF
in
68
Ω
D1
4.7 KΩ
33 pF
V
o
4.7 KΩ
D2
68
Ω
33 pF
Figure 14. Fabrication of Agilent
Diode Pairs.
In high power applications,
coupling of RF energy from the
detector diode to the reference
diode can introduce error in the
differential detector. The
HSMS-282K diode pair, in the six
lead SOT-363 package, has a
copper bar between the diodes
that adds 10 dB of additional
isolation between them. As this
part is manufactured in the
SOT-363 package it also provides
the benefit of being 40% smaller
than larger SOT-143 devices. The
HSMS-282K is illustrated in
Figure 15 — note that the ground
connections must be made as
close to the package as possible
to minimize stray inductance to
ground.
HSMS-2825
or
HSMS-282K
RF
in
HSMS-282K
differential
amplifier
V
o
4.7 KΩ
Figure 18. Temperature Compen-
sated Detector.
HSMS-2825
matching
network
HSMS-2825
Figure 16. Voltage Doubler
Differential Detector.
In certain applications, such as a
dual-band cellphone handset
operating at both 900 and
1800 MHz, the second harmonics
generated in the power control
output detector when the handset
is working at 900 MHz can cause
problems. A filter at the output
can reduce unwanted emissions
at 1800 MHz in this case, but a
[3]
Hans Eriksson and Raymond W. Waugh, “A Temperature Compensated Linear Diode
Detector,” to be published.
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
