Specifications and information are subject to change without notice
WJ Communications, Inc
• Phone 1-800-WJ1-4401 • FAX: 408-577-6621 • e-mail: sales@wj.com • Web site: www.wj.com
Page 4 of 5 February 2006
AP504
DCS-band 4W HBT Amplifier Module
Product Information
The Communications Edge
TM
MTTF Calculation
The MTTF of the AP504 can be calculated by first
determining how much power is being dissipated by the
amplifier module. Because the device’s intended application
is to be a power amplifier pre-driver or final stage output
amplifier, the output RF power of the amplifier will help
lower the overall power dissipation.
In addition, the
amplifier can be biased with different quiescent currents, so
the calculation of the MTTF is custom to each application.
The power dissipation of the device can be calculated with
the following equation:
Pdiss = Vcc * Icc – (Output RF Power – Input RF Power),
Vcc = Operating supply voltage = 12V
Icc = Operating current
{The RF power is converted to Watts}
While the maximum recommended case temperature on the
datasheet is listed at 85 ˚C, it is suggested that customers
maintain an MTTF above 1 million hours.
This would
convert to a derating curve for maximum case temperature vs.
power dissipation as shown in the plot below.
Maximum Recommended Case Temperature vs. Power Dissipation
to maintain 1 million hours MTTF
50
60
70
80
90
4567
89
10
11
12
Power Dissipation (Watts)
To calculate the MTTF for the module, the junction
temperature needs to be determined. This can be easily
calculated with the module’s power dissipation, the thermal
resistance value, and the case temperature of operation:
Tj = Pdiss * Rth + Tcase
Tj = Junction temperature
Pdiss = Power dissipation (calculated from above)
Rth = Thermal resistance = 9 ˚C/W
Tcase = Case temperature of module’s heat sink
From a numerical standpoint, the MTTF can be calculated
using the Arrhenius equation:
MTTF = A* e
(Ea/k/Tj)
A = Pre-exponential Factor = 6.087 x 10
-11 hours
Ea = Activation Energy = 1.39 eV
k = Boltzmann’s Constant = 8.617 x 10
-5 eV/ ºK
Tj = Junction Temperature (ºK) = Tj (ºC) + 273
A graphical view of the MTTF can be shown in the plot
below.
MTTF vs. Junction Temperature
1.E+05
1.E+06
1.E+07
130
140
150
160
170
180
Junction Temperature (°C)
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