AN1526 Freescale Semiconductor / Motorola, AN1526 Datasheet - Page 4

• Manufacturer Part Number: AN1526
• Description: RF Power Device Impedances: Practical Considerations
• Manufacturer: Freescale Semiconductor / Motorola

at low quiescent current compared to the collector current
at peak output power there is a question at which bias point
to take the s–parameters.
computed from the s–parameters at a typical bias current
for class–AB operation and the same measurements at a
collector current corresponding to operation of the device
at rated output power under class–C conditions.
from the s–parameter data introduced by changing the bias
point. For this particular transistor, it indicates the
simultaneous conjugate match impedances, taken at a
collector current more in line with the current under class–C
conditions, are a better match to the conjugate of the class–C
impedances. Comparison of the output simultaneous
conjugate match impedance with the relatively higher
optimum load line impedance from the load–pull
measurements, illustrates how the load line has been shifted
to increase the efficiency of the amplifier. A rough calculation
of the collector efficiency from (Re(Z
Re(ZM1)]) gives a value of 68% at 870 MHz, very close to
the actual value of 70% from the efficiency contours with
the same load–line. Note maximum power transfer cannot
usually be achieved with a simultaneous conjugate match
because of non–linear current limiting characteristics of the
device.
CURRENT METHODS OF ENSURING
CONSISTENT DEVICE IMPEDANCES
with regard to the long term consistency of the device,
minimum gain requirements and consistent impedances.
Most of the recent devices characterized for the land mobile
environment utilize a broadband fixture to demonstrate
performance over a range of frequencies. The recently
introduced MRF650 goes a step further and specifies gain,
efficiency and input return loss at three frequencies of
operation in a specified test fixture. Motorola has found over
the years that this is the most cost effective way of producing
RF power devices with minimum variability. Of course, new
testing and characterization techniques are constantly being
4
Since class–C circuits are biased at cut–off and class–AB
Below is a comparison of the device impedances
This data shows a large shift in the impedances computed
Users of RF power transistors have two main concerns
(MHz)
(MHz)
Freq.
Freq.
806
838
870
806
838
870
0.95 + j3.15
1.00 + j3.31
1.07 + j3.37
3.29 + j3.95
3.83 + j3.18
3.74 + j2.02
Input Z
Input Z
[S
[S
Table 3. MRF873 Impedance Data Computed from S–Parameters (I
11
11
Table 4. MRF873 Impedance Data Computed from S–Parameters (I
]
]
Conjugate.
0.48 – j3.19
0.50 – j3.41
0.57 – j3.48
Conjugate.
1.13 – j2.98
1.06 – j2.90
1.02 – j2.86
I/P Simul.
I/P Simul.
Freescale Semiconductor, Inc.
Match
Match
For More Information On This Product,
opt
)/[Re(Z
Go to: www.freescale.com
Impedance
2.41 – j7.24
1.90 – j6.79
1.35 – j6.41
Impedance
8.94 – j2.31
8.09 – j4.32
5.99 – j5.72
Output
Output
opt
) +
MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION
1.19 + j7.15
0.94 + j6.61
0.71 + j6.27
2.86 + j5.52
2.10 + j5.14
O/P Simul.
O/P Simul.
Conjugate
Conjugate
1.57 +j4.69
Match
Match
evaluated. The engineer unfamiliar with RF power devices
will probably ask, “If the fixture is used to produce a
consistent device, what produced the fixture?” The answer
lies in the device development process.
sample devices are typically evaluated in narrowband
tuneable fixtures. During this evaluation period, the device
designer is balancing a multitude of performance parameters
with customer and manufacturing requirements. By the end
of the evaluation period, devices from quite a wide
distribution will have been constructed. These devices are
then used to design a broadband fixture to be used in
characterization and factory testing of the production part.
A typical broadband fixture and circuit schematic are shown
in Figures 20 and 21 for the MRF873 RF power transistor.
Broadband performance for this fixture is shown in Figure 25.
Specifications for the device are based on this fixture and
the devices that defined it. A portion of these evaluation
devices are locked away and referred to as “master
engineering correlation units.” Motorola’s philosophy is that
in the event of a damaged or irreplaceable fixture, these
master devices are sufficient to duplicate the fixture. To be
sure, there is considerable data taken on the original fixture.
This data is not limited to RF performance but also includes
such information as broadband source and load impedance
sweeps. The engineer new to RF power design must
understand that the impedances presented in the data sheet
ARE NOT the impedances of the broadband fixture.
Data Book Impedances
output power and nominal supply voltage. The transistor is
operated in a test fixture with a range of tuning either by
on–board trimming capacitors or external tuners or a
combination of both methods. The impedances seen in the
data sheet represent the average of several devices placed
in a narrowband fixture and tuned, usually for maximum gain,
at a specific output power, supply voltage and frequency with
reflected power minimized to at least –25 dB input return
loss. The device is removed and the SOURCE and LOAD
impedances at Fc are measured with the reference plane
at the device package edge. A piece of information NOT
During the development of an RF power transistor,
Data book impedances are normally taken at the rated
2.93 – j1.39
2.92 – j1.10
2.92 – j0.81
2.93 – j1.39
2.92 – j1.10
2.92 – j0.81
Data Book
Data Book
Z
Z
OL
OL
From Load Pull
From Load Pull
Optimum O/P
Optimum O/P
C
Impedance
3.39 + j0.75
Impedance
3.39 + j0.75
3.60 +j1.26
3.60 +j1.02
3.60 +j1.26
3.60 +j1.02
C
= 50 mA)
= 2 A)
1.145
1.62
1.61
1.71
1.12
1.12
K
K
0.336
0.270
0.197
0.941
0.871
0.693
B1
B1