AN1526 Freescale Semiconductor / Motorola, AN1526 Datasheet - Page 3

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

devices is a much stronger function of load impedance than
shown for this small device.
transistors should only be used as an approximation for a
first cut circuit design. In a broadband amplifier design it is
often difficult to obtain a good match over the full frequency
range and in certain circumstances the input or output is
deliberately
increase at lower frequencies to provide a level gain
response. Good design would opt for a load–line where the
lower gain corresponds to a higher efficiency operating point.
MRF873 DEVICE IMPEDANCE COMPARISON FOR
DIFFERENT MODES OF OPERATION
difference in the way a high power RF device is specified.
Many devices are specified and characterized for class–C
or –AB operation. Common questions when using a device
differently from the way it is characterized in its data sheet
are: “What will the gain be?”, “What are the impedances?”
In general, power gain is highest when the device is operated
in class–A and slightly lower when the device is operated
in class–AB. Power gain is lowest when operating in class–C
decreasing as more reverse bias is applied to the base and
the transistor conduction angle decreases. The designer
should beware, a transistor designed to be rugged, i.e.,
capable of withstanding a specified output mismatch under
class–C conditions, will be LESS RUGGED with forward bias
applied to the base. Device impedances depend not only
on the internal structure of the device, but also how that
device is operated. For small–signal operation with the
device biased in class–A, the optimum device input and
output impedances, for a stable device, are the simultaneous
conjugately matched impedances which can be derived from
the s–parameters.
a function of the output power, the collector bias voltage and
the output reactance of the transistor. The required peak
MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION
Device impedances published by vendors of RF power
Device characterization techniques are not the only
For power operation the optimum output impedance is
Frequency
Frequency
(MHz)
806
838
870
mismatched to compensate for the gain
Table 2. Small–Signal S–Parameter Data for the MRF873 at V
Table 1. Comparison of Input Impedance for Different Operating Modes
50 mA
50 mA
50 mA
Frequency
2 A
2 A
2 A
I
C
(MHz)
806
838
870
Freescale Semiconductor, Inc.
For More Information On This Product,
0.963
0.877
0.961
0.858
0.958
0.861
S
11
S
Conjugate Match
11
Simultaneous
(I
0.478 – j3.19
0.503 – j3.41
0.568 – j3.48
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CQ
172.8
170.9
172.4
172.7
172.3
175.3
= 50 mA)
0.006
0.024
0.005
0.022
0.004
0.018
S
12
S
12
output power and the collector bias voltage determine the
operating load line. The output reactance of the device under
these conditions is conjugately matched to achieve
maximum power transfer, although this condition may be
modified, at the expense of gain, to attain higher efficiency.
where V
and V
under the frequency of operation. The value of this
parameter is particularly difficult to measure, but the normal
range is 1.0 to 2.5 Volts depending on the geometry, epitaxial
doping and thickness. A good approximate value for 12.5
Volt devices is 1.5 Volts and for 24 Volt transistors is 2 Volts.
for the internal collector node of the transistor, neglecting
the junction and parasitic device capacitance. These are in
parallel with the load line resistance. For transistors,
operating at VHF, and above the internal collector lead
inductance of the package becomes significant, and is in
series with the previously defined parallel R
For the CS–12 package the internal collector lead series
inductance can be represented by a 0.65 nH lumped
inductor. Some devices have internal collector matching,
transforming the internal load line impedance to higher value
for ease of broadband matching.
methods, assuming a simultaneous conjugate match, and
large–signal measurements, shows dramatic shifts in input
impedance (see Table 1 above). More subtle, but
measurable differences, can be seen in the change in input
impedance between class–C and class–AB data. The
small–signal s–parameter data is given in Table 2 below for
a collector bias current of both 50 mA and 2 A.
(I
1.33 + j3.34
1.43 + j3.41
1.50 + j3.32
CQ
Class–AB
The load line resistance is given approximately by:
The load line resistance is the optimum load impedance
Comparison of the impedance data taken by small–signal
7.72
27.9
4.12
17.5
8.35
6.31
= 50 mA)
CE(sat)RF
P
CC
out
is the required peak power,
is the collector supply voltage,
0.437
1.567
0.436
1.639
0.435
1.592
S
R
21
L
is the collector–emitter saturation voltage
=
S
V
21
CC
CC
1.10 + j3.26
1.19 + j3.24
1.24 + j3.34
– 4.95
– 3.30
– 17.7
– 21.7
– 11.0
2 * P
12.7
– V
Class–C
= 12.5 Vdc
CE(sat)RF
out
0.910
0.697
0.928
0.723
0.948
0.789
)
S
2
22
S
22
– 163.5
– 174.5
– 164.5
– 169.9
– 165.4
– 166.8
L
, C
obo
network.
3
(1)