Communication Concepts AN762 Application Note - Manual

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1 RF Application Reports
LINEAR AMPLIFIERS FOR MOBILE OPERATION
Prepared by: Helge O. Granberg
RF Circuits Engineering
INTRODUCTION
The three versions of the amplifier described here are
intended mainly for amateur radio applications, but are
suitable for other applications such as marine radio with
slight modifications.
100 W is obtained with two MRF455’s. MRF460 or
MRF453 is also adaptable to this design, resulting in
approximately 1.0 to 2.5 dB higher overall power gain than
the values shown. The MRF454 devices which can be
directly substituted with MRF458’s for slightly lower IMD,
deliver the 140 W, and two MRF421 devices are used in the
180 W version.
The use of chip capacitors results in good repeatability,
making the overall design suitable for mass production.
There are several precautions and design hints to be
taken into consideration regarding transistor amplifiers:
1. Eliminate circuit oscillation. Oscillations may cause over-
dissipation of the device or exceed breakdown voltages.
2. Limit the power supply current to prevent excessive dis-
sipation.
3. Adopt protective circuitry, such as fast acting ALC.
4. Ensure proper attachment of the device to a heatsink using
Silicone grease (such as Corning 340 or GC Electronics
8101) to fill all thermal gaps.
THE TRANSISTORS
The MRF421 with a specified power output of 100 W PEP
or CW is the largest of the three RF devices. The maximum
dissipation limit is 290 Watts, which means that the
continuous collector current could go as high as 21.3 A at
13.6 V operated into any load. The data sheet specifies 20 A;
this is actually limited by the current carrying capability of
the internal bonding wires. The values given are valid at a
25
°C mount temperature.
The minimum recommended collector idling current in
Class AB is 150 mA. This can be exceeded at the expense
of collector efficiency, or the device can be operated in Class
A at an idling current of approximately one fourth the
maximum specified collector current. This rule of thumb
applies to most RF power transistors, although not specified
for Class A operation.
The MRF454 is specified for a power output of 80 W CW.
Although the data sheet does not give broadband
performance or IMD figures, typical distortion products are
[ – 31 to – 33 dB below one of the two test tones (7) with
a 13.6 V supply. This device has the highest figure of merit
(ratio of emitter periphery and base area), which correlates
with the highest power gain.
The maximum dissipation is 250 Watts, and the maximum
continuous collector current is 20 A. The minimum
recommended collector idling current is 100 mA, and like
the MRF421, can be operated in Class A.
The data sheet specification for the MRF455 is 65 W CW,
but it can be operated in SSB mode, and typically makes
– 32 to – 34 dB IMD) in reference to one of the two test tones
at 50 W PEP, 13.6 volts. It contains the same die as MRF453
and MRF460, but is tested for different parameters and
employs a smaller package. The MRF455/MRF453/MRF460
has a higher figure of merit than the two devices discussed
earlier. Due to this and the higher associated impedance
levels, the power gain exceeds that of the MRF454 and
MRF421 in a practical circuit. The minimum recommended
collector idling current is 40 mA for Class AB, but can be
increased up to 3.0 A for Class A operation.
It should be noted that the data sheet figures for power
gain and linearity are lowered when the device is used in
multi-octave broadband circuit. Normally the device input and
output impedances vary by at least a factor of three from
1.6 to 30 MHz. Therefore, when impedance correction
networks are employed, some of the power gain and linearity
must be sacrificed.
The input correction network can be designed with RC
or RLC combinations to give better than 1 dB gain flatness
across the band with low input VSWR. In a low–voltage
system, little can be done about the output without reducing
the maximum available voltage swing.
At power levels up to 180 Watts and 13.6 V, the peak
currents approach 30 A, and every 100 mV lost in the emitter
grounding or collector dc feed also have a significant effect
in the peak power capability. Thus, these factors must be
emphasized in RF power circuit design.
THE BASIC CIRCUIT
Figure 1 shows the basic circuit of the linear amplifier.
For different power levels and devices, the impedance ratios
of T1 and T3 will be different and the values of R1, R2, R3,
R4, R5, C1, C2, C3, C4 and C6 will have to be changed.
The Bias Voltage Source
The bias voltage source uses active components
(MC1723G and Q3) rather than the clamping diode system
as seen in some designs. The advantages are line voltage
regulation capability, low stand-by current, (
‰1.0 mA) and
wide range of voltage adjustability. With the component
values shown, the bias voltage is adjustable from 0.5 to
0.9 Volts, which is sufficient from Class B to Class A
operating conditions.
In Class B the bias voltage is equal to the transistor V
BE
,
and there is no collector idling current present (except small
collector-emitter leakage, I
CES
), and the conduction angle
is 180 °.
MOTOROLA
SEMICONDUCTOR
APPLICATION NOTE
Order this document
by AN762/D

Motorola, Inc. 1993
AN762