Application Note
AN4003
Closed Loop Control Maximizes
MAPM-030400-010C00 and MAPM-030400-060C00 Module Performance
Rev. 1
I. Introduction
The MAPM-030400-010C00 and MAPM-030400-060C00 RF amplifier modules work together to form the basic
building block of a 30 MHz to 400 MHz amplifier that can supply over 60 W of output power. With the addition
of a few components, gain flatness over frequency and temperature can be maximized over its entire frequency
range to create a compact amplifier suitable for digital, AM and FM applications.
II. Description of the Amplifiers and Open Loop Performance
The MAPM-030400-010C00 Pre Amplifier is designed for a fixed 13 dBm input power. The ALC pin is then
used to adjust the RF power level from 0 W to over 10 W with a 0 V to 10 V control voltage. The module itself
consists of two stages of RF amplification. The first stage is push-pull cascode that can supply about 2 W. A
gain-equalization network is included between the stages. The final stage of the preamp is a conventional
Class AB silicon bipolar RF power amplifier with transformer-based matching. ALC is accomplished by setting
the current through the cascode chain as well as the bias current in the Class AB stage. This module is con-
tained in a compact aluminum housing.
The MAPM-030400-060C00 Power Amp consists of a single stage DMOS transistor with broadband coaxial
transformer-based matching. It is in a copper housing for maximum heat transfer.
When both modules are cascaded with a 1.75 dB pad between them, the power transfer curves look like Figure
1. The RF output power at a fixed 4.0 V ALC voltage ranges from 10 W to 94 W – a variation of 9.7 dB. The
saturated RF output power ranges from 73 W to over 100 W.
Pout Vs ALC by Frequency
120
60 MHz
Pre-amp driving power amp
1.75 dB isolation between Amps
Vcc = 27 V
Pin = 13 dBm
90 MHz
250 MHz
225 MHz 175 MHz
100
400 MHz
80
30 MHz
300 MHz
Pout (W)
350MHz
60
400 MHz
350 MHz
300 MHz
250 MHz
175 MHz
125 MHz
90 MHz
60 MHz
30 MHz
40
20
0
0
2
4
6
ALC (Volts)
8
10
12
Figure 1.
Varying ALC voltages results in gain variation. Closed-loop control can improve gain
flatness over frequency and temperature.
1
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changes to the product(s) or information contained herein without notice.
Application Note
AN4003
Closed Loop Control Maximizes
MAPM-030400-010C00 and MAPM-030400-060C00 Module Performance
Rev. 1
III. Control Loop Circuit Description
The schematic of the control loop circuit is shown in Figure 2. The Pre Amp (AMP6) and Power Amp (AMP7)
are connected together with an optional pad between them. Both are connected directly to the 27 V DC supply.
The RF output of the Power Amp is connected to the “In” port of the directional coupler. The RF Output is at the
“Out” port of this coupler. The FWD coupled port is connected through a 6 dB pad to the input of the detector.
The output of the detector is connected to op amp AMP3 which is configured as a non-inverting buffer. The out-
put of the buffer is connected to the control amp AMP2.
The Audio and Power Set inputs are connected to AMP4 which are configured as an inverting summer. Since
the input of the op amp is a virtual ground, the inputs are well isolated, eliminating the need for a coupling ca-
pacitor in the Audio input. Because this is an inverting summer, AMP5 is configured as a unity gain inverter to
create a positive reference voltage at AMP2. (Because of this double inversion, a negative supply is required
for the op amps.) AMP2 compares reference voltage to the detected voltage and sets the Pre Amp ALC voltage
to make these voltages equal. R7 and C1 are required to prevent control loop oscillations. Zener diode D1 is
used to clamp the ALC voltage at 10 V and -0.7 V, to ensure safe limits for the ALC input of the Pre Amp.
Figure 2.
A practical control loop for the MAPM-modules using readily-available op-amps.
2
Visit www.macomtech.com for additional data sheets and product information.
•
North America
Tel: 800.366.2266 •
Europe
Tel: +353.21.244.6400
•
India
Tel: +91.80.43537383
•
China
Tel: +86.21.2407.1588
M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make
changes to the product(s) or information contained herein without notice.
Application Note
AN4003
Closed Loop Control Maximizes
MAPM-030400-010C00 and MAPM-030400-060C00 Module Performance
Rev. 1
IV. Directional Coupler and Detector: Critical for Proper Loop Operation
The directional coupler and detector are the most critical parts of the control loop system – the feedback loop
can generally take care of any other variations in the components. However, any variations in the coupling fac-
tor or distortion in the detected output will directly affect the RF output signal level.
The RFTronics RFCP-20512-40-S coupler chosen for this project has a very flat frequency response from 30
MHz to 400 MHz. The coupling is a nominal 40 dB, with better than +/- 0.5 dB variation across the band. The
coupler is rated to handle 150 W, well below our anticipated 100 W maximum. This coupler is a dual directional
coupler, and the reflected power port can be used for VSWR protection as discussed in Section VII.
The RF detector must have a response that is directly proportional to the RF waveform. Furthermore, for distor-
tion-free AM, the detector must have a linear response. A linearly responding detector, such as the Analog De-
vices AD8361 chosen for this project, works quite well, even though its dynamic range is 30 dB. Residual distor-
tion is about 2%, and much of that may have been from the function generator used in the setup. In this circuit, a
6dB pad inserted at the input avoids overdriving the detector.
If desired, a standard diode detector can be used. As an experiment, the AD8361 was replaced with a Hewlett-
Packard 8473C detector. The detector buffer op amp (AMP3) was configured as an inverting amplifier with a
gain of 50 dB and an input impedance of 50 ohms. While not extensively characterized, this detector provided
adequate performance. A diode detector must have enough input power to drive the diode into the “linear” re-
gion, instead of the “square-law” region of operation. In our experiment, a 30 dB coupler adjusts the power to
drive the detector into linear operation.
V. Closed Loop Power Control
A photograph of the system setup is shown in Figure 3. Power transfer curves similar to Figure 1 were gener-
ated. The results are shown in Figures 4 and 5. Figure 4 shows most of the frequencies overlay each other.
The shape of the curve is due to the RF Voltage of the output following the Power Set Voltage. Figure 5 reveals
more information. This figure shows the RF power (in dBm) vs. frequency at various Power Set voltages. The
worst case flatness is 0.65 dB at a Power set voltage of 2.0 V.
Figure 3.
The completed unit using the 10W and 60W modules on their heat spreader test fixtures. The ALC
loop control circuit, constructed on prototype board, is on the left.
3
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•
North America
Tel: 800.366.2266 •
Europe
Tel: +353.21.244.6400
•
India
Tel: +91.80.43537383
•
China
Tel: +86.21.2407.1588
M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make
changes to the product(s) or information contained herein without notice.
Application Note
AN4003
Closed Loop Control Maximizes
MAPM-030400-010C00 and MAPM-030400-060C00 Module Performance
V. Closed Loop Power Control — Cont.
Rev. 1
400 MHz
350 MHz
300 MHz
250 MHz
175 MHz
125 MHz
90 MHz
60 MHz
30 MHz
Figure 4.
Initial
testing of the ampli-
fier system. The
frequencies and the
shape of the curve
is due to the RF
output voltage
following the Power
Set voltage.
Figure 5, below.
RF power input vs.
frequency at
various Power Set
voltages. The
worst case flatness
is 0.65 dB at a
Power Set voltage
of 2.0 V.
Flatness = 0.55 dB; Average = 48.25 dBm
Flatness = 0.65 dB; Average = 46.78 dBm
Flatness = 0.43 dB; Average = 43.72 dBm
Flatness = 0.28 dB; Average = 38.66 dBm
Flatness = 0.44 dB; Average = 32.53 dBm
Flatness = 0.54 dB; Average = 26.92 dBm
4
Visit www.macomtech.com for additional data sheets and product information.
•
North America
Tel: 800.366.2266 •
Europe
Tel: +353.21.244.6400
•
India
Tel: +91.80.43537383
•
China
Tel: +86.21.2407.1588
M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make
changes to the product(s) or information contained herein without notice.
Application Note
AN4003
Closed Loop Control Maximizes
MAPM-030400-010C00 and MAPM-030400-060C00 Module Performance
Rev. 1
VI. Closed Loop Amplitude Modulation
Initial AM modulation tests at 20 W of carrier power at 400 MHz showed very good performance at 95% down
modulation, see Figure 6. During initial testing at 350MHz, excessive distortion and clipping was observed. The
ALC loop tried to increase power, but the amplifier is saturated.
The test was repeated without the 1.75 dB pad between the Pre Amp and Power Amp, and a better function gen-
erator was used to reduce residual distortion in the sine wave audio input signal. The results are summarized in
Table I. While there was some improvement, clipping still occurred during the RF peaks at 350 MHz and 60 MHz.
However, these results are very good for a broadband amplifier at 95% down modulation. The distortion at this
modulation level ranged from 1.7% to 5.8%. At 350 MHz, the distortion improved to 3.9% when the modulation
was reduced to 90%. At 60 MHz, the distortion improved to 2.7% at 90% down modulation. While not a design
requirement for this amplifier, the AM frequency response was very good. The corner response frequency ex-
tended past 100 kHz at 90% modulation.
VII. Efficiency Improvement and Other Future Work
Table I shows that the overall DC to RF conversion efficiency of the amplifier is fairly poor at 27 V. It was no better
than 31% across the band, with an average of 27 %. The last three columns of Table I show one method of im-
proving the overall efficiency at the cost of some distortion. By lowering the DC supply voltage until the distortion
reached 4%, the efficiency improved across the board at every frequency and the average was improved to 34 %.
This method works by taking advantage of the control loop. Reducing the DC supply voltage lowers the saturated
power. The ALC voltage increases as necessary to compensate for any difference in gain and overcome any dis-
tortion due to the compression. By operating the modules in saturation, efficiency is improved.
A practical circuit to use this technique will continually lower the supply voltage during operation. If the efficiency
improving circuit senses that the ALC voltage has hit 10 V, it will quickly increase the supply voltage increasing the
saturated power and decreasing the ALC voltage required. Once the ALC voltage is under 10 V, the supply volt-
age can be lowered again, repeating the cycle indefinitely.
Figure 6.
Good AM performance
at 400 MHz with 20W of carrier
power. Modulation is 95%.
5
Visit www.macomtech.com for additional data sheets and product information.
•
North America
Tel: 800.366.2266 •
Europe
Tel: +353.21.244.6400
•
India
Tel: +91.80.43537383
•
China
Tel: +86.21.2407.1588
M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make
changes to the product(s) or information contained herein without notice.
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