A 1kW S-Band Pulsed Power Amplifier Using a Pallet Solution
By Eric Hokenson, Carlos Guerrero, Keith Barkley, Tyco Electronics, M/A-COM Power Hybrid Products

Abstract
Pallets are 50 W power modules that can be easily paralleled and chained in high power amplifiers with minimum design effort. This paper describes a demonstration pulsed power S-Band amplifier assembled with five PHA2729-300M pallets and M/A-COM transistor drivers capable of supplying over 1000 Watts peak RF Power over the frequency range of 2.7-2.9 GHz. The paper discusses design aspects and amplifier performance.

PHA2729-300M Pallet
The M/A-COM PHA2729-300M pallet consists of two silicon bipolar discrete transistors attached to a circuit board as shown in Figure 1. The circuit board used for the PHA2729-300M pallet is Rogers RT/Duroid® LM6010.5 laminate which contains the following electrical properties: dielectric constant (er) = 10.5, board height (h) = 0.025", 1 oz./ft2 electrodeposited copper foil. In addition, the bottom of the circuit board is clad with 0.187" of copper to provide good electrical grounding, thermal heat transfer, and mechanical integrity. Two channels are machined on the bottom of the board to allow for mechanical stress relief. The heat dissipates directly below the transistors in the middle of the board, and is not affected by the stress relief channels.

The major advantage of using M/A-COM pallets is that the large signal input and output impedances of the transistors are matched over the operating frequency band. The circuit transforms the impedance of the transistors to a nominal 50 W interface at both the input and output of the pallet. Thus, the amplifier manufacturer can simply “drop-in” the pallets onto their circuit board without performing empirical circuit tuning needed to get good broadband impedance matching, and thus acceptable RF performance. In addition, since the pallet combines the power of two discrete transistors, using pallets instead of individual transistors often saves space inside the power amplifier.

Power Combining
In most cases, pallets are designed by combining the RF power of two discrete transistors using a Wilkinson power combiner.[1]-[2] A Wilkinson combiner provides in-phase, equal power combining, and is used as a power divider as well. On the PHA2729-300M pallet the input power is equally divided to provide the required input level to the discrete transistors. Likewise, the output power of both transistors is combined in the same manner. Figure 2 provides a schematic of a 2-way Wilkinson coupler.

Theoretically, the Wilkinson technique can be used for any number of ports. The branch line impedances are determined as follows:

Equation 1
Thus, for a 2-way Wilkinson coupler with 50 W input and output impedances, the branch line impedances are 70.7 W.

The Wilkinson coupler requires that the 70.7 W transmission lines are a quarter wavelength long at the center frequency. Since the distance between the two input ports is ?/2, the power arriving at each port is 180° out of phase with the other port, and therefore cancels. This provides the isolation between the ports. Also, a 100 W balance resistor is applied across the output portions of the schematic. The balance resistor is used to absorb any imbalance of power between the input ports of the Wilkinson combiner.

1KW Amplifier Design
The 1kW amplifier was designed using five PHA2729-300M pallets arranged in a one-driving-four configuration. One pallet is used as a driver, and the other four are combined in parallel to provide over 1kW of peak RF output power. The driver stages are assembled using several M/A-COM bipolar power transistors. A diagram of the entire 1kW amplifier chain is illustrated in Figure 3.

The RF signal produced by the signal generator is a pulsed waveform of 100µs pulse width, 10% duty cycle. This RF waveform represents a typical pulse format used to test devices for S-Band radar applications.

The power transistors used in the amplifier chain are built in the common base configuration, and operated in the Class C mode. Typically, when operated under pulsed RF conditions, the devices are operated common base and Class C to attain higher stability and collector efficiency respectively. In addition, biasing the transistors in this manner simplifies the circuit design by only requiring a single voltage source for each unit. All power transistors utilized in the amplifier chain will be biased with a Collector Voltage supply of 36v.

The Wilkinson power combining technique is used to combine the power of the pallets. Likewise, the Wilkinson technique is also used for dividing the power equally to the input of the four pallets. In practical amplifier designs, other methods of power combining such as the hybrid model may be employed. The Wilkinson technique was used primarily due to its simple design and relatively low cost. Two sets of 2-way Wilkinson combiners rather than a single 4-way combiner are used due to the high impedance branch lines that would be required for a 4-way coupler. Lower impedance lines simplify etching circuit board transmission lines, and also have less transmission losses compared to higher impedance lines.

The 2-way Wilkinson splitter and combiner were made by etching the 50 and 70.7 W lines onto a circuit board, and using a 100 W chip resistor for the balance resistor. Figure 4 shows the final two stages of the 1kW power amplifier. As indicated earlier, the output power from one PHA2729-300M pallet is used to drive the four PHA2729-300M pallets in the final stage.

The board material used for the Wilkinson circuits is Rogers RT/Duroid 5870. The 5870 material contains the following electrical properties: er = 2.35, h = 0.031" [0.787mm], 1 oz./ft2 copper. You will note that the circuit board used for the Wilkinson divider and combiner circuit differs from the material used on the pallet. However, this particular circuit board was chosen for its low dielectric constant and increased board height. Figure 5 illustrates the difference in line widths for the different circuit board material and transmission line impedances. Thus, the 5870 board was chosen to allow for thicker circuit lines, and therefore, less variability with respect to etching tolerance.

Measurement Results
Several PHA2729-300M pallets were built and evaluated. The RF test data for the five pallets used for the 1kW amplifier board are presented in Figure 6. The pallet number defined in Figure 6 refers to its position in the 1kW amplifier board, as shown in Figure 4.

The 1kW pallet amplifier board shown in Figure 4 was evaluated under the same RF test conditions used to measure the individual pallets. A bi-directional coupler placed at the input of the driver pallet (pallet #1) through which the input and reflected power measurements were obtained with an HP437B digital power meter. The entire unit was operated into several high power attenuator loads and the output power was measured with an HP437B power meter. The supply current was measured with three different multi-meters to observe the load sharing of the units under test. Meter #1 was used to measure the DC current of the driver unit, and meters #2 and #3 measured the current drawn by the two units at the top and bottom of the board respectively. The total current of the amplifier board was then determined by adding up the current values measured from each meter.

Some empirical tuning was required on the Wilkinson power divider and combiners to provide adequate power sharing between the pallets, and thus improve the overall gain and efficiency of the board. Of course, thermal and electrical grounding is very important, and some improvements and modifications were required to improve the system performance. However, no tuning on the pallets themselves was required.

Figures 7 and 8 demonstrate the performance of the 1kW pallet amplifier board over the entire operating frequency range. The data presented in these figures represent the gain and power added efficiency (PAE) of the amplifier board when operated at 1000 watts of peak RF power. Figure 9 illustrates the gain of the amplifier board with respect to the output power.

Conclusion
The design of a high power S-Band solid-state amplifier using M/A-COM pallet technology has been demonstrated. The pallet offers some distinct advantages over chaining multiple discrete transistors as it eliminates the need for empirical impedance matching. The pallets were combined rather easily using a very a simple Wilkinson technique for power sharing. The amplifier was evaluated under conditions commonly used for S-Band radar applications, and performed well.

References
[1] Ostroff, E., Borkowski, M., Thomas, H., Curtis, J., Solid-State Radar Transmitters, Artech House, Inc., Norwood, MA, 1985.
[2] Dye, N., Granberg, H., Radio Frequency Transistors: Principles and Practical Applications, Butterworth -Heinemann, Newton, MA, 1993.
[3] Stratakos, Yorgos, Betzios, Panayotis, S-Band High Power Solid-State Amplifier Design and Development, Microwave Journal, Vol. 47, No. 5, May 2004.

Tyco Electronics
www.tycoelectronics.com
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