Upcoming generations of cellular radio systems such as GSM EDGE or UMTS require increasing power levels to meet linearity requirements of transmitted signals. To reduce overall costs, equipment manufacturers aim at reducing the number of transistors in their RF power amplifiers. The consequence is a constant demand for bigger transistors, reaching today the range of 100 to 200 W for single-ended devices.

As power increases, the input and output impedances of RF power transistors decrease, making more difficult the matching to 50 . In the same time, amplifier manufacturers request "easy to use" devices and no tuning in volume production. This implies efficient input and output internal prematch to enable higher terminal impedances, and easy to manufacture matching circuits on printed circuit boards. This article describes the MRF9100, 100 W LDMOS transistor for 900 MHz cellular applications, designed for ease of use.

Ease of Use

The matching networks from the die level to the 50 load can be divided in two parts. The first part or prematch is made internally inside the device. The second part is made outside the device by the application circuit.

From the RF transistor user's point of view, the transformation ratio between the device and the 50 load is a key parameter. The sensitivity and repeatability of the matching circuit are a direct function of this ratio.

On the component side, the characteristics of the technology are taken into account. At 1 GHz, power LDMOS transistors require an internal input prematch, the die input impedance being too low. For higher power, input and also output prematch are necessary to take all benefits from the technology.

In practice, the limitation at 1 GHz is the room available inside the package to design both the input and the output prematch. The picture is different at 2 GHz, where all power devices are input and output prematched.

The other criteria regarding impedances are their variation with frequency. From the user point of view, a low spread with frequency means a flat response in frequency over a broad band, achievable with a simple circuit. A large impedance spread will end up with a narrowband amplifier, even with a low transformation ratio.

On the component side, a small frequency spread implies a control of the quality factor Q (commonly limited to one or two) of the prematch cell. There is a trade-off between the impedance level increase and the associated frequency spreading.

The input and output of the LDMOS die being capacitive, an inductive shunt as a prematch cell offers the best results. However, for stability purpose, the series inductance - shunt capacitance prematch cell is usually used on the gate side.

A high impedance level and a limited spread in frequency are the criteria for the user of the transistor. On the component side, the ease of use translates in an input and an output prematch made by an inductive shunt. The outcome is a simple application circuit, with low sensitivity and offering a flat broadband frequency response.

Design of the MRF9100 - 100 W GSM900

The MRF9100 device is composed of two LDMOS blocks of 50 W each, as shown in Figure 1 . The NI-780 ceramic package also includes the output prematch, the input prematch being integrated.

Motorola LDMOS technology enables the integration of passive elements, thus, the input prematch is designed on chip. Compared to a standard MOS chip capacitor and wire prematch circuit, the saving of room provided by the on die prematch circuit is obvious and allows the design of the output prematch in this kind of package.

The input impedance at the 50 W block level is transformed to 4 via two cells of prematch. The first cell is an inductive shunt, the second being serial inductance - shunt capacitance.1 The stability of the die has been optimized, enabling the use of an inductive shunt for the first prematch cell.

The output prematch is an inductive shunt, made with bonding wires and a decoupling capacitor. The number of wires has been optimized to withstand a 10:1 all phases 100 W CW ruggedness test. The device has been measured in operation with an infra-red microscope in order to validate the design of the output match (see Figure 2 ). Note that this inductive shunt type of prematch circuit requires an inductance value roughly four times larger at 1 GHz than at 2 GHz to compensate for the capacitive nature of the transistor on the drain side.

MRF9100 Performance

The performance under 26 V of the MRF9100 device is benchmarked to the MRF9080, an 80 W LDMOS transistor using the same package and designed for the same 900 MHz applications.2 The input prematch of the MRF9080 is conventional, serial inductance - shunt capacitance, wires and chip technology. There is no output prematch.

The device impedances have been measured with a TRL technique3 and are listed in Table 1 . At the input for the MRF9100, the impedance is almost real, near 2 . The spread with frequency is low. In opposition, the input impedance of the MRF9080 varies significantly in both real and imaginary part. At the output, the impedance of the MRF9080 is stable with frequency near 1.2 . The presence of the output prematch for the MRF9100 translates into an increase of the real part in the range of 1.5 , the same impedance level as a 60 W transistor. In addition, the decreasing imaginary part with frequency will compensate for the frequency spreading due to the low pass matching networks of the application circuit.

The input impedance is broadband, covering the 860 to 960 MHz band, and the small-signal gain is flat, as can be seen in Figure 3 . The performance's flatness remains the same in power, as shown in Figure 4 . The device is capable of covering TDMA/EDGE/GSM applications in the 860 to 960 MHz frequency band with a single circuit. Therefore, the power amplifier designer can choose to design a wide band circuit or to optimize the amplifier for a given application.

The comparison with the MRF9080 (80 W, GSM900) puts in evidence the bandwidth magnification offered by the MRF9100. The integrated prematch triples the input return loss bandwidth at -10 dB level, while the broadband P1dB and efficiency performances benefit from the output prematch.

Application Circuit

Input and output matching circuits are simple. The small reactive part allows the first shunt capacitors to be pushed away from the lead reference plane of the transistor, as shown in Figure 5 . The mounting of the transistor in the user's application will be facilitated.

The matching networks are low pass, with transformation ratio of 25, making them easy to manage on a printed circuit board. The sensitivity of the circuit is roughly twice as low as the MRF9080 one. The user's amplifier can be manufactured in volume without any tuning. Also, higher terminal impedances allow for more compact matching on printed circuit boards and greater assembly tolerance at the device interface.

Conclusion

The ease of use parameters of a high power RF transistor have been defined. The integrated prematch innovative technology used for the MRF9100 design has led to meet these criteria of impedance quality of the device. The component's ease of use has been demonstrated through the broadband frequency response and the performance flatness achieved in a simple application circuit.

This device is a first step into a new type of power transistor where performance is no longer the only criteria of choice for the user. The ease of use enables a cost reduction for implementation of the component in the user's application. A full line of broadband RF power MOSFET devices utilizing integrated matching is expected to be introduced throughout 2003.

References

1. B. Becciolini: "Impedance Matching Networks Applied to RF Power Transistors," Motorola Application Note AN721D, 1993.
2. Motorola SPS, "MRF9080 Datasheet."
3. J.J. Bouny: "Impedance Measurements for High Power RF Transistors Using the T.R.L. Method," Microwave Journal , Vol. 42, No. 12, October 1999.

Motorola Semiconducteurs S.A.S., Toulouse, France.Circle No. 301