LDMOS Transistors for FM Broadband Applications

STMicroelectronics Inc.
Montgomeryville, PA

High efficiency and high gain amplifiers for FM transmitters are possible today with laterally diffused metal oxide semiconductor (LDMOS) technology. Presently, LDMOS RF power transistors are the proven mainstay in the high volume cellular base station power amplifier business, which demands low cost RF power transistor solutions. These LDMOS devices are unmatched from DC to 1 GHz and can be used in a variety of applications, making them a good candidate for high performance, low cost FM drivers.

Fig. 1 The broadband 4:1 transformer's input return loss Two of the key performance attributes of LDMOS devices are improved thermal resistance and reduced source inductance to the outside world. The source at the chip surface is connected to the substrate by diffusion of a highly doped p-type region. As a result, insulating material such as BeO is not necessary, which improves the junction-to-case thermal resistance. In addition to lowering the thermal resistance, the elimination of the BeO also reduces the device’s cost. LDMOS transistors have excellent high frequency response because of their high ft as well as superior gain due to the low feedback capacitance and reduced source inductance. The bipolar parasitic has been nullified due to the company’s unique design of LDMOS die, guaranteeing good ruggedness, efficiency and high current handling capability. This article describes how LDMOS products behave in the FM broadcast frequency range. The device used for this characterization is the model SD57045 28 V, 45 W LDMOS transistor.

Table 1 The SD57045 Transistor's Input and Output Impedances
Frequency (MHz)	Z_input (W)	Z_output (W)
88	10.8-j7.6	7.5-j0.15
95	10.6-j8.36	7.8-j0.34
108	10.5-j9.87	8.1-j0.61

Fig. 2 The FM broadband power amplifier Circuit Design
The input and output impedances of the SD57045 device are listed in Table 1 . With respect to these impedances, two 4:1 transmission line auto transformers were designed using 1/8-wave, 25 W semirigid coax. To achieve this transformation across the band a capacitor is added at the low impedance port of each transformer to cancel the leakage inductance. Figure 1 Fig. 3 The power amplifier's layout shows the frequency response. Simple L sections are utilized to make the final transformation from the low impedance port of the transformers (12.5 W) to the measured impedances of the device. This design uses printed series inductors on 30-mil glass Teflon circuit board.

The gain of any power FET is extremely high from DC through the low HF band. The use of a feedback network is necessary to suppress the low frequency gain and provide a nominal amount of gain at the frequency of interest. The feedback also helps to increase the input impedance. Fig. 4 Drain current vs. gate voltage Because LDMOS devices have such high gain at low frequencies, a low value, high power, flange-mount resistor must be included in the design. The capacitor in the feedback path (C3) provides negative feedback at low frequencies. This component was designed to be self-resonant far below the FM band. At 100 MHz, the capacitor appears slightly inductive, which helps to reduce the amount of feedback in the band of interest.

Fig. 5 Gate-source voltage vs. case temperature Unbalanced transformers offer an efficient method of transforming 50 W to low impedances. In addition to the RF advantages, auto transformers have a zero impedance point over a broad bandwidth, which offers an ideal point to feed DC to the gate and drain circuits. It is important to use a bypass capacitor precisely at the zero impedance point of the transformer to prevent high frequency oscillations. The value of this capacitor must be selected so that the self-resonant frequency is above the frequency of interest. Depending on the application, additional low frequency bypass capacitors isolated with lossy elements (ferrite beads) may be required to keep power supply noise out of the gate and drain circuits. The resulting circuit schematic and layout are shown in Figures 2 and 3 , respectively, where L3, L4 and L7 are inductive sections, L2 and L6 are 4:1 transformers (1.7", 25 W) and the board material is 30-mil, 2-oz copper with an e_r = 2.55.

Fig. 6 The class A safe operating region Characterization Results
The SD57045 LDMOS device used for this application has absolute maximum ratings at a 25°C case temperature of 65 V for the drain-to-source and drain-to-gate voltages, ±20 V for the gate-to-source voltage and 5 A for the drain current. Maximum power dissipation at 70°C case temperature is 93 W and maximum operating junction temperature is 200°C. Junction-to-case thermal resistance is Fig. 7 Power output and efficiency vs. input power 1.4°C/W. Figure 4 shows the SD57045 device’s drain current vs. gate voltage characteristics, Figure 5 shows the gate-to-source voltage vs. case temperature and Figure 6 shows the transistor’s class A safe operating region. The power output and efficiency vs. input power is shown in Figure 7 , and power gain and efficiency vs. power output is shown in Figure 8 . These data were obtained at 95 MHz and indicate that the device is capable of 45 W power output at a power gain of 22.5 dB and an efficiency of 63 percent.

Fig. 8 Power gain and efficiency vs. power output Conclusion
This article demonstrates that it is possible to utilize a low cost, 900 MHz cellular device as a commercial FM driver. LDMOS technology is a viable solution for frequencies covering the high HF through high UHF bands. Additional information can be found at the company’s Web site at www.st.com.