LDMOS FETs Take On BJTs in Pulsed Applications
Through L-band
By Tiefeng Shi, Chia-Ling Hsu, David Burdeaux, and Pascal Gola, Freescale Semiconductor, RF Division

Silicon bipolar junction transistors (BJTs) technology has been a dominant RF power device in solid-state pulsed amplifiers up to L-band frequencies for many years. They can currently achieve at least 1-kW of peak RF power in push-pull operation. Of the technologies that are potential alternatives to silicon BJTs in this application – VDMOS, GaAs, SiC, LDMOS, and most recently GaN, only LDMOS has thus far proven competitive. That is, LDMOS can deliver the required RF output power, high efficiency, gain, and linearity, along with ruggedness over wide-ranging operational conditions. As a result, LDMOS FETs are making significant headway in replacing BJTs in solid state amplifiers operating in Class C mode at frequencies up to 1400 MHz. A comparison of the two technologies (Table 1) shows the strengths of each. Two of the 50-V LDMOS devices, for 450 and 1200 to 1400 MHz, designed and manufactured by Freescale show the performance LDMOS can achieve for pulsed applications.

Freescale has for several years been working on increasing the frequency range of the 50V technology, and has produced a family of devices for operation at frequencies from UHF through L-band. The first example is the MRF6VP14300H, which delivers 330 W from 1200 to 1400 MHz with a 300 µs pulse width signal and 12% duty cycle at 150 mA quiescent current bias. It was the first reported LDMOS device to deliver this performance at this frequency. Power density is 1.89 W/mm, drain efficiency is more than 59%, and gain is 14 dB in Class C operation. Rise time is less than 60 ns and pulse droop only 0.4 dB. Thermal resistance is 0.13ºC/W and junction temperature remains below 100º C at a flange temperature of 65º C. Power gain and efficiency versus output power at 1.4 GHz is shown in Figure 1.

The MRF6VP14300H employs internal matching for both the input and output side of the transistor to transform the die level impedance, which makes it easy for the designer to match the transistor input and output impedances to 50 ohms across the 1.2 to 1.4 GHz band. The internal input match consists of a two-section, lowpass T-network, and the internal output match consists of a single shunt inductor in series with a capacitor, which can improve output bandwidth.

The device can handle 3-dB input power overdrive and can withstand a 5:1 VSWR mismatch with full phase variation under a 3-dB overdrive condition. With the built-in enhanced ESD protection circuit, the device can operate in Class C with 14 dB of gain. The gate threshold voltage is typically around 1.6 V. Figure 2 shows the test results of Vgs sweep from 1.11 V to 2.23 V, and the operation class shifts from C to B to AB. Input power ranges up to 15 W, which is 3 dB higher than the typical maximum input power of 7.5 W. Power gain in Class C is 3 dB lower than for Class AB although power gain and output power nearly converge when the input power is greater than 12 W.

Figure 3 shows the gain and output power at Vgs of 0.84 V to 1.11 V and 12% duty cycle. Quiescent current is 3 mA. The device is operated in Class C at different Vgs and gain is close at higher input powers. However, the device provides isolation when input power is lower than 500 mW at Vgs of 0.84 V, which is very important for the stability of the host system. The pulse waveform of a pulsed transmitter is its core “ingredient’, so slow transistor rise time and power droop within the pulse will produce a commensurate droop at the transmitter output, with a serious negative effect on system performance. This is important as well because many transistors are combined to produce the final output. Rise time of the MRF6VP14300H is less than 60 ns and pulse droop less than 0.4 dB from 1.2 to 1.4 GHz.

1 kW Performance at 450 MHz
The MRF6VP41KH is another example of Freescale’s 50-V capabilities, this time at 450 MHz. It can deliver 1 kW peak RF output with a 300-us pulse width (20% duty cycle) input signal. Efficiency is greater than 60% and gain is 17 dB at 450 MHz in Class C operation. It will handle a VSWR of 5:1 with 300 and 500 µs pulses at a 20% duty cycle. In fact, this device can be used in CW applications, where it can deliver 1kW at 352MHz.

The MRF6VP41KH is designed in a push-pull configuration. There is no internal-matching capacitor at the input and output (device output capacitance is less than 150 pF), so that an external matching network can be used to achieve high performance over a wide bandwidth. While LDMOS FETs have been very successful in wireless infrastructure applications in Class AB mode (where high linearity is critical), their performance in class C is also quite remarkable.

An on-chip circuit that provides considerable protection from electrostatic discharge (ESD) makes the device less susceptible to stray voltage during design and production. It provides the additional benefit of accepting a wide range of gate voltages from -6 to +10 VDC, which improves its performance when operates in Class C mode. The device has been built to accommodate both flanged (Figure 4) and earless packages to suite specific applications.

A comparison of the measured performance of the LDMOS device in Class AB, Class B, and Class C operation is shown in Figure 5. Output power and power gain versus input power are shown for a 300-µs pulse width and 20% duty cycle at 450 MHz under different Vgs conditions, effectively shifting operation from Class C to Class B. The device shows good linearity before the P1dB point and power gain remains near constant over a 3-dB range of input power (10 to 20 W). Power gain converges after input power increases to more than 22 W. Figure 6 illustrates efficiency at different Vgs bias conditions and is greater than 60% with input power greater than 18 W at 450 MHz. The droop performance at 450 MHz of the MRF6VP41KH is less than 0.3 dB at room temperature when delivering 1 kW of RF power. Rise time is less than 500 ns and fall time is less than 100 ns.

Summary
The silicon BJTs has earned its solid position in lower-frequency pulsed power amplifiers (and other applications as well) through its ability to deliver high power with very good performance in nearly all the parameters that matter most. However, LDMOS devices are encroaching on the territory that is currently the BJT’s near-exclusive domain, and will increase their penetration of this market because of their superior long-term “roadmap” to greater performance and higher frequencies as well as competitive specifications in nearly every area. The two devices described in this article are currently in production at Freescale, and more information is available at our website.

FREESCALE SEMICONDUCTOR
www.freescale.com/rfpower
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