Broadband, high-efficiency power amplifiers serve a wide range of military applications in present and future communications and navigation systems. But amplifier designers are limited in performance by the active devices available to them. Fortunately, the growing availability of wide-bandgap semiconductor devices, such as silicon carbide (SiC) and gallium nitride (GaN) transistors, is clearing the way for broadband, high-efficiency power amplifiers (PAs) capable of serving multiple electronic systems. Several distributed power-amplifier prototypes at power levels from 10 to 40 W have been developed recently at Rockwell Collins based on GaN pseudomorphic-high-electronmobility- transistor (pHEMT) devices. What follows are the results of designing and testing a 10-W distributed GaN power amplifier with maximum power output of more than +40 dBm and a poweradded efficiency of 30 to 70 percent over the bandwidth of 20 to 2000 MHz.

Both military and commercial high-frequency electronics designers have recognized the potential of wide-bandgap device technologies for broadband, high-power amplifiers. In particular, GaN HEMT devices offer capabilities that should fit well in future high-performance amplifiers from UHF through Q-band frequencies. Many electronic warfare (EW) applications require high-power, high-efficiency amplifiers that cover frequency bands from VHF through L-band.

Most of the amplifiers used in these applications are currently narrowband designs, with multiple units required to cover the required bandwidth and number of waveforms. Development of a broadband power amplifier with a frequency span wide enough to serve more than one waveform could potentially reduce the cost and size of the system. In addition, the increasing use of software-definedradio (SDR) architectures in tactical radio systems has prompted a need for “common building blocks,” such as linear broadband amplifiers capable of handling multiple waveforms as created under software control.

Broadband power performance can be achieved with a negativefeedback approach or a distributed amplifier approach. Negative feedback amplifiers can achieve decade bandwidth performance, but with limited efficiency. Distributed RF power amplifiers feature broadband performance and low sensitivity to circuit and device tolerances, leading to their widespread use in wideband communications systems for many years. Distributed amplifiers are capable of greater than a decade of bandwidth. Such a broadband amplifier, with 9 dB gain and +/-1 dB gain flatness from 1 to 13 GHz was realized in monolithic form on GaAs substrate in the early 1980s.1 A 2-to-20-GHz GaAs distributed power amplifier with 30 dB gain has also been demonstrated.2 However, most early work on distributed power amplifiers was focused on achieving high gain versus frequency rather than high efficiency. Those early amplifiers were not designed for high power output or high efficiency.

In contrast, recently a novel approach using drain-line tapering methods achieved power-added efficiencies greater than 50 percent with a distributed amplifier architecture, while still preserving the low VSWR and broadband characteristics of distributed amplifiers. Also, distributed amplifiers capable of 1 to 6 W output power using discrete LDMOS or GaAs HEMT devices were realized using low-temperature-cofired-ceramic (LTCC) substrates.3,4 However, the maximum power output was limited by the operating voltage of the active devices. Fortunately, the recent development of GaN HEMT technology has provided a tremendous opportunity for RF designers to develop high-power, high-efficiency broadband power amplifiers.

In conventional amplifiers, the gainbandwidth product is proportional to the ratio of the FET transconductance, gm, and the input capacitance, Cin. Placing multiple FETs in parallel will not increase the gain-bandwidth product. A distributed amplifier overcomes this obstacle to wide bandwidth by adding the individual transconductance values of multiple FETs without adding their input and output capacitances, and defeats the limitations of finite gain-bandwidth product by absorbing the parasitic capacitances into the transmission lines. Figure 1 shows the basic topology of a conventional distributed power amplifier and its simplified equivalent circuit. In this circuit, microstrip lines are periodically loaded with the complex gate and drain impedances of the transistors, forming lossy transmission-line structures. In this simplified model, the forward and reverse gains are given by:

where

Gpf = the forward power gain;

Ggb= the reverse power gain;

gm = the transconductance of the active devices;

l = the electrical length of each unit;

Zod,og = the characteristic impedances of the artificial transmission lines;

ßg,d = the phase constants over each unit length; and

N = the number of cells that forms the transmission lines.

See associated figure

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