Bespoke MMICs aid radar, phased array and oscilloscope applications

Traveling-wave amplifiers - also known as distributed amplifiers - are not new products. Their roots can be traced back to 1936 when they were built using valve technology. Today they are available as standard products featuring amplification up to 40 GHz that are being used in broadband communication networks and pieces of electrical measurement equipment.

However, these off-the-shelf products are not ideal for radar, phased array, or oscilloscope applications because in these cases the performance specifications need to be specifically tailored. Targeting these needs, here at the Institute of Electronics, Communications and Information Technology (ECIT), at Queen s University, Belfast, we are designing and building bespoke traveling-wave GaAs amplifiers with very low gain variations with frequency and very low phase distortion. Our designs include amplifiers with a small variation in gain over their frequency range, which are used for testing the electromagnetic compatibility of various RF components, and for addressing the problems associated with signal reflection from antennas. We also produce amplifiers with low phase distortion that are suitable for applications that do not need a wide frequency response but do require a faithful reproduction of the pulse s shape. This feature makes these amplifiers ideal for use in radar, phased arrays, oscilloscopes, and also in optical receivers, where they can be used to amplify the photodiode s signal without distortion.

Traveling-wave amplifier MMICs operate by absorbing parasitic shunt capacitances - which come from the insulator that separates the gate and the conducting channel - into artificial transmission lines. This removes the active device s main cause of frequency dependence, and also ensures a very low phase distortion during amplification.

One of our traveling-wave amplifiers is shown in "ECIT s traveling-wave amplifiers". It features a 0.5-18 GHz bandwidth, 12 dB gain with less than ±1 dB ripple and a 17 dBm 1-dB compression point - defined as the output power for which the gain has dropped by 1 dB. The design occupies 3.0 × 1.25 mm of die area and was fabricated using the D01PH power process at OMMIC, the France-based compound semiconductor foundry owned by Philips.

Some of the performance characteristics of these amplifiers can be seen in figures 1 to 3. The measured gain from 10 of our MMICs produced from a single batch is shown in figure 1. The pairs with the highest and lowest gain were then selected, and their output power recorded as a function of input power (see figure 2). The phase performance of three of our amplifiers can be seen in figure 3.

The original circuits were designed without any on-chip DC blocking or bias circuitry to improve yield and minimize performance variation. However, this approach requires DC blocking capacitors to be added off-chip (see figure 4a), alongside a more complex biasing network, and leads to large modules that are not easy to assemble.

Because it is planar, a traveling-wave amplifier is well-suited for complete integration as a GaAs MMIC, and we have integrated all the necessary circuit components onto the die. This approach offers many advantages, including less board space, lower module cost, and a reduction in the number of solder joints. The increased integration also means that no external capacitors are required, and the external inductors that are difficult to source because they require a flat frequency response from 0.5-18 GHz can be replaced with cheaper versions that just have to block low frequencies. The level of integration has been extended even further by constructing traveling-wave MMICs with an on-chip bias network that do not require an off-chip bias "tee". This enables the first stage to directly feed the second one, eliminating the need for an interstage network (see figure 4b).

We have also built traveling-wave amplifiers that do not require a negative gate bias for a non-mobile communications provider that did not have a negative voltage supply. Our MMIC made the off-chip charge pump that was used with the amplifier redundant, and cut the module size by half. The changes also introduced some downsides - a higher DC current consumption and reduced drain efficiency - but these drawbacks were not a major concern because the MMIC was powered by mains electricity.

Building on this existing experience we are now developing traveling-wave amplifier MMICs with an improved 1 dB compression point. Conventional designs are restricted by artificial transmission lines that are relatively lossy, which causes input signal strengths to decrease significantly as the signal travels along the gate line, and means that consecutive stages are driven with declining power. The consequence is that only the first few stages receive sufficient power to be driven into gain compression. Although adding further stages can increase the saturated output power, it does not improve the 1 dB compression point.

To reduce this effect we have redesigned our circuits to minimize gate line losses and have developed a traveling-wave amplifier design with a predicted 1 dB compression point of 22 dBm. This year our commercialization division plans to launch these improved devices commercially and extend the frequency range of our existing amplifier designs to meet customer demand.

Our development and commercialization of traveling-wave MMIC amplifiers has shown us that for certain applications these products are a better choice from a performance and manufacturing perspective than standard products, even if the dies themselves are more expensive than the off-the-shelf circuits. This is because the different characteristics of the distributed amplifier vary in their importance from application to application, and bespoke designs can target the specific needs that they serve. It is an approach that is helped today by computer-aided design packages that enable the characteristics of a traveling-wave amplifier to be fine-tuned precisely. These factors lead us to conclude that for relatively low-volume high-performance circuits, ECIT s bespoke MMICs not only improve the amplifier s performance and reduce its size and complexity, but create a more cost-effective circuit that does not require expensive hybrid assembly steps.