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Amplifiers Gain Broadband Power

May 30, 2014
These solid-state amplifiers offer distortion-free performance in coaxial and drop-in packages for design flexibility in broadband applications.
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Amplification is almost always needed in high-frequency systems, whether transmitting or receiving. When broadband amplification is needed, finding a solution can be most challenging, since a large number of simultaneous amplifier performance parameters, such as output power, gain, and even noise figure, must be satisfied over a wide frequency bandwidth.

Fortunately, the new model ZHL-5W-422+ coaxial amplifier from Mini-Circuits is an example of a broadband amplifier that not only delivers outstanding electrical performance with 5-W CW output power from 500 to 4200 MHz, but includes protective circuitry and numerous features to make its installation and use almost foolproof. The 50-Ω amplifier is well suited for military, commercial, test, and even amateur-radio applications, and can be supplied with or without a heat sink.

Model ZHL-5W-422+ designates a coaxial amplifier with fan and heat sink, which is also available as model ZHL-5W-422X+ as the coaxial amplifier in its housing alone, without fan and heat sink (Fig. 1). The Class-A linear amplifier, which is based on GaAs FET active devices, provides adequate continuous output power for a variety of transmit operations. These include frequency-modulated (FM) radios, television transmitters, point-to-point radio transmitters, amateur radio, and laboratory use.

1. Model ZHL-5W-422+ is a solid-state amplifier with fan and heatsink, while model ZHL-5W-422X+ is just the amplifier (right) for use from 500 to 4200 MHz.

This is an amplifier with instrument-grade gain performance, with 20-dB minimum gain and 25-dB typical gain across its frequency range. Although these are not staggering gain numbers, what is impressive is the flatness of the gain with frequency, typically holding well within ±1 dB gain from 500 to 4200 MHz (Fig. 2). The amplifier is rated for worst-case gain flatness of ±1.7 dB across its operating frequency range.

2. The small-signal gain of the model ZHL-5W-422+ amplifier remains flat through 4.5 GHz.

Perhaps not as apparent, this is an amplifier that also features impressive directivity of better than 50 dB across its full operating frequency range (exceeding 60 dB at some frequencies). In addition, it is well matched for use in a wide range of applications, with a low input VSWR of no worse than 2.3:1 and typically at 1.70:1.

The low-input VSWR enables well-matched connections between signal sources, such as a feedline from an antenna, and the input port of the amplifier. Such “compatible impedances” help minimize mismatches and irregularities in amplitude and phase performance. As a result, with low-VSWR amplifier input ports that are well matched, the specified gain flatness can actually be achieved. In addition, for applications where phase irregularities due to impedance mismatches might be a problem—such when amplifying phase-modulated signals—these amplifiers help to minimize phase deviations across a wide bandwidth.

With or without the heat sink, the amplifier is rated for reasonable 5 W output power from 500 to 4200 MHz, with +35 dBm typical output power at 1-dB compression and +37 dBm typical output power at 3-dB compression. The linear amplifier boasts output third-order intercept point of at least +40 dBm and typically +45 dBm, and output second-order intercept point of at least +50 dBm and typically +55 dBm (Fig. 3). Two-tone output levels were measured with high-quality commercial test equipment, using two test tones set 1 MHz apart in frequency.

3. The upper trace shows output power at 1-dB compression and the lower trace is output power at 3-dB compression for the model ZHL-5W-422+ amplifier.

With its generous output-power levels, it also achieves high efficiency, boasting 30% typical operation under saturated output-power conditions and respectable noise figure across its frequency range (typically at 7 dB). The amplifier is designed for use from a 2-A supply at +28 VDC. It can handle input power levels as high as +20 dBm without damage. Models ZHL-5W-422+ and ZHL-5W-422X+ are RoHS compliant and supplied with SMA connectors.

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Output Power Pitfalls

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Healthy output power in an amplifier can bring with it many risks; these come in the form of overheating, backup damage, and other potential problems as a result of short-circuit or open-circuit connections to a powered amplifier. No audio through RF/microwave amplifier is immune from damage due to faulty connections, but the model ZHL-5W-422+ has been designed to handle as many problems as possible.

With or without heatsink, the amplifier is unconditionally stable and features reverse polarity protection against “wrong-connection” hookups. The amplifier will not suffer damage when operated with an open or short output load under full CW output power. The amplifier, which is designed for an operating temperature range of -20 to +50°C, incorporates automatic shut-off protection when its base-plate temperature exceeds +85°C.

With the many challenges of broadband amplification at RF/microwave frequencies, perhaps the new model GVA-123+ amplifier from Mini-Circuits is even more impressive, packed into a tiny SOT-89 package for applications from 10 MHz to 12 GHz. Suitable for use in cellular base stations, in test equipment, in satellite-communications (satcom) equipment, and in avionics systems, this is an amplifier that practical defines gain flatness, with specified performance of ±0.7 dB gain flatness from 0.05 to 8 GHz. While its gain does drop below 8 GHz (Fig. 4), this is a compact amplifier that offers 16.9 dB typical gain at 2 GHz, with typical return loss of 20 dB at that frequency and almost ruler-flat gain from 10 MHz to 8 GHz. 

4. Model GVA-123+ is a broadband amplifier in an SOT-89 package with gain within ±0.7 dB through 8 GHz.

Of course, achieving desired results with RF/microwave amplifiers in broadband applications requires an understanding of amplifier distortion and noise behavior. Mini-Circuits has worked closely with leading test-and-measurement equipment suppliers such as Agilent Technologies to establish proven measurement techniques for characterizing its broadband amplifiers under a variety of operating conditions.

Broadband amplification was once primarily considered a requirement of measurement applications. Nevertheless, as more commercial and defense systems move to higher-order digital modulation schemes, wider signal bandwidths are needed—and amplifiers are needed to process those bandwidths. From Mini-Circuits, this is just a sampling of the latest crop of broadband solid-state amplifiers with good output power, reasonable gain, and excellent gain flatness, all available in a variety of different packages for different requirements. 

Mini-Circuits, P.O. Box 350166, Brooklyn, NY 11235-0003; (718) 934-4500, FAX: (718) 332-4661.

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