Amplifiers Gain Power And Minimize Noise

Feb. 24, 2009
Improving solid-state device technologies are providing designers with more options for low-noise and high-power amplifiers, although tube amplifiers are far from forgotten.

Amplifiers come in many shapes and sizes, from audio through optical wavelengths. For RF/microwave communications amplifiers, the two main duties have to do with strengthening low-level signals for receivers and boosting high-level output signals for transmitters. Although they differ in function, as well as in size and power requirements, both types of amplifiers benefit from continuing improvements in transistor technologies.

Depending on output requirements, amplifiers range from tiny chips to complete subsystems with digital interfaces. In general, the trend toward higher levels of integration, such as embedding an amplifier on a chip with other transceiver components, still favors small-signal designs. Large-signal or power amplifiers are still mostly designed around discrete transistors and with discrete matching components. Power transistors are simply larger than low-noise or small-signal transistors. They dissipate more heat than low-noise transistors and require larger supporting (impedance-matching, power-supply) passive circuit elements, making a power amplifier larger than a low-noise amplifier (LNA). Power amplifiers operate on current in the ampere (A) range, compared to LNAs that only require milliamperes (mA).

At one time, both microwave low-noise and power amplifier designs were dominated by their use of GaAs field-effect transistors (FETs). But the development of other transistor architectures, such as GaAs heterojunction bipolar transistors (HBTs) and high-electron-mobility transistors (HEMTs), has given amplifier designers alternatives to traditional high-frequency silicon bipolar transistors and GaAs FETs for high-frequency LNAs. For power amplifiers at higher frequencies, GaAs FETs are still the device of choice. At lower frequencies, however, silicon laterally diffused, metal-oxide-semiconductor (LDMOS), silicon-carbide (SiC), and gallium-nitride (GaN) transistors offer impressive power densities.

Specifiers of high-frequency amplifiers have probably never had as wide a selection. Many suppliers offer both LNAs and power amplifiers, while others may specialize in one type or the other. Most companies feature selections

of amplifiers for specific bands or for broadband use, where one amplifier can be used for different applications. For example, long-time amplifier developer Microwave Solutions offers both power amplifiers and LNAs in specific frequency bands. The firm's model MSH-5617902 is a power amplifier designed for C-band applications from 5.9 to 6.4 GHz. It delivers +40 dBm output power at 1-dB compression and third-order intercept point of +49 dBm, while providing 40 dB gain with current consumption of 7 A from a +12 VDC supply. While not designed for low noise, it achieves a respectable noise figure of 8 dB.

In contrast, the company's model MSD-5611402 is an LNA designed for the same 5.9-to-6.4-GHz band. While it provides only +18 dBm output power at 1-dB compression, it boasts a low noise figure of 1.6 dB while drawing only 120 mA from a +12 VDC supply.

CTT, known for its power amplifiers, also offers LNAs from 0.5 to 20.0 GHz based on GaAs PHEMT technology. It's AM LNA family includes the model AMM/020-1022 with 22-dB gain and 1-dB noise figure from 0.5 to 2.0 GHz.

Illustrating the small sizes possible for small-signal LNAs, Hittite Microwave sells both chip and packaged LNAs such as the HMC617LP3, which fits into a QFN package measuring just 3 x 3 mm. Based on GaAs psuedomorphic HEMT (PHEMT) semiconductor technology, it features a low noise figure of 0.5 dB from 550 to 1200 MHz with 16 dB gain.

Packed into a 3 x 3 mm LFCSP housing, the model ADL5521 LNA from Analog Devices also leverages GaAs PHEMT technology for coverage from 0.4 to 4.0 GHz. It achieves 0.8-dB noise figure at 900 MHz, with 20.8-dB typical gain.

Mini-Circuits supplies a surface-mount LNA module measuring just 0.59 x 0.39 x 0.12 in., its model TAMP-960LN+. Ideal for GSM receivers, it operates from 824 to 960 MHz with a noise figure of a mere 0.55 dB at 915 MHz with 18-dB gain and +16.5-dBm output power.

For raw bandwidth, the model UA1L30VM amplifier module from Centellax features 30-dB typical gain from 0.01 to 30.00 GHz with noise figure of 4.5 dB. AML Communications also offers wideband amplifiers, including its AML240L2201 with 22-dB minimum gain from 2 to 40 GHz.

While LNAs tend to be housed in surface-mount packages and compact coaxial housings, power amplifiers start as coaxial modules and can be as large as an enclosure full of 19-in. rack-mount units. For example, the model RF1000 power amplifier from Milmega combines MMIC and SiC device technologies for extremely RFhigh RF output power from 200 to 1000 MHz. It yields 1000 W saturated output power, but weighs 130 kg and occupies 18 spaces in a 19-in. rack.

Some companies, such as Microwave Power, have developed proprietary circuitry for thermal management and integration. Based on an approach that applies monolithic fabrication techniques to ceramic substrates, the firm manufactures amplifiers for use from 1 to 40 GHz with as much as +57.5-dBm saturated output power and 12-dB noise figure.

In terms of power and integration, Ophir RF recently introduced its model 7000 amplifier system. It integrates four separate channels, covering 1 to 30 MHz, 30 to 500 MHz, 500 to 1000 MHz, and 1.0 to 2.5 GHz. The three lower-frequency channels deliver 100 W typical saturated output power while the microwave channel offers 50 W typical saturated power. The fan-cooled system measures 19 x 8.75 x 26 in. and weighs 95 lbs.

While more and more transistor-based RF/microwave amplifier suppliers are offering solid-state "replacement" units for traveling-wave-tube amplifiers (TWTAs), TWTAs are hardly obsolete. AR Worldwide offers a wide range of solid-state and TWTA amplifiers for system and test applications. Model 2000T8G18, for example, generates 2000 W CW output power from 7.5 to 18.0 GHz. The high-power amplifier weighs 2600 lbs (1182 kg). The somewhat more compact model PTC6442 TWTA from TMD generates 500 W minimum output power from 7.5 to 18.0 GHz with 57-dB gain, but weighs a mere 105.6 lbs (48 kg).

Xicom Technology recently introduced several lighter-weight TWTAs. The firm's C-band model XTRD-1250CL and Ku-band model XTRD-1250KL TWTAs operate in the 5.85-to-6.425-GHz and 13.75-to-14.50-GHz satcom uplink bands, respectively, and occupy 6 rack units while hitting the scales at just 115 lbs (see figure). They deliver 1250 W of peak output power.

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