Amplifiers Gain In Performance

AMPLIFIERS HAVE changed over the years, with tubes no longer the dominant active device. Except for extremely high-power applications, or those calling for the linearity possible with traveling-wave-tube amplifiers (TWTAs), transistor amplifiers fill most of the signal-boosting chores in commercial, industry, and military RF/microwave applications.

Back in 1965 (and still true today), TWTAs provided dependable gain and output power in many commercial and military systems, including satellite communications and electronic warfare systems. Perhaps sensing the growing interest in using solid-state amplifiers in many systems, the Electron Dynamics Division of Hughes Aircraft Co. in Torrance, CA was promoting not only their capabilities in TWTAs from 1 to 18 GHz and power levels of 10 and 20 W, but also developments in solid-state amplifiers through 2 GHz (to 5 W from 0.5 to 2.0 GHz as well as from 0.5 to 1.0 GHz). The solid-state amplifiers, based on silicon bipolar technology, employed hybrid microstrip circuitry and an integral AC-to-DC power supply. The amplifiers provided at least 30 dB gain over their octave or more of bandwidth (Fig. 1).

Around that same time, the LEL division of Varian Associates in Copiague, NY was selling a line of solid-state intermediate-frequency (IF) preamplifiers and post amplifiers designed for phase and gain matched signal processing in receiver systems. By combining frequency mixers and preamplifiers within a common housing, a single component could process received signals from 0.5 to 40.0 GHz at IF center frequency of 60 MHz and 3-dB bandwidth of 20 MHz, applying as much as 24-dB RF-to-IF signal gain or as much as 30-dB gain reduction to control amplitude levels. The IF units boasted impressive (for that time) noise figure of 3 dB.

In the February 1973 issue, Managing Editor Dick Davis wrote about diode reflection amplifiers for radar and communications based on Trapatt and Impatt diodes. Based on developments in these semiconductors, various organizations were pursuing solid-state amplifiers with usable output-power levels at microwave and millimeter-wave frequencies. Bell Laboratories had developed an Impatt amplifier capable of 1 W CW output power and 30 dB gain at 6 GHz. The amplifier had been designed to replace 10-W TWT amplifiers in 6-GHz short-haul communications links, as part of AT&T's TH series microwave repeaters.

That report included details of work by Hughes' Electron Dynamics Division on a building-block Impatt diode amplifier with a gain-bandwidth product of 16.6 GHz and as much as 260 mW output power at 60 GHz. To achieve higher output power at that frequency, a pair of Impatt amplifiers was combined by means of a 3-dB hybrid coupler to achieve more than 150 mW total output power and more than 20 dB gain at 60 GHz (Fig. 2). With a 2-GHz bandwidth, the Impatt amplifier was designed for communications, boosting phase-modulated millimeter-wave signals at data rates to 1 Gb/s.

RCA Laboratories was also involved with Impatt diode amplifiers at that time, combining either Gunn-diode amplifiers or transferred electron amplifiers (TEAs) with Impatt amplifiers to achieve higher output powers with lower noise figures. By combining two low-noise TEA preamplifier stages, each with 10 dB gain, with another TEA stage with 8 dB gain, the first three stages of a four-stage amplifier were formed. The output stage was an Impatt amplifier, which delivered total output power of 300 mW at 8 GHz with a noise figure of about 14 dB but efficiency of only about 3 percent (Fig. 3). The goal of RCA's design project was to achieve several watts of output power at that frequency by combining additional Impatt amplifier stages.

Not to be outdone, the Special Microwave Devices Operation of Raytheon Company, in Waltham, MA, was building GaAs Impatt diode amplifiers with a unique approach to circuit design. Instead of the usual microwave integrated circuits (MICs) fabricated on alumina substrates, Raytheon's engineers had developed an approach that fabricated microstrip circuits on yttrium-iron-garnet (YIG) ferrite substrate materials. The use of the ferrite substrate eliminated the need to embed a ferrite puck into a dielectric circuit board substrate for coupling into and out of the Impatt avalanche diode. The YIG substrate was 40 mils thick with a dielectric constant of 15. The design goal was to create a four-stage amplifier with 3 W output power and 20 dB gain at 5.4 GHz with a 25-percent bandwidth.

In support of solid-state amplifier advances, device suppliers continued to push the power levels and performance of their semiconductors. For example, in 1973, TRW Semiconductor of Lawndale, CA advertised gold-metalized amplifier transistors ranging in power from 1 W at 2 GHz to 10 W at 2 GHz and 5 W at 3 GHz. At the same time, Microwave Semiconductor Corp. (MSC) of Somerset, NJ offered Class A ballasted microwave power transistors from 500 MHz to 3 GHz, with as much as 12 dB power gain and 1.6 W at 1 GHz; 6 dB power gain and 1.5 W output power at 2 GHz; and 4-dB power gain and 1 W output power at 3 GHz. The common-emitter transistors were supplied in hermetic packages.

The firm's model MSC 1330 power transistor was designed for 30 W CW output power at 1.3 GHz as a result of 8.5-dB gain and a +28-VDC supply. It could also generate 70 W output power with 10-dB power gain under pulsed signal conditions, running with 10-s pulse widths at 10 percent duty cycle at 1.3 GHz and a +40-VDC supply. In July 1974, Power Hybrids, Inc. (PHI) introduced their model PH2020 matched NPN planar (bipolar) transistor with 20 W CW output power at 2 GHz (Fig. 4). In small quantities, the device was priced at $245 each. It featured a fishbone chip design to maintain low current densities in the emitter fingers, with 10 ballasting resistors in each transistor cell (Fig. 5); each finger had one emitter ballasting resistor. Since the resistor is in series with the emitter, it provided degenerative ballasting-type feedback to minimize cell-to-cell thermal differences and improve device stability. It was supplied in a 25-mil-thick beryllium-oxide package with average thermal resistance of less than 3C/W. The device, which was suitable for L-band telemetry and radar systems, yielded 5.5 dB power gain at 2 GHz and +28 VDC with 38 percent typical drain efficiency. Of course, not all amplifiers are designed for high output power, and low-noise amplifiers (LNAs) are crucial parts of receiving systems as well. For example, Watkins-Johnson Co. of Palo Alto, CA, advertised their model WJ-5004-200 LNA in the 1970s, touting a noise figure lower than that of TWTs. The LNA featured a 4.5-dB noise figure from 2 to 4 GHz with +10-dBm output power. It measured 1.3 x 2.3 x 2.9 in. with power supply (Fig. 6).

As detailed in a February 1973 article, the company's technical staff pushed the use of emerging GaAs FET devices in their amplifiers. Until that time, silicon bipolar transistors were commonly used in LNAs, with typical noise figures of 2 dB at 1 GHz, 3 dB at 2 GHz, 5 dB at 4 GHz, and 6.5 dB at 6 GHz. The bipolar was the basis for low noise amplifiers to about 6 GHz. Compared to bipolars, the GaAs devices of that time offering maximum frequency of oscillation (fmax) in excess of 50 GHz, with the potential for about 10 dB gain through 10 GHz and amplifier noise figure of about 4.3 dB at 10 GHz in narrowband operation. The article explored the design of an LNA capable of a full octave bandwidth from 4 to 8 GHz based on GaAs FET devices. It promised a noise figure of 7 dB across the full bandwidth.

At that same time, Avantek of Santa Clara, CA offered their model AMT-6005M LNA, an MIC amplifier designed for use as a receiver preamplifier. It featured noise figure of 10 dB from 4 to 6 GHz and 28 dB gain with 1.5 dB gain flatness. The LNA could produce +10 dBm output power at 1-dB compression. It measured just 2.3 x 1.3 x 0.6 in. and featured thin-film microwave-integrated-circuit (MIC) construction based on a sapphire substrate (Fig. 7).

In May 1998, in an attempt to work with TWTs, CTT, Inc. of Santa Clara, CA developed solid-state amplifiers with equalization circuitry designed to correct for the gain/output variations over frequency of a TWT. The low-noise driver amplifiers were available in bands of 6 to 18 GHz, 7 to 17 GHz, and 7 to 19 GHz in packages measuring only 0.96 x 0.66 x 0.32 in. (Fig. 8). A customer would provide a designated gain or power window, and the CTT amplifier would be customized to provide the appropriate equalization for the supplied curve.

Finally, not all amplifiers were meant to be linear in nature. In 1993, AEL Defense Corp. of Lansdale, PA introduces a number of successive-detection logarithmic amplifiers (SDLAs) based on GaAs heterojunction-bipolar-transistor (HBT) device technology that could process input ranges as wide as -65 to +5 dBm. Available for bandwidths of 0.2 to 0.8 GHz, 0.5 to 2.0 GHz, and 2 to 6 GHz, these SDLAs were supplied in packages measuring just 0.40 x 0.28 x 0.10 in. with an amplifier chip mounted on a chip carrier The amplifiers provided logarithmic linearity of 1.5 dB across the wide dynamic range, using devices from a mature GaAs HBT process from Rockwell International in Thousand Oaks, CA. The SDLAs could also be used as limiting amplifiers.