Microwave Memories: Tunnel-Diode Amplifiers Represent A Bygone Era

Going back through the first few years of this magazine, then known as MicroWaves, offers more than just a trip down memory lane. It provides an opportunity to see the industry and the world as it was 50 years ago. The ad and graphical depictions in articles are vastly differentnot to mention their content, which has vastly changed.

For example, the July 1962 cover story highlighted a product that has become largely overshadowed by gallium-arsenide (gaas) field-effecttransistor (fet) circuits: the tunnel-diode amplifier (see figure). Tunnel diodes are still widely used in detector circuits. But their limited linearity in amplifier circuits prompted the development of alternative technologies, such as gaas and silicongermanium (sige) devices.

Titled "tunnel Diode amplifier has 3.5-db noise figure," the article described a product that was no doubt the pride of Micro state electronics corp. (Murray hill, nJ) at that time. The nta-1300b L-band amplifier was developed specifically for use in phased arrays, troposcatter systems, and relay links. It was part of a series employing the firm's high-cutoff-frequency, gallium-antimonide tunnel diodes. Although gallium-antimonide tunnel diodes were apt to be more temperature sensitive than germanium devices, the article noted, rf and direct-current (Dc) temperature-compensation networks assured stability from 32 to +130F. With optional heaters, that range could be extended to 40 to +140F.

The amplifiers featured nominal gain of 17 db and a 1-db bandwidth of 10 percent at L-band. Multiple units could be supplied with gain tracking to a fraction of a decibel and phase tracking to 2 deg. individual units offered 0.2 db gain stability and phase stability down to 1 deg. In addition, the tunnel diode provided "absolute stability" from Dc to the cutoff frequency, thanks to networks that employed lumped, stripline, and coaxial circuits along with four-port circulators. any simultaneous input or output mismatch (including open or short circuit) could be tolerated without danger of oscillation. In addition, all units provided a voltage standing wave ratio (VsWr) of 1.50:1.

Under a sub-headline titled, "small size aids retrofit," the typical amplifier size is toutedless than 100 cubic inches. For satellite applications, however, it was noted that "miniature" versions were available. (If only we had the measurements on them!) Weight for the standard versions was in the range of 1 to 4 lbs. The amplifiers were available in small quantities on 60-day delivery at prices ranging from $2000 to $5000. Just imagine how much pressure the engineers of this amplifier felt as they strove to meet a design schedule that today seems incomprehensibly long. and they did it all manually, without the aid of computer-aided-engineering (cae) and other software design aids.

That era of design has passed away, although the tunnel diode is still going strongeven if it is now found more commonly in detectors than amplifiers.

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