Circuit: A 100-dB RF Amplifier

Published in the July/August 1977 edition, this project set out to improve on existing RF amplifier designs by extending their usable dynamic range while keeping intermodulation distortion under control. The result was a compact amplifier covering approximately 100 kHz to 30 MHz, making it suitable for various applications.

As we celebrate 65 years of Elektor, it is both informative and inspiring to look back at projects from decades ago. For instance, consider this RF amplifier designed to handle an impressive 100 dB dynamic range. The components may belong to 1970s, but the engineering challenge behind the circuit will sound very familiar to RF engineers today.

Published in the July/August 1977 edition, this project set out to improve on existing RF amp designs by extending their usable dynamic range while keeping intermodulation distortion under control. The result was a compact amplifier covering approximately 100 kHz to 30 MHz, making it suitable for various applications.

The Circuit

A 3N211 dual-gate MOSFET sits at the center of the design. Rather than simply designing an amplifier for maximum fixed gain, the circuit exploits the device's second gate to control amplification. Varying the second-gate bias allows the gain to be adjusted across a very wide range, producing the claimed 100 dB dynamic range.

The designers paid particular attention to intermodulation performance. The article describes testing the amplifier with two signals and examining the resulting intermodulation products on a spectrum analyzer. Negative feedback is also employed to improve linearity, while transformers and tuned networks provide the necessary RF coupling and impedance matching, the author notes.

Why Look at It Today?

Today, engineers have access to RF ICs, next-gen simulation tools, and spectrum analyzers that engineers in 1977 could only dream of. Yet the fundamental problems illustrated by this 100-dB RF amplifier project have changed surprisingly little. This circuit is more than an interesting artifact from the Elektor archive. It shows how engineers tackled an RF problem with the components and measurement tools available almost half a century ago. Today, the question is: How would you build it now?

The 100-dB RF Amplifier Project

The original article, “RF Amplifier with 100 dB Dynamic Range,” appeared in Elektor July/August 1977. Check out the article.

Editor's Note: Because the article first appeared in a 1977 edition of Elektor, some of the parts and components might not be available. Still, we think you’ll find the article inspirational and informative.