Lossless Feedback Amplifier Design

Abstract

Lossless feedback has been applied to three types of microwave amplifiers:

1. High-gain amplifiers (HGAs), Mason, 1954
2. Low-noise amplifiers (LNAs), Vendelin, 1975
3. High-power amplifiers (HPAs), Vendelin, 2005

The LNA and HPA are duals of each other. This paper will give a tutorial review of all three feedback amplifiers, with special emphasis on the HPA with maximum output power Class A and a simultaneous output match (S22 = 0).

This was an invited paper at RFIT (RF Integrated Technology) in Singapore, December 1, 2005. It has been expanded to include FBPA designs at 2 GHz with 0.8 W GaAs MESFETs. This work was also published in IEEE Trans MTT, April 2006, by Zuo-Min Tsai et al.

Introduction

The application of lossless feedback to microwave amplifiers has been well known for many years. Beginning with Mason's classic paper, which was further reviewed by Gupta, the concept of unilateral gain was introduced in 1954, which was found to be invariant to common-lead and the highest stable gain achievable by the transistor. Later, in 1975, Vendelin (and others) showed LNAs could be improved by using lossless feedback, which was found to be normally a common-lead inductance in the source (emitter). The amplifier would give Fmin and S11=0, a very useful improvement. Today we can add HPAs may also be improved by using lossless feedback.

After you have read the paper, please feel encouraged to share your feedback with Mr. Vendelin in the Active and Passive Circuits Forum.