INTERNATIONAL JOURNAL OF CIRCUIT THEORY AND APPLICATIONS Int. J. Circ. Theor. Appl. 2008; 36:375–386 Published online 20 August 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/cta.441 A time-delay technique to improve GBW on negative feedback amplifiers Luis Nero Alves ∗, † and Rui L. Aguiar Electronics Telecommunications and Informatics, University of Aveiro, Aveiro, Portugal SUMMARY This paper presents a new method to improve the GBW (gain-bandwidth product) on negative feedback amplifiers. The proposed method is based on the introduction of time-delay elements in the feedback loop, which can be exploited to retrieve significant bandwidth enhancements. This delayed feedback concept is analyzed, and considerations are presented for first-order amplifiers, based on theoretical analysis. The concept is simulated and further demonstrated in a practical example using a series-shunt feedback amplifier with a TL081 operational amplifier (OA) and a 36-m-long coaxial cable as a delay element. Measured experimental results show a maximum bandwidth improvement of almost 90%, from a theoretical maximum of 141%. Copyright 2007 John Wiley & Sons, Ltd. Received 17 November 2006; Revised 4 July 2007; Accepted 7 July 2007 KEY WORDS: feedback amplifiers; delays; delayed feedback; gain-bandwidth product 1. INTRODUCTION Traditionally, negative feedback amplifiers (NFAs) (Figure 1) have a roughly constant product of the gain times the bandwidth (GBW). This claim is generally true for first-order amplifiers disregarding second-order effects caused by the feedback network, or even the presence of more poles. It is common practice to disregard time delays that may appear in the feedback path, although these are commonly associated with excess phase elements [1–4] that may promote instability. However, it is possible to exploit these delays to increase system bandwidth above the value predicted by the traditional GBW product, while maintaining stability. This paper presents a demonstration of the possibility of using time-delay elements for bandwidth enhancement in NFAs. Section 2 presents the concept of negative delayed feedback. Amplifiers using this concept are then compared with traditional NFA realizations. Section 3 presents a possible ∗ Correspondence to: Luis Nero Alves, Electronics Telecommunications and Informatics, University of Aveiro, Aveiro, Portugal. † E-mail: nero@av.it.pt Copyright 2007 John Wiley & Sons, Ltd.