Novel approaches in voltage-follower design

Abstract

The aim of this research programme was to design and develop novel voltagefollowerslbuffers, suitable for radio frequency (RF) applications. The emphases throughout has been on improving key characteristics, in particular distortion, operating bandwidth, input and output impedances, offset-voltage and power supply demands of the design. The majority of the results of this work have been reported by the author in the technical literature (I] to (6). Initially this research focuses on the investigation of the underlying operating principles of the voltage-follower to provide an in-depth understanding of its operation. This study concentrates on establishing reasons for the poor distortion, low input and high output impedances and increased offset-voltage and confirmed that these designs have inherently poor performance in these parameters. The analysis is carried out using both theoretical modelling and computer simulation, using the wellestablished software package ORCAD PSpice. Despite the availability of high performance computer simulation tools, it becomes apparent that 'hand' calculations in the design process, generally based on DC and small-signal transistor parameters, are essential. Therefore a detailed analysis of the transistor-models used throughout this research is carried out with PSpice data. Using the analytical results of the conventional voltage-follower as a benchmark, various novel circuit techniques investigated. Several new circuits are proposed with respect to improving the previously mentioned key characteristics. The first technique comprises local feedback and single-valued current biasing and 111 consists of emitter-followers exclusively throughout the signal path, keeping the distortion of the input signal to low levels [1 J, (2). The second technique is based on local feedback with double-valued current biasing, increasing somewhat the power dissipation but reducing, notably, the distortion of the configuration [3J, [4J, [5J, [6J. The final technique employs the emitter-followers throughout the signal path in combination with global feedback and double-valued current biasing, which presents significantly better results, on certain parameters, than conventional and existing configurations. It is anticipated that this work will be published in the near future