Introduction

Wide-band amplifier circuit design is one of the most difficult and varied areas in the whole subject of electronic circuit design. There are the wideband intermediate frequency amplifiers associated with radio, television and radar receivers, there are the wide-band pulse amplifiers of radar and nuclear instrumentation systems, there are the direct-coupled wide-band amplifiers found in oscilloscopes, and many other electronic systems, and there are the wide-band repeater amplifiers found in cable communication systems.

A number of excellent texts consider many of the areas listed above. Maclean's book [1] deals mainly with the repeater kind of amplifier, and is particularly strong on the computer aided optimisation of a design for the best noise performance and low sensitivity to component tolerance. Kovács' book [2] covers a much wider field and attempts to answer the question which will be central to this chapter: how is the first step in design, the choice of the circuit shape, the initial circuit idea, to be made? This is, perhaps, in contrast to Carson's well-known book [3], which contains no practical circuits but is excellent on theory.

This chapter will concentrate on one small area in the field of wide-band amplifier circuit design: wide-band direct-coupled amplifiers. These are needed for the deflection amplifiers of the conventional analog oscilloscope and, of course, for the digitising oscilloscope when this kind of instrument is to be designed for use with very low level signals. In both cases, the input stages of the deflection amplifier call for high input impedance, wide bandwidth and well-defined gain.