The design of wide-band transistor feedback amplifiers

Abstract

A design technique is developed which apparently overcomes all the limitations of common-emitter transistor video amplifiers. This technique is based on the use of the impedance mismatch which occurs between stages having alternate series and shunt feedback. It is shown that the realizable gain-bandwidth product is in excess of 0.9ωT, the gain and bandwidth are insensitive to transistor parameter variations, and large output voltages may be obtained. The equations for both gain and bandwidth are developed in a form which is particularly suited to practical design work, and are accurate despite their comparative simplicity.In addition to the main treatment, the design of terminal stages and of multi-stage feedback loops is considered in detail, and some aspects of the theory of noise in feedback amplifiers are discussed.Two complete design examples are described. These are a 20dB 25Mc/s amplifier for 75Ω lines using two OC170 transistors, and a vidicon head-amplifier which achieves 3 × 10−9 A noise at the input in a 5 Mc/s bandwidth.