Wide-Band Feedback Amplifiers

Abstract

An approach to the problem of the design of transistor wide-band feedback amplifiers is given. Choice of feedback circuit configurations for the stabilization of amplifier circuit transconductance, transresistance, voltage gain, and current gain are discussed in terms of an idealized phase-reversing amplifier. A composite stage including two transistors which may be made to approach this idealized amplifier is described, including considerations of biasing, asymptotic high-frequency cutoff characteristic, and composite hybrid parameters. The design of a three-transitor amplifier for a bandwidth of 8 mc having a closed-loop gain of 36 db and over 30 db of feedback is discussed. Local feedback is made an integral part of the design to enable the amplifier to accommodate a wide range of transistor parameters without individual adjustment. Shaping of the open-loop frequency characteristic is accomplished largely by the transistors themselves. The circuit configuration permits the variation of return ratio with frequency to be controlled primarily in one stage, whereby the gain-phase characteristic may be readily tailored to provide a preassigned transient characteristic. The design procedure for the amplifier is discussed, including maximization of return ratio and shaping of the high-frequency characteristic. Signal-flow diagrams are used to derive the frequency dependence of the return ratio. Measurements of magnitude and phase of the return ratio and the closed-loop gain and transient response are shown.