Some Notes on Wideband Feedback Amplifiers

The extension of the passband of wideband amplifiers is a highly important problem to the designer of electronic circuits. Throughout the electronics industry and in many research programs in physics and allied fields where extensive use is made of video amplifiers, the foremost requirement is a passband of maximum width. This is necessary if it is desired to achieve a more faithful reproduction of transient wave forms, a btter time resolution in physical measurements, or perhaps just a wider band gain-frequency response to sine wave signals. The art of electronics is continually faced with this omnipresent amplifier problem. In particular, the instrumentation techniques of nuclear physics require amplifiers with short rise times, a high degree of gain stability, and a linear response to high signal levels. While the distributed amplifier{sup 1} may solve the problems of those seeking only a wide passband, the requirements of stability and linearity necessitate using feedback circuits. This paper considers feedback amplifiers from the standpoint of high-frequency performance. The circuit conditions for optimum steady-state (sinusoidal) and transient response are derived and practical circuits (both interstage and output) are presented which fulfill these conditions. In general, the results obtained may be applied to the low-frequency end. The fundamental limitation in feedback amplifiers arises from the over-all phase shift in the amplifier and in some cases, the feedback circuit as well. As the shift in phase approaches 180 degrees on either side of the mid-band, the feedback becomes positive, resulting in regeneration and possible oscillation. The relationships between attenuation and phase shift necessary for amplifier stability have been formulated and published{sup 2}. It is the phase shift and its attendant difficulties that make feeback over more than three stages impractical for video amplifiers; and while three-tube feedback{sup 3} is feasible on theoretical grounds, it is difficult to design practical circuits which use the feedback to the best advantage. Fig. 1 shows two of the most satisfactory three-tube feedback loops which have been used for video purposes. The circuit in Fig. 1a accomplishes the feeback through the common plate impedance of T{sub 1}. The one serious shortcoming of this circuit is that the gain of T{sub 1} is not stabilized. This tube merely serves to drive the feedback loop encompassing T{sub 2},T{sub 3}, and T{sub 4}. For this reason the circuit is inadequate when used in those applications where the virtues of feedback; e.g., stability, linearity, are required.