Transistor stages for wide-band amplifiers

The risk of instability when negative feedback is applied to a multistage amplifier is minimized by dividing the amplifier into sections, each with its own feedback. If there are not more than two 90° phase-shifts in each feedback loop, the circuit will be stable. Since the interaction between cascaded stages depends on their input and output impedances, various methods of controlling these impedances have been examined. Several practical wide-band amplifier circuits are described, and it is shown that such circuits can be made to accept wide variations in transistor characteristics. When operation is required near the cut-off frequency of the transistor, a transformer coupling is proposed. The relatively poor low-frequency response of this circuit is improved by the addition of an RC coupling. Bias currents in the transistor circuits are fixed by a d.c. feedback which is decoupled at signal frequencies. Operation at very low frequency (<1 c/s) is considered.