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Introduction to the Filter Amplifier

The previous discussion has centered around the transient response of amplifiers. The only function of the amplifier was to provide gain with as little distortion of the waveform as possible. Suitable test waveforms were the step and square wave, and hence there was developed a technique of evaluating amplifier performance in terms of the response to such waveforms. The steady-state response of the amplifier was purely incidental.

In amplifying portions of many systems, however, there is an additional requirement. Not only is an amplifying unit expected to produce gain over a bandwidth sufficient for the information contained in the signal, but it must also reject signals outside the appointed band. This is primarily a steady-state matter; i.e., we are talking in terms of the frequency response of the system. It may of course be true that the desired signal which is to be amplified consists primarily of transient waveforms and that the transient response must therefore not be entirely ignored. Nonetheless, we shall seek to allow for this through use of the interrelationships developed in the preceding chapters between steady-state and transient response.

Our principal concern will be with bandpass amplifiers, rather than with lowpass or highpass. In one sense we considered the lowpass amplifier in the preceding chapters, but there were no requirements that the amplifier reject frequencies above any prescribed frequency limit. Bandpass amplifiers are fundamental to all types of systems using the radio-frequency spectrum on a frequency-separation basis (as opposed to time separation, e.g., multichannel pulse-time modulation), including radio, radar, and carrier on wire lines.

There are several aspects of the design requirements, and their relative importance will vary from one application to another. These are the center frequency /o, the bandwidth B, and the gain magnitude A0 at band center. It shall be the objective here to present techniques which make

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