TABLE OF CONTENTS Amplifiers loaded with a pair of coupled resonance circuits, individually tuned to the center frequency of the band, were extensively used to obtain a reasonably flat frequency characteristic in a limited band, since the early days of radio. Although the resonance circuits are tuned to the same frequency, the transfer function of the circuit exhibits two pairs of conjugate poles having different imaginary parts, as in Fig. 4.18(a).
In earlier realizations, the main application area of these double-tuned amplifiers was to intermediate frequency (IF) amplifiers of all types of super-heterodyne receivers. The standard IF frequency was around 450 kHz with a bandwidth of 9 kHz for AM receivers and 10.7 MHz with a bandwidth of 150 kHz for FM receivers. The advantages of this approach were a reasonably flat response within this relatively narrow band, with only one amplifying stage, and the ease of the tuning procedure. Magnetic coupling is usually preferred to couple the resonance circuits, but it is possible to show that any approach that provides interaction among the resonance circuits gives similar results and it is possible to use the same basic equations, after an appropriate parameter conversion.
With the recent availability of computer-based tools, on-chip inductors can now be designed with reasonable precision together with their parasitics, and consequently, the capacitively or inductively coupled resonance circuits can be considered as components of integrated circuits.
The basic form of a magnetically coupled circuit is shown in Fig.
