Band-pass amplifier filters

An amplifier-filter circuit for amplifying and filtering an input signal, comprising:

• an amplifier circuit, having inverting and non-inverting inputs and a high gain factor, for providing an output signal representative of the difference between signals applied to the inverting and non-inverting inputs;
• feedback resistance means, connected between the output and the inverting input of the amplifier means, for providing a selected resistance;
• means for applying the input signal to the amplifier means non-inverting input;
• active filter circuit means, responsive to a signal applied to an input thereto and having a bandpass transfer function centered around a selected resonance frequency, for providing a filtered output signal at an output thereof;
• second means for applying the signal at the amplifier means inverting input to the active filter circuit means input;
• and
• coupling resistance means, connected between the active filter circuit means output and the amplifier circuit inverting input, and having a resistance less than said first feedback resistance means;
• the active filter circuit means, second means for applying, and coupling resistance means being connected between the amplifier circuit inverting input and ground so as to provide a variable impedance which appears as a series-resonant tuned filter having a resonance frequency substantially equal to said selected resonance frequency.