Design of a High-frequency Positive-feedback Bandpass Amplifiers

At present, the detection frequency of optical quantum noise detectors is generally low. In order to enable the detector to measure the quantum noise of light field at high frequencies, a design scheme of high frequency positive feedback active band-pass amplifier circuit was proposed. The transfer function of the amplifier circuit was derived from the circuit structure, and the expressions of the resonant frequency, Q value and gain of the amplifier circuit were obtained. Taking the circuit design with a center frequency of 80 MHz, a bandwidth of 3 MHz and a gain of 200 as an example, the resistance, capacitance and other parameters in the amplifier circuit were determined according to three characteristic parameter expressions, and the correctness of the theoretical analysis and design scheme was verified by using Multisim software simulation. Based on the high-speed and low-noise operational amplifier OPA847, a positive feedback band-pass amplifier circuit was built according to the design parameters, and its working performance was tested. The results show that the circuit built according to the design scheme has a center frequency of 80 MHz, a bandwidth of 2 MHz, and a gain of 14 dB. The experimental results of the center frequency, bandwidth, and gain are in good agreement with the design values. A little error comes from the impedance of the operational amplifier itself, the resistance value deviation in the actual circuit, and the distributed capacitance and inductance of the circuit board.Using this design scheme to determine the parameters of resistance and capacitance in the amplifier circuit, the gain can be adjusted without affecting the central frequency of the amplifier circuit, so that the detector can achieve the effect of high frequency, narrow band and high gain.