What is the basic principle of parametric amplifier?

    The communication distance requirements of modern radio are gradually increasing, and there are many factors that affect radio communication, such as geographical environment, electromagnetic environment, and climate environment. This makes the signal received by the receiver extremely weak. In order to receive such weak electrical signals, the sensitivity of the receiver must be improved. In order to improve the sensitivity of the receiver, the first-stage amplifier in the receiving device must be improved. People have developed various low-noise amplifiers, and parametric amplifiers are one of them. Let us briefly introduce the basic principles of parametric amplifiers.

    Amplifiers use triodes or multi-electrodes. Most of them use DC power supply. However, parametric amplifiers are made of diodes, and their power supply is not DC, but a high-frequency oscillator. In order to explain its working principle, let's first talk about the role of capacitors.

    The picture above is a parallel plate capacitor. Let's assume that plate B is fixed and plate A can move up and down. If this capacitor is charged, plate A will have a positive charge (+Q) and plate B will have a negative charge (-Q). At this time, if someone wants to pull plate A upwards to make it farther away from plate B, then this person must use force to overcome the attraction between the positive and negative charges on the two plates. In other words, people must "do work" to pull plate A apart. As a result of this work, the energy stored in the capacitor increases.

    How do we know that the energy stored in the capacitor has increased? We know that the voltage (V) across the capacitor and the capacitance (C) of the capacitor and the charge (Q) on the capacitor plates have the following relationship, namely Q=CV or V=QC. When we pull the plates of the capacitor apart, the capacitance C decreases, and since the charge Q remains unchanged, the voltage V across the capacitor increases. In other words, the energy stored in the capacitor has increased. However, if the capacitor is not charged, that is, there is no charge on both plates, there is no charge attraction, so no work needs to be done whether the plate A is pulled apart or pushed closer. At this time, the capacitor neither loses energy nor gains energy. This energy conversion relationship can be used to make an amplifier.

    In the actual parametric amplifier circuit, as shown in Figure 1, only the power of the pump source frequency fP, the signal frequency fs, and the sum frequency or difference frequency fP±fs is allowed to exist. All other combined frequencies are suppressed so that their power is zero. In Figure 1, VD is a variable capacitance diode; Us is the signal source, Rs is its resistance, Bs is the bandpass filter of the signal frequency fs; UP is the pump power supply, Rp is its internal resistance, Bp is the bandpass filter of the pump source frequency fP; Bi is the bandpass filter of the sum frequency or difference frequency, and Ri is the load. In this case, the Menley-Lowe formula can be simplified to:

    Where P0,1 is the signal power Ps; P1,+1 is the power of the sum frequency fP+fs; P1,-1 is the power of the difference frequency fp-fs. If Bi in Figure 1 is a sum frequency filter, then the "±" in equations (1) and (2) are both "+". At this time, the figure shows an up-conversion parametric amplifier. Since its signal spectrum is not inverted, it is also called a non-inverting parametric amplifier. Its input frequency is fs, the output frequency is fS+fp, and the output load is Ri of the sum frequency filter branch. It can be seen from equation (1) that the signal power Ps is a positive value, and the amplification factor is equal to the ratio of the sum frequency fP+fs to the signal frequency fs power. This amplifier has stable operation and high gain, and is mostly used in up-converters. If Bi in Figure 1 is a difference frequency filter, then the "±" in equations (1) and (2) are both "-". At this time, it is also an up-conversion parametric amplifier, but its signal spectrum is inverted, so it is also called an inverting up-conversion parametric amplifier.