The RF front-end LNA circuit is an important part of the front-end circuit design of radio equipment. Because the actual radio propagation environment is usually harsh, LNA must be considered in the RF front-end circuit. LNA as a key circuit in the RF module, the noise size directly affects the performance of the receiver. On the other hand, the design of the LNA is also one of the most challenging aspects of radio-related circuit design. This is mainly manifested in that it also needs to satisfy high gain, low noise, good input-output matching, and unconditional stability at the lowest possible operating current.
CDMA2000, one of the standards for mobile communications, is an important technical standard for third-generation mobile communication systems and has received extensive attention worldwide. In fact, the design of the LNA circuit is one of the main contents of the base station circuit design under this standard. At the same time, due to the particularity of mobile communication equipment applications and the CDMA2000 multiple access method, suppressing interference/noise, increasing gain, and maintaining its stability is the basic starting point of related circuit design. Therefore, the design problem of the CDMA2000 base station LNA circuit can be combined to discuss some key issues and solutions for the RF LNA circuit design under general conditions. This also provides a practical background for the discussion of general issues.
2. Some Problems in the Design of RF LNA CircuitsThe design of a low-noise amplifier circuit should first focus on low noise and high gain requirements. In the case of high gain, starting from the reliability of its performance, the stability of the circuit must be taken into account. In order to realize the comprehensive consideration and compromise processing of these factors in the actual circuit design, the relevant issues are first discussed one by one.
2.1 Noise problems and their solutions
The main sources of noise that cause the noise characteristics of the amplifying circuit to deteriorate include flicker noise, thermal noise, and shot noise. Among them, the flicker noise is also called 1/f noise, its value is related to the semiconductor material, and the power spectral density is inversely proportional to the frequency. Therefore, it is not necessary to consider too much in the case of radio frequency. Thermal noise is the noise introduced by the electron thermal motion. It is mainly controlled by the ambient temperature and has nothing to do with the current. In the case of RF, this noise is affected by the increase in the frequency of the external field, and its magnitude will increase significantly as the frequency increases. Splatter noise is not directly affected by temperature. The amount of shot noise is proportional to the operating current. In summary, the suppression of noise in RF low-noise amplifier circuits should mainly consider the suppression of thermal noise and shot-scatter noise.
In general, noise problems can be effectively solved by selecting low-noise devices, reducing circuit insertion loss, improving linearity, and reducing circuit power consumption [1]. Note that, including the selection of low noise devices, most of the above methods are related to the specific design of the circuit. Therefore, in the actual design, the above processing methods should be applied flexibly according to the requirements of related indicators.
2.2 Gain Problems and Their Solutions
In order to achieve a high gain of the circuit (generally in the several tens of dB), a multi-stage amplifier circuit is generally required, such as the two-stage amplifier circuit shown in FIG. 1 . This processing method has been widely used in practice.
Figure 1 Two-stage amplifier circuit model
In order to balance noise considerations, the noise and gain of multi-stage amplifier circuits should be considered together. For m-class amplifiers, the total noise figure is determined by the noise figure NF and gain Av of each single-stage circuit [2].
Since the amplification of each single-stage amplifier circuit is generally much greater than 1, it is obvious that the total noise figure of the multi-stage amplifier circuit is determined mainly by the pre-stage circuit, and the noise level of the first-stage circuit is the main factor that affects the entire circuit. For this reason, the noise of the preceding circuit should be suppressed by selecting low-noise devices. At the same time, compromises such as the selection of amplification stages and single-stage magnifications are taken into account.
In order to obtain the maximum gain output of the LNA circuit, the input and output must be impedance matched. For an actual LNA amplification circuit, the input and output impedances are generally designed to be 50 Ω. From the noise point of view, the best transmission of energy can be achieved by conjugate matching between the antenna feed end and the LNA circuit input.
2.3 Stability issues and their solutions
The stability of the circuit is also the LNA circuit must be considered, especially in the design of the amplifier, the stability of the amplifier must be ensured to avoid possible self-excitation, the stability criterion is as follows [2]: Definition
Where Δ=S11S22-S12S21
If K'1 and |Δ|'1, then the circuit is absolutely stable to any signal source and load impedance; if K'1 or |Δ|'1, there is potential instability in the circuit [3]. For some signal sources or load impedances, self-excitation can occur. This does not mean that the circuit cannot work, but it requires careful selection of signal source and load impedance. Therefore, in the circuit design, the S-parameters of the relevant components must be measured.
Improve the stability of the circuit can usually use four impedance connections, the connection is shown in Figure 2. The circuit in Figure 2(b) is connected to the input of the circuit with a transfer resistor to improve stability. The AFS simulation results of the improved and the improved kf are shown in Figure 3 (where k represents the stability factor).
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