RF amplifier, what are the main parameters of RF amplifier

RF amplifier, what are the main parameters of RF amplifier?

Description of the main parameters of RF amplifier

Operating frequency range (F): refers to the operating frequency range in which the amplifier meets the various indicators. The actual operating frequency range of the amplifier may be greater than the defined operating frequency range.

Power gain (G): refers to the ratio of the amplifier output power to the input power, and the unit is usually "dB".

Gain flatness (ΔG): refers to the range of amplifier gain variation over the entire operating frequency range at a certain temperature.

Noise factor (NF): Noise factor refers to the ratio of the input signal-to-noise ratio to the output signal-to-noise ratio of the amplifier, and the unit is usually "dB". The noise factor is expressed by the following formula: NF = 10lg (input signal-to-noise ratio / output signal-to-noise ratio) When the noise factor of the amplifier is relatively low (for example, NF < 1), the noise factor of the amplifier is usually expressed in terms of noise temperature (T). The relationship between noise factor and noise temperature is: T = (NF-1) T0 or NF = T/T0 + 1 T0-absolute temperature (290K)

1dB compression point output power (P1dB): The amplifier has a linear dynamic range, within which the output power of the amplifier increases linearly with the input power. This amplifier is called a linear amplifier, and the ratio of these two powers is the power gain G. As the input power continues to increase, the amplifier enters the nonlinear region, and its output power no longer increases linearly with the increase in input power, that is, its output power is lower than the value expected by the small signal gain. The output power value when the gain drops to 1dB lower than the linear gain is usually defined as the 1dB compression point of the output power, expressed as P1dB. Typically, when the power exceeds P1dB, the gain will drop rapidly and reach a maximum or fully saturated output power, which is 3-4dB greater than P1dB.

Third-order intercept point (IP3): The simplest way to measure the nonlinear characteristics of an amplifier is to measure the 1dB compression point power level P1dB. Another popular method is to use two adjacent signals 5 to 10MHz apart. When these two signals with frequencies f1 and f2 are added to an amplifier, the output of the amplifier not only contains these two signals, but also contains intermodulation products (IM) with frequencies mf1+nf2, where m+n is the order of the intermodulation products. At moderate saturation levels, the third-order component closest to the fundamental frequency usually plays a dominant role. Because the third-order terms play a dominant role until the point where the distortion is very serious, the third-order intercept point (IP3) is often used to characterize the intermodulation distortion. The third-order intercept point is an important indicator to describe the linearity of the amplifier. The typical value of the third-order intercept power is 10-12dB higher than P1dB. IP3 can be obtained by measuring IM3, and the calculation formula is:

IP3=PSCL+IM3/2; PSCL——single carrier power;

If the third-order intermodulation point is known, the fundamental wave to third-order intermodulation suppression ratio and the spurious level of the third-order intermodulation point can be estimated by the following formula:

Fundamental wave and third-order intermodulation suppression ratio = 2[IP3-(PIN+G)] Third-order intermodulation spurious level = 3(PIN+G)-2IP3.

Input/output standing wave ratio (VSWR): Microwave amplifiers are usually designed or used in microwave systems with 50 ohm impedance. The input/output standing wave indicates the degree of matching between the input impedance and output impedance of the amplifier and the required impedance of the system (50 ohms).

It is expressed as follows:

VSWR = (1+|Γ|)/(1-|Γ|); where Γ= (Z-Z0)/(Z+Z0)　

VSWR: input to output voltage standing wave ratio; Γ: reflection coefficient; Z: actual impedance at the input or output of the amplifier; ZO: required system impedance.

Working voltage/current: refers to the power supply voltage required when the amplifier is working and the current value required to be supplied when the amplifier is working.

Definition of amplifier gain window: In this product manual, the gain definition of the amplifier adopts the definition method of gain window (excluding narrowband power amplifiers). The definition method of gain window is to clearly define the fluctuation and variation range of the amplifier gain based on three gain indicators: the maximum gain allowed by the amplifier (Gmax), the minimum gain allowed by the amplifier (Gmin), and the gain fluctuation of the amplifier (ΔG).

Linearization technology of RF power amplifier The
nonlinear distortion of RF power amplifier will generate new frequency components. For example, second-order distortion will generate second harmonics and two-tone beat frequency, and third-order distortion will generate third harmonics and multi-tone beat frequency. If these new frequency components fall within the passband, they will directly interfere with the transmitted signal. If they fall outside the passband, they will interfere with the signals of other channels. For this reason, the RF power amplifier must be linearized, which can better solve the problem of signal spectrum regeneration. The principle and method of basic linearization technology of RF power amplifier is nothing more than taking the amplitude and phase of the input RF signal envelope as a reference, comparing it with the output signal, and then generating appropriate correction. There are three common technologies for achieving RF power amplifier linearization: power back-off, pre-distortion, and feedforward.

1. Power fallback

This is the most commonly used method, which is to use a larger power tube as a low-power tube. In fact, it improves the linearity of the power amplifier at the expense of DC power consumption.

The power back-off method is to back off the input power of the power amplifier from the 1dB compression point (the amplifier has a linear dynamic range, within which the output power of the amplifier increases linearly with the input power. As the input power continues to increase, the amplifier gradually enters the saturation region, and the power gain begins to decrease. Usually, the output power value when the gain drops to 1dB lower than the linear gain is defined as the 1dB compression point of the output power, expressed as P1dB.) by 6-10 decibels, and work at a level far less than the 1dB compression point, so that the power amplifier is away from the saturation region and enters the linear working area, thereby improving the third-order intermodulation coefficient of the power amplifier. In general, when the fundamental power is reduced by 1dB, the third-order intermodulation distortion is improved by 2dB.

The power back-off method is simple and easy to implement. It does not require any additional equipment and is an effective method to improve the linearity of the amplifier. However, its disadvantage is that its efficiency is greatly reduced. In addition, when the power back-off reaches a certain level, when the third-order intermodulation reaches below -50dBc, further back-off will no longer improve the linearity of the amplifier. Therefore, in situations where linearity requirements are very high, it is not enough to rely solely on power back-off.

2. Pre-distortion

Predistortion is to add a nonlinear circuit in front of the power amplifier to compensate for the nonlinear distortion of the power amplifier.

The advantages of pre-distortion linearization technology are that there is no stability problem, a wider signal bandwidth, and the ability to process signals containing multiple carriers. Pre-distortion technology is low-cost and consists of several carefully selected components packaged into a single module, which is connected between the signal source and the power amplifier to form a pre-distortion linear power amplifier. The power amplifier in the handheld mobile station has adopted pre-distortion technology, which uses only a small number of components to reduce the intermodulation product by a few dB, but it is a very critical few dB.

Pre-distortion technology is divided into two basic types: RF pre-distortion and digital baseband pre-distortion. RF pre-distortion is generally implemented using analog circuits, which have the advantages of simple circuit structure, low cost, easy high-frequency and broadband applications, but the disadvantages are that the spectrum regeneration component is less improved and the high-order spectrum component is difficult to offset.

Digital baseband predistortion is a promising method because it can be implemented with digital circuits due to its low operating frequency. It is highly adaptable and can offset high-order intermodulation distortion by increasing the sampling frequency and the quantization order. This predistortion consists of a vector gain regulator that controls the amplitude and phase of the input signal according to the content of the lookup table (LUT). The magnitude of the predistortion is controlled by the input of the lookup table. Once optimized, the vector gain regulator will provide a nonlinear characteristic opposite to that of the power amplifier. Ideally, the output intermodulation product should be equal to the output amplitude of the dual-tone signal through the power amplifier and opposite in phase, that is, the adaptive adjustment module is to adjust the input of the lookup table to minimize the difference between the input signal and the output signal of the power amplifier. Note that the envelope of the input signal is also an input to the lookup table. The feedback path samples the distorted output of the power amplifier, which is then sent to the adaptive adjustment DSP through A/D conversion to update the lookup table.

3. Feedforward

Feedforward technology originated from "feedback", which is not a new technology. It was proposed by Bell Labs in the United States as early as the 1920s and 1930s. Except that the calibration (feedback) is added to the output, the concept is completely "feedback".

The feedforward linear amplifier is composed of two loops through couplers, attenuators, synthesizers, delay lines, power dividers, etc. After the RF signal is input, it is divided into two paths by the power divider. One path enters the main power amplifier. Due to its nonlinear distortion, the output end has third-order intermodulation interference in addition to the main frequency signal that needs to be amplified. A part of the signal is coupled from the output of the main power amplifier, and the main carrier frequency signal of the amplifier is offset through loop 1, leaving only the inverted third-order intermodulation component. After the third-order intermodulation component is amplified by the auxiliary amplifier, it is offset by loop 2 to offset the intermodulation component generated by the nonlinearity of the main amplifier, thereby improving the linearity of the power amplifier.

Feedforward technology provides the advantages of higher calibration accuracy without the disadvantages of instability and bandwidth limitation. Of course, these advantages come at a high cost. Since the power level is high during output calibration, the calibration signal needs to be amplified to a higher power level, which requires an additional auxiliary amplifier, and the distortion characteristics of the auxiliary amplifier itself should be above the indicators of the feedforward system.

The offset requirements of the feedforward power amplifier are very high, and the amplitude, phase and delay must be matched. If there are power changes, temperature changes and device aging, the offset will fail. For this reason, adaptive offset technology is considered in the system so that the offset can keep up with the changes in the internal and external environment.