Linear RF Power Amplifier Technology

RF power amplifiers to produce the distortion will be a new frequency components, such as to produce second-order harmonic distortion and two-tone beats, for the third-order distortion will produce the third harmonic and polyphonic beats. These new frequency components, such as fall within the passband, will launch a direct interference with the signal, if the fall outside the passband signals will interfere with other channels. This should be the RF power amplifier linearization, it can solve the spectrum signal regeneration. RF power amplifier linearization technology, the basic principles and methods are nothing more than input RF signal envelope amplitude and phase as a reference, and compare the output signal, and then generate the appropriate correction. The RF power amplifier linearization, there are three common techniques: power back to back, pre-distortion, feed-forward.

1, the power regression

This is the most commonly used method, that is, the larger power tubes used for low-power to control the use, in fact, at the expense of DC power to improve the amplifier linearity.

Regression method is to power amplifier input power from the 1dB compression point (amplifier has a linear dynamic range, in this range, the amplifier output power increases linearly with the input power. With the continuing increase of input power, amplifier gradually into the saturated zone, power gain began to fall, usually the gain down to 1dB lower than the linear gain value when the output power is defined as the output power of 1dB compression point, with P1dB said.) back back back 6-10 dB work in much less than 1dB compression point level, so that the amplifier away from saturation into the linear working area, thereby improving the power amplifier's third order intermodulation coefficient. In general, when the fundamental power reduced 1dB, the third-order intermodulation distortion to improve 2dB.

Power regression method is simple and easy to achieve, without adding any additional equipment, is to improve the amplifier linearity effective way to the disadvantage of greatly reduced efficiency. In addition, when the power fall back to a certain extent, when the third order modulation to achieve-50dBc below to continue to rollback will no longer improve the amplifier linearity. Therefore, the high linearity requirements of the occasion, completely on the power rollback is not enough.

2, pre-distortion

Pre-distortion is a nonlinear increase in the power amplifier before the power amplifier circuit to compensate for nonlinear distortion.

Pre-distortion linearization techniques, it has the advantage of stability does not exist, there is a wider signal bandwidth, can handle with multi-carrier signal. Predistortion lower cost, by a few carefully selected components packaged into a single module, and even between the signal source and amplifier, to form pre-distortion linear amplifier. Handheld mobile Taichung amplifier pre-distortion techniques have been used, it is only a small number of components to reduce the intermodulation products of a few dB, but is critical of several dB.

RF pre-distortion technology into pre-distortion and digital baseband pre-distortion are two basic types. RF pre-distortion to achieve the commonly used analog circuit with a circuit of simple structure, low cost, easy-to-high-frequency, broadband applications, advantages, disadvantages that renewable component to improve the low frequency spectrum, high frequency components to offset the more difficult.

Digital base-band pre-distortion as low frequency, can use digital circuit, adaptable, and can be increased by increasing the sampling frequency and quantized step several ways to offset the high order intermodulation distortion, is a promising Methods. This predistorter gain regulator from a vector composition, according to look-up table (LUT) to control the content of the input signal's amplitude and phase pre-distortion of the size of the input from the lookup table to control. Once gain regulator vector optimization, will provide a contrast with the amplifier nonlinear characteristics. Ideally, when the output intermodulation products should be two-tone signal through the amplifier output range of the same phase on the contrary, adaptive adjustment module is to adjust the input lookup table, so that the input signal and the amplifier output signal of the difference between the minimum . Noted that the envelope of the input signal is an input lookup table, the feedback path to sample the output power amplifier distortion, and then through the A / D transform into the DSP in the adaptive regulation, and then to update the lookup table.

3, feedforward

Feedforward technology originated in the "feedback", it is not a new technology should be back in the twenties and thirties on the Bell Labs by the United States put forward. In addition to calibration (feedback) is the increase in output, the concept completely, "Feedback."

Feedforward amplifier via coupler, attenuator, synthesizer, delay lines, splitters composed of two loop. RF signal input, divided by two-way splitter. All the way into the main power amplifier, because of its nonlinear distortion, in addition to the need to enlarge the output of the frequency signal, there are third-order intermodulation interference. From the main amplifier output coupling part of the signal through the loop a main carrier frequency offset signal amplifier so that only the third order intermodulation RP. Third-order intermodulation by the auxiliary amplifier, offset by the main amplifier nonlinear loop 2 produced intermodulation, thus improving the amplifier linearity.

Feedforward technology provides both the advantages of high accuracy calibration, no bandwidth is limited by instability and shortcomings. Of course, these advantages are paid for with a high cost, because the output calibration, higher power level, calibration signal be amplified to a higher power level, which requires additional auxiliary amplifier, and requires the auxiliary amplifier itself distortion should be at the top of feed-forward system of indicators.

Feedforward power amplifier offset requirement is very high, need to be amplitude, phase and delay matching, if power changes, temperature changes and device aging offset porosity, causing failure. To this end, consider the adaptive offset in the system technology to offset the change to keep up with internal and external environment.