A new RF power amplifiers for the predistorter

In a variety of RF power amplifier linearization technology, feed-forward technology with high linearity and bandwidth, but the circuit structure complicated, costly and inefficient, he was used mainly for large power amplifiers, in the direct release stations, pre-distortion techniques have certain advantages, his low-cost, power consumption, simple circuit structure. RF power amplifier distortion in the machine, the amplifier performance is good or bad depends on the characteristics of predistorter. Good predistorter can greatly improve the power amplifier linearity, and better spectral regrowth suppression. In this paper, a way to generate IM3 and IM5 were pre-distortion device, he could well improve the 3rd order and 5th order intermodulation.

A predistorter circuit structure

Basic harmonic generator circuit shown in Figure 1.

A new RF power amplifiers for the predistorter

The harmonic generator consists of two Schottky diodes with the same characteristics (this paper, the company Skyworks SMS3922), capacitors, resistors, and 180 ° 3dB coupler composed of the bridge. Two different diode bias voltage increases, so working in different non-linear zone, to produce the distortion signal. Capacitor resistance with certain band signal, harmonic generator in the input and Canada, two-tone signal Vice Vi:

Vi = Acosω1t + Acosω2t (1)

Where: A is a two-tone signal amplitude; ω1, ω2 is a two-tone signal frequency and ω1 ≠ ω2. And assuming that the nonlinear harmonic generator transfer function y = f (x), output V0. That is: V0 = f (Vi). To expand into power series of the form:

A new RF power amplifiers for the predistorter

By controlling the diode bias voltage, can be appropriately regulated type (2) of the coefficients bi (i = 1,2,3 ...), so that equation (2) were independently able to generate IM3 and IM5 signals, in the pre-distortion line nature of the circuit, with two harmonic generators, one is used to generate IM3 signal, another was used to generate IM5 signals. This RF power amplifier can be controlled separately by the 3rd order intermodulation products generated by 5-order intermodulation.

In the ADS in the circuit of Figure 1 Two-tone test simulation. Adjust the diode bias voltage, the output can be generated by the IM3 and IM5. The simulation results are shown in Figure 2 (a) and (b) below. From the simulation results can be seen that the output frequency component also contains, in order to eliminate the fundamental frequency component on, but also on the Figure 1 circuit is improved, plus all the way to the consumer circuit. The input plus a splitter, then the way the basic harmonic generator, the other way through the attenuator and phase shifter adjustment, then with the fundamental harmonic generator output through the combiner synthesis, shown in Figure 3 said.

A new RF power amplifiers for the predistorter

A new RF power amplifiers for the predistorter

Properly adjusting attenuator and phase shifter, making a road on the base frequency signal and the harmonic generator output baseband signals are equal in amplitude, the phase difference of 180 °, so you can eliminate the signal to baseband. The simulation results shown in Figure 4 (a) and (b) below.

A new RF power amplifiers for the predistorter

2 results

To demonstrate the research effectiveness of predistorter, the predistorter applied to 5W CDMA RF power amplifier, and two-tone test for ADS in the simulated circuit diagram shown in Figure 5.

A new RF power amplifiers for the predistorter

The simulation study, the main amplifier is used in Freescale's MRF9045 RF devices, vector modulator by the attenuator and phase shifter, used to adjust the signal's amplitude and phase distortion, the main amplifier to generate the third order and five order intermodulation components are equal in magnitude, the phase difference of 180 °, to maximize the cancellation of Third-and fifth-order intermodulation. ADS carried out in two-tone harmonic balance simulation (2-Tone HB), to test the power amplifier linearity. In the case without pre-distortion in the amplifier output power is 5W, the distortion of large, two-tone intermodulation IMD3 and IMD5 are shown in Figure 6 (a) below. When used in this article pre-distortion techniques, IMD3 and IMD5 be significantly improved as shown in Figure 6 (b) below.

A new RF power amplifiers for the predistorter

From the simulation results can be seen in the adoption of this article pre-distortion techniques, IMD3 and IMD5 were improved 14dBc and 9dBc.

3 Conclusion

In the simulation experiment confirms the validity of this predistorter. Be used 5W CDMA RF power amplifiers, the two-tone test showed that, IMD3 and IMD5 be improved, were improved 14dBc and 9dBc.

This article describes the use of harmonic generator to achieve a linear pre-distortion technology, because by adjusting the bias voltage of two diodes, to produce respectively IM3 and IM5, it is easy to end as the adaptive control of using the adaptive algorithm more accurate adjustment, IM3 and IM5 make a better improvement.