With the rapid development of communication systems, all countries are now accelerating the pace of 4G network development. The advantages of higher speed, better quality, and more flexibility make 4G networks unbeatable superiority. High data transmission rate and multimode transmission have a great influence on the communication system. The complex modulation method undoubtedly puts forward a higher linearity requirement for the core module power amplifier in the base station, and also requires a higher channel bandwidth. Research has also become a hot area of ​​research today. DXY Dingxin has also made significant investments in microwave radio frequency research for a long time, covering wireless communications (relay stations, base stations, RRUs, WLANs), WIFI, CMMB, ISM (industry, science, medical), etc. The company's microwave radio frequency experiments The room has become the application engineering laboratory of the Institute of Microelectronics of NXP and Tsinghua University. The reverse Doherty power amplifier designed in this paper is one of the products.
As the core component of a transceiver, PA accounts for about 1/3 of its system energy consumption. Traditional base station power amplifiers improve linearity by simply sacrificing power amplifier efficiency through power backoff. In order to reduce the operating costs of communication operators, complying with the development trend of green communications and improving the efficiency of PA are urgently needed. The traditional DPA can also provide sufficient efficiency within a certain range of power backoff, which means that the research on DPA is particularly important. Compared with the traditional DPA, the 1/4-wavelength transmission line after the peak-amplifier of the IDPA structure compensates the phase and also reduces power leakage, further improving power amplifier efficiency.
Second, the working principle of IDPAThe traditional DPA includes a carrier power amplifier and a peak power amplifier. The carrier power amplifier operates in Class B or Class AB. The peak power amplifier operates in Class C. The carrier power amplifier is always working, and the peak power is turned on only when the peak power is on. When the carrier power amplifier reaches a saturation state and the peak power amplifier is about to start working, the efficiency reaches a theoretical maximum value of 78.5% for the Class B power amplifier. As the input signal is gradually increased, the peak power amplifier is turned on, thereby reducing the apparent impedance of the carrier power amplifier, which is equivalent to connecting the carrier power amplifier in series with a negative impedance. At this time, the carrier power amplifier is always in a saturated state, and its output voltage is constant. As the output impedance continuously decreases, the output current of the carrier power amplifier also becomes larger. When the peak power amplifier reaches saturation, its efficiency is also maximized. A Class B amplifier can only reach its maximum efficiency when it is at its peak, and this Doherty amplifier can only reach its maximum efficiency at half the peak value.
The 1/4 wavelength transmission line behind the carrier power amplifier plays a role in impedance conversion, and also causes the two power amplifiers to have a 90° phase difference. To make the two power amplifier outputs in phase, it is necessary to add a 90° phase shift before the peak power amplifier. The DPA block diagram is as follows
Compared with the traditional DPA structure, the 1/4 wavelength transmission line of the IDPA structure is added behind the peak power amplifier, and at the same time as phase compensation, it can also reduce power leakage and further improve power amplifier efficiency. The IDPA structure is shown below.
For DPA technology, the peak amplifier only works when the carrier amplifier reaches saturation, so that the efficiency of the amplifier can be maintained at a high level within a certain range of back-off, and will not drastically decrease.
During the small signal phase, the carrier amplifier is operating normally and the peak amplifier is off.
At this point, the peak amplifier is equivalent to the open circuit state, and the impedance Z2 is infinite, which prevents the power leakage of the carrier amplifier. The 1/4 wavelength transmission line after the carrier power amplifier functions as an impedance transform, ie Z0*Z0=Z1*Z3, Z4*Z4=R*Z3. When Z0=R=50 ohms and Z4=0.707R, Z3=25 ohms, Z1=2Z0=100 ohms.
During the large signal phase, the peak amplifier is turned on and can be seen as the active load of the carrier power amplifier.
Assuming that the carrier power amplifier and the peak power amplifier are equal power inputs, then the output impedance Z1 of the carrier power amplifier gradually decreases from 2R, so that more power can be output, so that the efficiency of the IDPA is always maintained at a high level, and the power does not fall back. Within a certain range, efficiency has dropped dramatically. Because the circuit network of the two power amplifiers is the same, there will be the same output power, the same size of current output. At this point Z1 = Z2 = Z3 = Z0 = R = 50 ohms.
Third, IDPA designThe power amplifier selected for this design is NXP's BLF8G27LS-100P. The carrier amplifier operates in Class AB, the gate voltage is selected as 1.99V, the peak amplifier operates in Class C, and the gate voltage is selected as 0.67V.
Input and output matching circuits are designed for different saturation power and efficiency. The drain adopts a bilateral balanced feed structure. The circuit is composed of RF filter capacitor, envelope frequency decoupling capacitor and 1/4 wavelength microstrip line. The length of this microstrip line can be adjusted according to the performance of the power amplifier to shorten the envelope frequency as much as possible. Voltage. Through the optimization of the bias circuit network, the memory effect is further reduced and the video bandwidth VBW is increased to improve the wideband characteristics of the power amplifier.
The optimal load impedance of the power amplifier tube changes with the input power. Under specific power and efficiency requirements, the input and output matching network and the bias circuit network can be adjusted to achieve a good compromise between power and efficiency. . The physical circuit of the design after optimization is as follows
Testing AM-AM curves at 2500MHz, 2595MHz, and 2690MHz in the 2500MHz-2690MHz frequency range
2500MHz AM-AM
2595MHz AM-AM
2690MHz AM-AM It can be seen from the above AM-AM curve that the gain fluctuation is about 0.1dB in the range of -30dBm to -16dBm input power, and the gain linearity of the power amplifier is good.
WCDMA single-carrier signal test with a peak-to-average ratio of 10.3dB
Freq/MHz250025952690
Gain/dB14.214.614.1
P-1/dBm50.149.4649.82
P-3/dBm50.9751.3550.62
Peak power/dBm51.2351.4350.81
I/A (28Vdc/Pout = 43.5dBm) 1.971.951.96
Eff40.58%41.0%40.79%
The above table shows gain, power and efficiency in the frequency range of 2500MHz-2690MHz. The gain fluctuation is 0.5dB. The change of power and efficiency in the whole frequency band is in good agreement, which shows that the PA can meet wider bandwidth requirements.
Test ACP with WCDMA Quad Carrier Signal with 19.84MHz Bandwidth
Total Carrier
Power(dBm)ACP Lower
(dBc)ACP Lower
(dBc)PAR(dB)
2595HMz41.46-31.87-33.699.8
V. ConclusionFor the peak-to-average ratio of 4G-LTE broadband signals, this article uses NXP (NXP) BLF8G27LS-100P power amplifier to design a reverse Doherty power amplifier, while maintaining the power amplifier efficiency at a higher value, but also got A good linearity shows that the power amplifier designed by DXY RF Labs can be applied to various communication systems such as 4G-LTE.
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