High efficiency and low harmonic distortion class E RF Power Amplifier

In recent years, with the rapid development of wireless communication, wireless communication in the central part - the wireless transceivers are increasingly demanding lower power consumption, higher efficiency and smaller size, but as a transceiver in the last level, the power amplifier in the transceiver power consumption has accounted for 60% to 90%, severely affected the performance of the system. Therefore, the design of a high efficiency and low harmonic distortion of power amplifier transceiver for improving efficiency, reducing power consumption, improving system performance are of great significance.

I use a SiGe BiCMOS process to achieve an integrated Class E power amplifier, the operating frequency of 1.8GHz, the operating voltage is 1.5V, the output power of 26dBm, and has high efficiency and low harmonic distortion, is suitable for FM / FSK, etc. constant envelope modulation signal power amplification. In order to achieve the design goal, the power amplifier using a number of special methods, including the use of two-stage architecture, the differential and complementary cross-coupled feedback configuration.

E Power Amplifier

Class E power amplifier works

Class E power amplifier is characterized by the transistor to switch, compared to traditional transistors as current sources A, B, AB power amplifier with higher power added efficiency (PAE, power added efficiency).

Figure 1 shows an ideal class E power amplifier schematic. Where, C for the FET junction capacitance and the external capacitors and, ron for the FET in the linear zone of the drain-source resistance.

High efficiency and low harmonic distortion class E RF Power Amplifier

Figure 1 E class power amplifier schematic

When the input voltage is greater than the threshold voltage, the FET operating in the linear area, the equivalent switch is closed, due to a very small drain-source resistance between ron, so VD is approximately 0; and when the input voltage is less than the threshold voltage, field effect of tube end, the equivalent of switching off, ID to 0. At this time, C starts charging, causing increased VD, VD in the filter tuning network from a fundamental, transfer to the load resistor. When the switch closes again, there are VD = 0 and dVD / dt = 0, which makes the FET on the same voltage and current do not appear to eliminate the discharge brought about because (1 / 2) CV2 depletion transistors ideal efficiency of 100%.

In addition to high efficiency, E class power amplifier and a benefit is scalability, that is, the output efficiency at the same time ensuring greater range of adjustment to output power. Because the equivalent FET switches, so will not affect the amplitude of input voltage output size. Similarly, the spot effect triode tube in the zone, the resistance between drain and source ron there will be power consumption PLOSS, class E amplifier is the main power loss. As PLOSS and VD2 proportional to, we can drain efficiency is expressed as:

(1) high efficiency and low harmonic distortion class E RF Power Amplifier

Where, C is a constant. Thus, by adjusting the voltage to ensure a certain output power, E class will be able to maintain a high efficiency power amplifier.

Problems

Class E power amplifier also has many limitations. For example, because VD VDD Congress than about three times, so when the design must take into consideration the impact of the breakdown voltage, this will make the output power range have limits. In addition, in order to reduce the loss caused by ron, as far as possible to increase the width to length ratio, but the greater area of the transistor, will cause a larger gate capacitance, making the need for a smaller input inductance coupling, This input signal will be higher demands, it is difficult to achieve accuracy by BiCMOS technology. And large gate leakage capacitance will cause output to the input of strong feedback, which led to the coupling between input and output. Finally, the single-ended output circuit for each cycle should gravitropism or silicon substrates a large discharge current, which may lead to substrate coupling current frequency and input and output signal the same frequency, thereby produced the wrong output signal.

Circuit Design and Improvement

Figure 2 shows the structure for the two differential amplifier, including M5, M8 sub-structure for the first differential amplifier, the second-class power amplifier for providing a large driving voltage; M1 and M2 sub-component second differential amplifier, The M6, M7 and M3, M4, respectively constitute one or two cross-coupled positive feedback structure.

Efficiency and low harmonic distortion class E RF Power Amplifier

Two coupled differential amplifier

Differential structure

Figure 2 shows a fully differential structure to address the impact of substrate coupling. As the differential structure, the double-ended output for each cycle will discharge twice gravitropic current, thus the frequency of the coupling current became twice the signal current, which eliminates the substrate coupling on the signal interference. In addition, the same power supply voltage, while providing the same output power when the stream of fully differential structure, the current of each switch is much smaller than the single-ended output, so not to increase the switching losses of the premise, you can use the size smaller transistors, which reduces the input signals.

LC oscillator

In order to reduce losses caused by ron, and to increase the switching speed, typically M1 and M2 of the aspect ratio will be doing bigger, so to will of the input signals higher demand.

Figure 2 shows a power amplifier using mode locking techniques, namely the LC oscillator structure, not only further reduce the switch size, and speed up the conversion rate of the switch. By the M3, M4 constitute the cross-coupled oscillator part, to provide negative resistance to compensate for the inductor L1, L2 loss caused, and the introduction of positive feedback input switch. Work so that when LC oscillator power amplifier input frequency, because of its output in M1 and M2's drain, will help enter the switch to complete in the shortest possible time, "open" and "off" state of change, which can further reduce the size of input switch. By adjusting the LC oscillator, results in output to input frequency oscillation occurs, thus speeding up the switch opening and closing speed, to reduce the switch aspect ratio purposes.

Furthermore, compared with single-port power amplifier output structure, as shown in Figure 2, the structure of cross-coupled power amplifier, in practical applications will be lower total harmonic distortion (THD). Because the use of a fully differential structure, the output port will substantially weaken the even harmonics, the harmonics in the output of odd harmonics dominant.

Simulation results and analysis

This circuit uses 0.35μm SiGe BiCMOS process simulation, because the SiGe transistors with higher cutoff frequency, consistent with the requirements of the operating frequency at 1.8GHz. In addition, CMOS process with good compatibility, can achieve highly integrated chips.

In the Cadence simulation tool adopted after SpectreRF, the output power and power added efficiency (PAE) versus frequency curve (Figure 3). When the power supply voltage of 1.5V, when in the 1.8GHz, PAE reached the maximum 45.4%, maximum drain efficiency of 66.2%, then the output power of 26dBm.

High efficiency and low harmonic distortion class E RF Power Amplifier

Figure 3 PAE and output power versus frequency curves

Can be seen from Figure 4, even harmonics in the output is not dominated, it is greatly weakened, compared to single-port power amplifiers, harmonic distortion of the device has greatly improved. When the input frequency of 1.8GHz, the power supply output current shown in Figure 5, by calculating the power output can be as 595.5mW. Figure 6 shows the drain voltage VD through the tuning network of the base after the second wave to retain part of the next wave, it can be calculated load (50Ω) the power to 394mW.

Low harmonic distortion and efficient class E RF Power Amplifier

Figure 4, the output harmonic

High efficiency and low harmonic distortion class E RF Power Amplifier

Figure 5 Supply Current

High efficiency and low harmonic distortion class E RF Power Amplifier