微波電路期中報告 論文 研討 : Linearity Improvement of High Efficiency Envelope Tracking Amplifier using Dynamic Gate Voltage Takeshi Okada, Yoji Isota, Takayuki Sasamori, Teruo Tobana Faculty of System Science and Technology, Akita Prefectural University 84-4, Tsuchiya-Ebinokuchi, Yurihonjo, Akita , Japan 報告人 : 碩研電子一甲 MA 林殿朗 Southern Taiwan University Department of Electronic Engineering

Abstract In the recent wireless communication systems, orthogonal frequency division multiplexing (OFDM) is widely used for its high spectral efficiency. OFDM signal envelope changes rapidly and the peak-to- average power ratio (PAPR) is very high. When amplifying OFDM signals, it is requested that the transmission RF power amplifier (PA) has enough high saturation power, to avoid the distortion of the signal. 1 Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012

Abstract In the recent wireless communication systems, orthogonal frequency division multiplexing (OFDM) is widely used for its high spectral efficiency. OFDM signal envelope changes rapidly and the peak-to- average power ratio (PAPR) is very high. When amplifying OFDM signals, it is requested that the transmission RF power amplifier (PA) has enough high saturation power, to avoid the distortion of the signal. 2 Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012

In general, maximum power efficiency of the PA can get in the compression or saturation region. But the linear amplification can get in the “backing off” region. This will decrease the power efficiency. Therefore, for the high PAPR signals, the average power efficiency is much lower than the peak efficiency. This trend becomes more remarkable in the next generation wireless broadband systems such as LTE or WiMAX. So, improvement of the power efficiency of the PA in the back-off region becomes very important. 3 Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012

4 Fig. 1. Block diagram of proposed ET amplifier that control both Vd and Vg Fig. 1 shows the block diagram of proposed ET amplifier. The voltage corresponding to the amplitude of the envelope is generated in the variable Vd supply part, and it is supplied to RF-PA. RF-PA is biased as class AB. The Vg is controlled together according to the amplitude of the input signal.

5 Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012 Fig. 2. IMD3 versus Pin with each Vg and Vd Fig. 2 shows the IMD3 characteristics in condition with the various Vg. These figures show when Vd is 2V, 6V, 10V, respectively. From these figures, we can see that when the input signal is small, IMD3 becomes small with high Vg. In the contrary, when the input signal is large, IMD3 becomes small with low Vg. So, there is an optimum gate voltage for each Vd.

6 Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012 Fig. 3. PAE versus Pin with each Vg and Vd Fig. 3 shows the PAE characteristics with each Vg and Vd. When Vd is high, PAE changes slightly when Vg changes. But, when Vd is low, PAE changes considerably when Vg changes. Based on these measurement results, we decide the value of Vg and Vd for each input power.

7 Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012 Fig. 4. IMD3 and PAE of each Vd at class AB operation Fig. 4 shows the result of PAE and IMD3 for each Vd. In these figures, we fixed the Vg value to -2.5V.

8 Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012 In each input power, we decided the Vd value to obtain maximum PAE. Under this condition, we can get the IMD3 for each input power. And this is considered to be a performance of a normal ET amplifier. In this case, normal ET amplifier can get the good PAE characteristics, 21% higher than the class AB amplifier (Vg=-2.5V, Vd=10V). In contrast, the linearity becomes worth, and the IMD3 is 32dB higher.

9 Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012 Fig. 5. PAE and IMD3 in each operation (simulation) Fig. 5 shows the simulated PAE and the IMD3 comparison with three types of the amplifier. One is the ET amplifier with variable Vg and Vd, and the other is the normal ET amplifier with variable Vd only, and the last one is class AB amplifier with constant Vg and Vd. The ET amplifier with variable Vg and Vd shows the good improvement in IMD3 of 33dB or less, and the degradation of the PAE is about 8%.

10 Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012 The highly efficient variable Vd supply part is critical to the ET amplifier, because the total system efficiency is the product of the variable Vd supply part efficiency and RF transistor drain efficiency. In this paper, we don’t use the variable Vd supply circuit and variable Vg supply circuit. So, we assume the ideal variable Vd and Vg supply circuit.

11 Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012 Fig. 6. Vd and Vg of ET amplifier Fig. 6 shows the Vd and Vg voltage of the ET amplifier. The variable Vd supply circuit can be realized, because the drain voltage is simply increase with Pin. But the gate voltage change is not simple. Realization of the variable Vg supply circuit will be a future work.

CONCLUSION To improve the linearity, we propose the ET amplifier controlling both the Vd and Vg. We measured the required Vg and Vd for the good efficiency and linearity. From the simulated results, improvement of the IMD3 is 33dB compared with a normal ET amplifier. The degradation of the PAE is about 8%. 12 Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012

心得 這是一個用 Gate 電壓改善追蹤訊號封值的高效率放大 器，在文中用了許多參數去做比較，得出一個較佳的 參數去做出這個高效率放大器，控制 drain 和 gate 電壓 來提高 IMD3 ，這比其他的追蹤訊號封值放大器高出約 33dB 。 13 Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012