The probability distribution of output-based 3G handset power amplifier design optimization

Abstract: This paper discusses the concept of a new power amplifier design: the power amplifier driver circuit in general have to work in low power output, high output power is needed only when start-power high-power amplifier circuit. By understanding the actual use of mobile phones in case the output power probability distribution, in the limited battery capacity to extend the talk time effectively with the technology can not only effectively reduce the average current amplifier, and the realization is simple.

The current global 3G mobile phone penetration rate is rising progressively, while the new phone also integrates more and more features. Today's cell phones already have multimedia features: including video camera, megapixels camera module, auto focus, camera flash and white for the light LED, MP3, MP4 and JAVA games. In order to improve network coverage, many dual-mode phones using GSM / GPRS + CDMA, GSM / GPRS + WCDMA or integrated embedded wireless LAN (WLAN) design of the online features. To achieve such a rich, powerful features, the biggest problem is cell phone battery power and talk time, how to save battery power consumption has become a mobile phone designers have to face the most pressing problems.
Optimization based on probability distribution of output power amplifier design 3G mobile phone
Figure 1: CDMA mobile phone system using the probability distribution of output power.

Transmission signal power amplifier for mobile phones (PA) has been the largest mobile phone power consumption of components, it effectively reduces the power amplifier, the average current consumption is expected to greatly extend the phone's talk time. In this article, we will explore the concept of a new power amplifier design, the power amplifier driver circuit in general have to work in low power output, high output power is needed only when a higher launch power power amplifier circuit. This technique not only effectively reduce the average current amplifier, and the operation is simple. Understand the actual use of mobile phones in the context of the probability distribution of output power is limited battery capacity to extend the talk time effectively with the important foundation.

The key specifications of handset power amplifiers

Linearity: the linearity of power amplifier will directly affect the performance of mobile phones, such as CDMA2000 system, the adjacent channel power ratio (ACPR) and WCDMA systems in the adjacent channel leakage ratio (ACLR). And other channels to avoid interference between or system, regardless of the circumstances, it can not meet the requirements of these specifications is a compromise.

Power-added efficiency: power-added efficiency (PAE) is obtained by the output RF power amplifier and the percentage of input DC power:

Based on the probability distribution of output power amplifier design optimization of 3G mobile phone

The traditional design has focused on 3G power amplifiers in high power mode to obtain high PAE. For example, using E-pHEMT (enhancement-mode pseudomorphic high electron mobility transistors) the traditional CDMA power amplifier +28 dBm output power can reach 40% of PAE, but in the medium and low power output will drop when the PAE is very low.

Based on the probability distribution of output power amplifier design optimization of 3G mobile phone
Figure 2: the best of traditional and compare the characteristics of power amplifiers.

When using the bypass 3G power amplifiers with high power levels of the design concept, rather than trying to model the efficiency of the high-power optimization, we will focus on how to improve real work situations include high, medium, low-power mode total talk time. If you can choose the drive circuit of the gain, power output capability and linearity and output circuits, including overall performance, including talk time can be greatly improved.

Gain Control: In the CDMA or WCDMA power amplifier bias control on the common can be divided into basically two types, namely analog bias control and digital bias control. Analog bias control is more complicated, partly because of the need to provide a stable base-band chip set, a variable analog voltage, and a more generous "on" or "off" digital control voltage is more difficult than many. In addition, analog control voltage to change often affect the degree of the amplifier gain stability, so that mobile phone output power adjustment more difficult, so the latest 3G power amplifier design is generally applied to digital control.

Amplifier how to extend the talk time

In most cases, to enhance the high-power mode power amplifier PAE usually only slightly improve the talk time. Figure 1 shows the actual CDMA system, mobile phone output power according to the probability distribution function was calculated to prove this hypothesis.

Based on the probability distribution of output power amplifier design optimization of 3G mobile phone
Figure 3: Bypass the concept of high-power-level circuit, adopt the traditional switch or amplifier using Avago's CoolPAM variable impedance transmission line in parallel.

Shown in Figure 1, regardless of cell phone use in urban or rural areas, most of it concentrated in the power output-10dBm to +10 dBm (low / medium power range) range, only in a few suburbs farther from the base station possible prolonged use of high-output power mode. If you only focus on improving the high-power mode 的 PAE, and in fact can not be substantially extended talk time, Suoyiwomen should focus on improving the Qiang, Di Shang power of PAE. Figure 1 also shows several different power amplifier module (PAM) in low, medium and high power mode, the relative current consumption. PAM PAM or simulation and intelligent compared to the traditional PAM Figure 1 in the middle and low-power range, the highest power consumption. Because of its gain range of nearly all power output remained constant, while the intelligent PAM modulation for digital bias, in the middle and low-power mode not only slightly lower gain, but also slightly improve the current consumption. Analog control PAM used in the variable gain control simulation of bias, although the talk time can be slightly improved, but it will cause a significant change in the gain. In addition, both the first and second generation of CoolPAM technology, have a unique digital bias control switch and the appropriate combination of gain, can CoolPAM amplifier in a significant reduction in low power mode current consumption, while reducing standby power consumption.

Average current and the call of time:

Average current = ∫ (PDF (current) dp

Talk time is proportional to the reciprocal of the average current. (Note: PDF for the probability distribution function, shown in Figure 1.)

PA is the average current mode of each possible output current multiplied by the-50dBm to +28 dBm output power range of the relative probability of integration should be used, then the call time and the average current proportional to the reciprocal.

According to this relationship, the average current is in fact under the system power output probability distribution, so received in accordance with the principles of 3G power amplifiers can be used to improve the design, low power PAE, and lower in the current low power consumption .

The design concept of the best 3G PAM

As can be seen from Figure 2, the traditional fixed-gain power amplifier is usually the form of design, +28 dBm output power in high-power mode can generally achieve 40% PAE, but in the +16 dBm output power Shique can only achieve 8% to 10%, and when the output power increases, most of the power amplifier design characteristics of linearity and ACPR will become even worse. Tebie time when the power amplifier near saturation, ACPR values will be greatly improved, but the right, the low-power mode, the difficulty to meet the ACPR Guigebingbu.
Based on the probability distribution of output power amplifier design optimization of 3G mobile phone
Figure 4: Different power amplifier in the urban and rural areas, the average power consumption comparison.

Optimized design concept of 3G PAM usually high, medium and low output power gain of two different designs. When the phone needs to work under high-power mode when the power amplifier can be set to high gain mode, but requires low output power situation, Ye can support low-Zeng Yi mode. By changing the amplifier gain, can greatly reduce, the low output power at work in the current consumption. It provides about 20% of the medium power PAE, but only to the traditional 8-10%. Although the low-power low-gain mode ACPR value will have some impact, but was able to provide sufficient margin to meet the ACPR of the constraints.

Design and Implementation of the best power amplifier

This level bypass power amplifier circuit (stage-bypass PA) application is actually very simple. Shown in Figure 3, when the phone require a high output power, parallel circuit of the second switch opens, the signal can be sent through two cascaded power stage amplification to achieve +28 dBm output power. When the phone only, the low power output, the switch can automatically turn off, through the bypass circuit and close the second stage bias. Therefore, the main design requirement is to provide first-class circuit, the low power required for the work of linearity and output power Rongliang.

This bypass feature to achieve the most simple way is to use transistors as switches, but the disadvantage is The transistors also need current to work, this will be additional to current Xiaohao increases the amplifier's overall power consumption, Yinci CoolPAM power amplifier is by Biangeng parallel transmission line impedance to achieve switching.

Comparison of the average current

Shown in Figure 4, compared with the conventional power amplifier, power amplifier optimized either in urban or rural, have lower average power consumption.

Average current is multiplied by each output power probability distribution function of the total combined calculated. For the traditional power amplifiers work in urban areas, the typical average current of about 118mA, in the suburbs, compared with 136mA. In both cases, compared with bypass circuit of 3G high-power amplifier can achieve approximately the average current of 36mA or less. If using a 850mA / H batteries, and baseband chipset, the average current consumption is 200mA, then the conventional power amplifier in the urban areas of talk time of 160 minutes, compared with 152 minutes in rural areas. With the elimination of additional power consumption of the bypass switching transistor CoolPAM power amplifier, if the same conditions, in urban areas up to 224 minutes talk time, even higher than conventional power amplifier 64 minutes.