Some people say that battery life is the most important user demand in mobile phones. Even when consumers are eager for advanced multimedia features, they are reluctant to get these features and give up long talk time and standby time. Even if cell phone designers extend battery life, they are facing “conflicting†demands and adding new features that consume more power. Although battery technology has continued to improve in recent years, there is no breakthrough technological innovation. The task of improving efficiency falls on IC design manufacturers, and lower power consumption allows for better power management.
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In mobile phones, the power amplifier (PA) that drives the antenna is the largest consumer of battery power. By increasing the efficiency of all output power levels of the mobile phone, power consumption can be effectively reduced, thereby extending battery life. This article describes the HELPTM (Low Power High Efficiency) used by ANADIGICS' CDMA and WCDMA power amplifiers to meet the power requirements of mobile phones.
To improve efficiency, we first need to assess the power levels required for most mobile phone communications in urban and suburban environments. In this regard, we can refer to the data published by the CDMA Development Team (CDG). The power level distribution chart published by the CDG shows that during most of the time when the mobile phone is turned on in both environments, most mobile phones operate at a much lower power than the maximum transmit power. This is because most mobile phone users are usually close to the mobile phone tower when they make a call, so the mobile phone only needs relatively low output power when working. For example, when the wireless standard requires a maximum output power of approximately +28 dBm, more than 80% of the calls require less than +10 dBm. Unfortunately, traditional PAs are significantly less efficient at low power levels, which increases current consumption. Increases efficiency when operating at low power levels, significantly extending battery life. A standard WCDMA amplifier delivers 42% efficiency at +28dBm output, and a efficiencies of only 8% at output +16dBm, with a quiescent current of approximately 50 mA.
Traditional RF power amplifiers are bipolar GaAs devices that convert power between high and low power levels with a switching threshold of +16 dBm. A common method is to use an external DC-DC converter to switch the voltage of the amplifier to maximize PA efficiency. The disadvantage of this approach is the addition of additional components and costs in the bill of materials (BOM) and wasted board space. ANADIGICS' HELPTM PAs, based on its patented InGaP-PlusTM technology, provide a better and cheaper solution. InGaP-PlusTM allows circuit designers to integrate high-performance HBTs and high-performance pHEMTs on the same substrate. This resulting BiFET technology can use HBT to build high linear amplifiers and use pHEMTs to build fast, low loss switches. PAs using BiFET technology can maximize efficiency without using an external converter to achieve low output power. The pHEMT switch allows different amplifier links to be selected in the PA, depending on the output power requirements. The benefit is that the efficiency is more than 2 times at medium output power and from 8% to 21% at 16 dBm. As a result of increased efficiency, the average power consumption will be reduced by 50%. Through the processing of three power level approaches, the 3rd generation HELP device (called HELP3) reduces power consumption by up to 75%, and the quiescent current is also significantly reduced. The HELP technology reduces the quiescent current from 50mA to 15 mA, while the quiescent current of HELP3 Only 7 mA (Table 1).
Table 1. shows the three related PAs specifications.
How is this achieved in practical applications? Considering a typical mobile phone in an urban environment, the receiving circuit and the baseband portion consume 125 mA. When the power consumption of other transmitting circuits is equal, the power consumption of the transmitting circuit will vary from the PA used (the power consumption when transmitting the circuit includes not only the PA but also other components such as the RF driving amplifier). The following are the current consumption of the three options in the call mode:
1. Mobile phone, using a traditional power amplifier (two-state PA):
The current consumption of the PA is 70mA, and the current consumption in the call mode is: 116mA (transmitting circuit) + 125 mA (baseband & receiving circuit) = 241 mA
2. Mobile phone, using HELP power amplifier (two-level BiFET):
The current consumption of the PA is 34 mA, and the current consumption in the call mode is: 81 mA (transmitting circuit) + 125 mA (baseband & receiving circuit) = 206 mA.
3. Mobile phone, using HELP3 amplifier (three-level, BiFET):
The current consumption of the PA is 18mA, and the current consumption in the call mode is: 68mA (transmitting circuit) + 125 mA (baseband & receiving circuit) = 193 mA.
The HELP and HELP 3 quiescent currents can be reduced by 70% and 86%, respectively, which will also help reduce power consumption in the handset during standby.
Of course, power consumption is not the only reason to choose a PA. Linearity, noise, and system-level performance such as support for high-end services (HSDPA) are all taken into account. In any case, the basic performance of the amplifier must comply with the specifications.
After meeting the basic performance requirements, start the power management of the PA. Higher integration, added more features, also saves space and reduces bill of materials. For example, one difference between HELP and HELP3 technology is that the HELP3 PA has a built-in voltage converter.
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