The role of mobile phone amplifier

What is an amplifier?

A power amplifier (PA), commonly known as a "loudspeaker", is the most basic device in an audio system. Its task is to amplify the weak electrical signal from the signal source (from the mixing console in a professional audio system) to drive the speaker to make sound.

Characteristics requirements of mobile phone amplifier

GSM/GPRS quad-band mobile phones transmit signals based on the ETSI/3GPP communication standards, so the characteristics of the power amplifier must meet the following requirements.

Generally speaking, the PA output power level in the GSM850MHz/900MHz frequency band is divided into 15 power levels, such as 5 (33dBm) ~ 19 (5dBm), and in the DCS1800MHz/PCS1900MHz frequency band, there are 16 power levels, such as 0 (30dBm) ~ 15 (0dBm). As the distance between the mobile phone and the adjacent base station and the reception condition of the mobile phone are changed, the PA output power level must be relatively adjusted and accurately set according to the instructions of the base station. There is not only a single output power.

Because the GSM/GPRS communication system is a TDMA system (Time-Division-multiple-Access), the transmission (up-Link) and reception (down-Link) of signals do not occur at the same time. The PA is mainly responsible for transmitting mobile phone signals to the base station. Its PA power operation response time must comply with the specifications of the ETSI/3GPP communication standard. As shown in Figure 1, it is divided into three time zones: (a) 28us (b) 542.8us (c) 28us. In zone (a), when the mobile phone wants to transmit signals to the base station, the PA must prepare for Power-ramping-up within 28us to keep the PA output power at a stable and fixed value so as to start the actual mobile phone signal transmission work. This is zone (b). When the signal transmission is completed, the PA Power must be turned off within 28us in zone (c) to save battery current consumption and enter the receiving mode (receiving base station signals). In addition, there are two points worth mentioning:

(1) In regions (a) and (c), the PA’s response speed must be fast enough to achieve full power output and no power output within 28us, respectively. In addition, the power output ramp-up or ramp-down rate must also be appropriately fast to achieve a very smooth power rise and fall curve. Otherwise, the so-called switching spectrum noise is likely to be generated, thereby affecting the call quality of nearby mobile phone users.

(2) In region (b), the PA output power must be kept very stable within a power variation range of ±1dB. Otherwise, the signal that the mobile phone wants to transmit will easily be modulated by the PA due to the unstable power of the PA itself, thereby generating modulation spectrum noise. This will greatly increase the BER (Bit-error-rate) of the mobile phone itself during calls, resulting in poor call quality.

Figure 1 PA output power vs. time diagram

Because the maximum distance between a mobile phone and a base station is about 35 kilometers, the output power of the PA is about 30~35dBm, so the PA needs a larger supply current, which can be as high as 1.6~2A. The output efficiency of a general PA is about 50%, plus the PA duty cycle (duty

cycle) to GPRS Class 12 will reach 50%

(4 time slots), so the PA module will generate a lot of heat in the IC itself, so there must be good heat dissipation treatment, otherwise the PA will be easily damaged by overheating.

Because mobile phones are often used in very bad environments, such as high-speed driving, severe weather conditions, etc., the phone's receiving sensitivity (Sensitivity) is required to be very high. At the same time, the output power required by the PA is also relatively high, so the requirements for the receiving frequency band noise power and other characteristics generated when the PA is working will be more stringent.

Generally, in the normal operation mode of a mobile phone PA, the impedance seen at the output end is a 50Ω load. However, when the user improperly uses the mobile phone, such as holding the antenna or even unplugging the antenna, the PA load impedance will completely deviate from the normal operating 50Ω load. This is called PA mismatch. In this case, the PA power cannot be delivered, which will cause more heat to be dissipated on the IC, which may easily cause the PA to burn out. In addition, because the PA itself is a high-power component, in addition to output power, it will also generate a lot of thermal noise. The PA itself will have a big stability problem. If a mismatch occurs again, it is more likely to cause the PA to oscillate, thereby generating noise of other frequencies (Spurious Oscillation noise), affecting other mobile phone system users. Therefore, the PA design itself must ensure that the PA will not oscillate or burn out under mismatch conditions.

As mentioned above, PA is a high-power component, and its output power is impedance matched by microstrip transmission lines, so RF signals are easily radiated through dielectric coupling and air to the PA peripheral mobile phone communication circuit, and even affect nearby mobile phone users. The most typical example is that the signal is coupled to the VCO in the PLL, which easily causes the VCO frequency to shift, which will greatly affect the call quality of the mobile phone itself, so the shielding and isolation of the PA design itself is a challenging topic.

PA Technology and Operation In recent years, PA design has mostly adopted a multi-chip module (MCU) structure due to the poor Q value of the IC output impedance matching circuit, as shown in Figure 2.

Figure 2 Functional block diagram of PA

According to the functions, it includes the following three parts:

50Ω input/output matching circuit: to enable proper impedance matching and power transmission with peripheral circuits such as voltage-controlled oscillators and switches.

CMOS power control IC: This IC can be said to be the core of the PA module. Its functions include: providing HBT IC DC bias (DC-Bias), band selection (Band-Selection between GSM and DCSBand), temperature compensation, self-power measurement (to ensure that the PA output power is the required value of the base station), output power adjustment (different output power level adjustment), power switch (switching between transmission and reception) and protection device circuit (to prevent damage to the mobile phone due to improper use).

GSM/DCS HBT PA IC: The main function of this HBT PA IC is to amplify the power of RF signals. Currently, mobile phone PAs are all made of heterojunction double-digit transistors (GaAs)

In application, PA must be equipped with an external control loop to make its operation fully comply with the E-GSM/DCS/PCS standards, as shown in Figure 3.

Figure 3 Power control loop

In use, a VDAC value is sent through the baseband to set the power value that the PA needs to transmit, and then a sensing Schottky diode is used to measure the output power of the PA. The measured value is compared with the value of VDAC to obtain an error current, which is then integrated by the integrator Cin to determine the output voltage of VC. Then, the voltage value of VC is used to adjust the amplification power of the PA. In this way, a stable control loop system is formed to ensure that the PA output power is the power value set by the baseband.