PA is an important component module of the communication system, which is responsible for amplifying the RF signal and power output.
In the mobile communication system, the output signal of PA is about 29 ~ 32 dBm about 1,000 mW . Compared with the power output of the transceiver chip about 0 dBm 1 mW , the output power of PA is 1000 times larger.
Figure 1: Typical cell phone communication system link
Why 'burn PA'?
1.Semiconductor devices in mobile phone PA chip
The semiconductor processes commonly used in PA design are GaAs HBT, GaAs pHEMT, SOI, Bulk CMOS. GaAs HBT has become the preferred process for the power output stage of RF power amplifier because of its high power density and low cost compared with GaAs pHEMT .
The HBT device has three limiting parameters, which are :
2.Voltage/current swing of PA
For RF PA HBT devices, the collector of the output transistor has the maximum voltage and current swing. In order to better characterize the relationship between voltage and current, the voltage and current are usually drawn in the same picture by means of Loadline load line, as shown in Figure 2.
Figure 2: Typical dynamic load line for a PA
The load line of the transistor reflects the relationship between the voltage and current of the transistor under different loads, which is generally drawn on the DC-IV curve. In the load line :
-
The slope of the load line reflects the magnitude of the load impedance.
-
In actual circuits, the presence of the imaginary part of the load creates a voltage/current phase difference that may make the load line behave as a hollow loop.
-
The choice of DC operating point voltage and current has an effect on the swing of the load line.
In PA operation, the presence of high supply voltage and high V.S.W.R will increase the PA output voltage and current swing. Figure 3 shows a typical power amplifier with dynamic load lines at 3.2V and 5V bias voltage, 50Ω and VSWR=10:1 respectively. When the voltage and current swing exceeds the tolerance value of the device, it will cause the device to burn out.
Figure 3: Variation of PA dynamic load line for different voltages and V.S.W.R.
How do I protect my PA from burning out?
1.Design Guarantee
PA needs to be rationally designed to ensure that ruggedness requirements are met.
-
Current Design Guarantee
-
Voltage Design Safeguard
-
Circuit design guarantee
Ruggedness test
The complete Ruggedness test environment is shown below:
Figure 5: Ruggedness test environment
The Ruggedness test needs to cover the following test conditions:
The above test items need to be cross-combined to ensure that the PA will not have Ruggedness problems under any conditions.
Generally, the worst conditions for Ruggedness occur at low temperatures under maximum input power, maximum voltage, and maximum VSWR.
Application assurance
The main safeguards needed in the application are as follows:
-
proper control of supply voltage
-
Proper Control of Output Power
-
Pay attention to power integrity and signal timing
Conclusion
PA Ruggedness design is a complex project, related to device physics, circuit design, and system application. In PA design, Ruggedness must be carefully designed to ensure that the cell phone will not be "burned" in various environments.
This article is excerpted from the RF Tube Official Account. Please contact for removal if there is any infringement.
