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With Supply Voltage Control, the collector or drain voltages of the last one or two amplifier stages are controlled. This limits the voltage swing of the RF power transistors.
Advantages:
- Output power is predictable after a simple calibration
- If is TDMA (GSM) the burst template (power versus time) can be easily shaped
- Control of PA is possible over a wide dynamic range
Disadvantages:
- A regulator (MOSFET) is required to switch the large supply current, adding size and cost. The MOSFET also has up to 100 milliohms of resistance, which reduces overall system efficiency.
Designers will sometimes use the supply-voltage control architecture in designs that already need an external MOSFET switch in the supply line. However, in designs not offering an additional MOSFET, the use of the supply-voltage control approach will probably be avoided.
Supply Current Control it use a Resistor to sense the current.
If the output power versus supply current characteristic is known for a given PA, the output power can be accurately set.
The advantage of this configuration is the possibility to integrate the power control circuit on the same chip with the PA, with only the sense resistor external.
Disadvantages:
- Power loss at higher power levels due to voltage drop across sense resistor.
- Limited dynamic range due to low sensitivity at low power levels. The voltage across sense resistor becomes small compared to noise and offset voltages of the error amplifier
- Sense resistor should be temperature stable and able to handle the high current, which requires a relatively large surface-mount component.
Thank your reply . I design mobile-phone .When I test the power level at PL 15 in DCS band ,the roll off of power level is very large ,about 3.6dbm between CHN512 and CHN885. I test the impedance from PA to FEM , that is very close to 50ohms . the PA control is current control .
How can I do ? Could you give me some advice ?
Hi
First of all don’t forget the spec GSM11.10 or 05.05 allow in DCS for PCL15 a conducted power tolerance of +/-5dBm, from nominal.
Conducted output power roll-off across frequency is normal, but 3.5dB is too much. Check the output match, or if this one is integrated in the PA check (using the Network Analyzer) for parasitic resonance’s between PA output and antenna connector (coupler, LPF, switchplexer, diplexer, TL etc).
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