An important part of a cell phone is the power amplifier (PA), which transmits power to the base station through the antenna. A cell phone needs to meet the standard for communication. One of the cell phone types is the GSM cell phone, which has to comply with the GSM specification (www.edgi.org). Important requirements for PAs in a GSM cell phone are:
* Adjustable and precisely setable power.
* PA is not allowed to transmit outside its frequency band
* PA is only allowed to transmit in its own timeslot, not in others
When a PA is used without control, it is hard the meet the GSM specifications because the PA isn't accurate enough. Reasons for inaccuracy are:
* The PA is a non-linear device
* The PA is not stable over temperature
* Gain-control function of PAs have quite a spread and are therefore not exactly known
Figure 1 shows the response from the voltage-control-pin to the output power of the PA. Some points in the PA response curve are rather steep, which can be a problem for low-resolution DACs, which usually set the Vapc. The power level can't be set with the required accuracy. A linear response in dBs would be desirable to solve this. The gain-control curve depends on temperature, which is a combination of ambient temperature and self-heating of the device. Due to variations in the production process, the PA's gain-transfer function is not known exactly. Especially the steep parts in the gain-control curve can cause a large error.
A PA controller can linearize the transfer function from the DAC to PA output power, stabilize the power control with respect to temperature and make the interface from the baseband chip to the PA easy.
Figure 1: Typical power amplifier response.
Principle of a PA control loop
The key benefit of a PA control loop is its immunity to changes in the PA's gain function. When a PA controller is used, the PA control loop response is no longer a function of the transfer function. PA output power is plotted against Vapc in Figure 2. The block schematic of Figure 2 shows how a PA control loop works. Any temperature dependency in the gain control function of the PA will be eliminated because the PA is in the forward path of the system. When the loop gain is sufficiently large, the accuracy of the loop depends on the elements in the feedback path, which are the coupler and the detector.
Figure 2: Block schematic of a PA control loop.
The detector connected to a coupler measures the PA output power. Its measurement is compared with a set value (Vramp), which is controlled by the DAC of a baseband chip. The error between the set value and the measured power is forced to be zero by the error amplifier, which sets the Vapc of the PA. Based upon the set value Vramp (controlled by a DAC), the control-loop will set the PA's gain control voltage to whatever level is necessary to produce the desired output power level. The closed loop response is not longer dependent on the PA.
Implementation of a PA control loop
A practical implementation of PA control loop circuit is given in Figure 3. The control loop consists of a limited number of components. Besides the PA and the coupler all elements of the block diagram of Figure 2 are combined in the PA controller. The PA controller consists of three blocks:
* RF detector
* Ramp converter
* Error amplifier
Depending on the measured power, the detector is sourcing an amount of current. At high power levels it's sourcing more current than at low power levels. The ramp converter is the reference for the whole loop and determines the output power of the PA. The ramp converter translates a voltage into a ramp current (Iramp). The ramp current is a sinking current. The control loop will act to keep both currents equal in magnitude. The error amplifier will correct the difference in currents by adjusting its output voltage.
Figure 3: Power amplifier control loop.
Assume that at startup, the power is low. The detector current is smaller than the ramp current. Voltage will build up across the capacitor; the output voltage of the error amplifier will increase (Vcomp voltage is constant). As a result of this, the PA output power will increase and the detector will source more current. This keeps going until the detector current is equal to the ramp current such that voltage across the capacitor and the output voltage will stabilize; The PA output has reached its set value. If the PA output power is too high, the detector current is larger than the ramp current. The voltage across the capacitor will decrease and the output voltage will decrease as well. The power drops until the two currents are equal again.
Closed loop gain response
The circuit as described so far, has been designed to produce a constant output power level. If the detector has a high dynamic range, the circuit can precisely set PA output levels over a wide power range. National Semiconductor's LMV243 PA controller has a dynamic range of 50 dB typical. To set a PA output power level, the reference voltage, VRAMP, is varied. Non-linearity in the gain transfer function of the PA doesn't appear in the overall transfer function, Pout vs. Vramp (see Figure 4). The shape of the curve is determined by the response of the detector. As stated earlier, the elements in the feedback path of the control loop determine the gain transfer function. The detector is the main element in the feedback path. Therefore, the detector needs to be accurate, temperature-stable, and preferably linear in dBs to get a better performance. The only requirement for the control loop is that the gain control function of the PA has to be monotonic. With a linear in dB detector, the relation between Vramp and PA output power is linear in dBs as well, which makes calibration of the system easy.
Figure 4: PA Output power vs. Vramp.
GSM mobile phones are using Time Division Multiple Access (TDMA) to transmit their data. A period contains 8 time slots, one for each cell phone. This means that a power amplifier is allowed to transmit during 1/8 of the time. To prevent interference between two cell phones, a time mask profile is set. To meet the GSM time mask, the output power of the PA needs to ramp up and down sufficiently fast.
By varying the capacitor C across of the integrator in Figure 3 the response time of the loop is changed. A small capacitor will have a fast response time but can cause ringing in the output. Implementing a larger capacitor will give the loop good stability but will increase settling time. An RC-combination can be used instead of a capacitor if necessary. As described before, the VRAMP input value sets the RF output power. By applying a certain ramp profile to the Vramp-pin, the power level (Pout/dBm) of the PA is set to obtain the required time mask; in this case a square pulse. A time mask of the PA's output power is displayed in Figure 5. The time mask meets the limits (displayed by doted lines) over a wide temperature range.
Figure 5: Time mask.
A PA control loop is a powerful tool in controlling the power amplifiers response to meet the GSM specification. A PA controller is introduced to create this PA control loop. The PA controller has a linear (in dB) response from the DAC of the baseband to the output power of the PA, which is very useful to improve the behavior of the PA. The PA control loop is accurate, temperature-stable and fast enough to meet the time mask specification. Calibration of the PA control loop is easy.