Benefits of RFMD® Power Flattening Circuit
By Bobby L. Johnson, Applications Engineer, RF Micro Devices

Introduction
Large variations of output power and current into a mismatched load can affect efficiency and possibly compromise the PA’s ability to maintain the minimum output power necessary to prevent dropped calls. It is increasingly more important to correct the power variation at the PA level in the handset. This becomes even more important at type approval in order to receive carrier compliance for TRP and SAR. The requirements TRP, Total Radiated Power, and SAR, Specific Absorption Rate, are tests that the carriers and the governmental agencies have placed on mobile phone manufacturers to better improve the quality of service and protect the user.

Increased current causes the handset to transmit more power. This excess power needs to be dissipated into the antennae. Power that does not get absorbed by the antennae is radiated and dissipated into the phone materials and/or the user; possibly exceeding the SAR absorption rate of 1.6 watts per kilogram. Likewise, power variation in the negative direction could result in the handset failing minimum TRP and dropping calls. This tradeoff can be a difficult balance to achieve. RFMD achieves this through the introduction of the Power Flattening Circuit. The RF3196 has an integrated power flattening circuit that prevents the PA from high current conditions when a mismatch VSWR such as 3:1 is presented to the output of the PA.

Advantages of RFMD Power Flattening
When a mismatch is presented to the output of the PA, its impedance is varied and could bring the load into high output power regions on the Smith Chart. As the output power increases, so does current consumption. The current consumption can become very high if not monitored and limited. When considering the architecture of the transmit chain and the limited isolation through the switch, any mismatching at the antennae can load the output of the power amplifier.

A mismatch can be created by a broken antenna, setting the phone on or near a metal object, or just by the position of the phone in relation to the user’s head. The design of the antennae and the power amplifier’s ability to deliver constant output power are key to how well the phone is affected under adverse conditions.

The power versus current ellipse is plotted as a function of phase where output power is on the y-axis and current on the x-axis. The thinner and narrower the ellipse is, the better the PA’s performance into mismatch. In Figure 1, the RF3166 was used in this test as the control part without power flattening to compare the results of the RF3196. The ellipse is wider and taller for the RF3166; this is because the output power is varying approximately 3.2dBm in output power and 1.74A in current.

With the addition of the power flattening circuit, it is apparent in Figure 2 that the RF3196 performance into mismatch is greatly improved. The output power variation is less than 1.5dBm and the current varies approximately 1Amp. Another advantage is that the max current drawn into mismatch is less that 2.1A, so there is the added advantage of improved efficiency.

The power flattening circuit monitors current through an internal sense resistor. As the current changes, the loop is adjusted in order to maintain output power. The result is flatter power and reduced current into mismatch, such as when a 3:1 load is presented to the output.

This is possible because of the linear relationship between output power and current. In Figure 3, is output power and current swept over phase into a 3:1 VSWR. It is apparent that the current and output power are increasing and decreasing together.

Power Flattening Implementation
The original RFMD Power Star® power control circuit uses a single feedback loop at the collector to keep the PA in constant saturation. The power flattening circuit adds a second loop to feedback a Vsense voltage. The Vsense voltage is sensed across an internal sense resistor on the module. This Vsense voltage is compared to a reference voltage. This reference voltage is set by design into 50ohms, where the current mirror ratio is set to control the amount of current in the feedback loop that adjusts the gain of the PA to correct for the swings in impedance. Figure 4 is a simplified diagram of the feedback loop with the sense resistor.

If the current through Rsense has increased the collector voltage, Vcc will be decreased. Likewise, if the current through Rsense decreases, Vcc will be increased. The Vcc voltage is controlled by internally adjusting the Vramp control voltage to keep the power flat. The constant sensing of the Rsense voltage and the adjustment of the collector voltage, depending on the current through Rsense, is what keeps the power flatter and improves current variation.

With the power flattening circuit implemented, the circuit’s operation is evident when the power and current are now plotted. The previous condition of a linear relationship between power and current is now reversed. In Figure 5, is the Power vs Current over phase.

Summary
Output power and current variation into a mismatched load can compromise the PA’s ability to maintain the minimum output power, control maximum radiated power, and meet the requirements of governmental agencies and cellular service providers. RFMD’s RF3196 PA with integrated Power Flattening Circuit senses a voltage across an internal sense resistor. This voltage is fed back to compare to a reference voltage that is set into 50 ohms. Then it adjusts Vramp to reduce current and keep the power flat during mismatch conditions. As TRP and SAR compliance is increasingly more important in the market, this feature makes the RF3196 the premiere power amplifier under adverse conditions.

RF Micro Devices
www.rfmd.com
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