Envelope Tracking Power Supply / DC Modulator

The DC modulator or power supply used in an RF envelope tracking amplifier is one of the most exacting areas of the design - it supplies the correct voltage to the RF amplifier at all times.

Envelope Tracking primer includes:
What is envelope tracking Block diagram PSU / DC modulator Envelope shaping Control signal Delay balancing

The DC modulator or envelope tracking supply provides the final RF amplifier with the required voltage needed to provide the required output power and little more.

By providing the final amplifier with only the required amount of voltage, dissipated power is considerably reduced as the final power amplifier device is not able to dissipate nearly as much power as if a steady supply was provided.

The design of a supply / DC modulator capable of providing exactly the required voltage level is an exacting design challenge.

Envelope tracking power supply basics

The function of the envelope tracking power supply is to take in the envelope information and then output a voltage to the RF amplifier that matches the waveform.

The function of the envelope tracking power supply is not easy to create. The requirements are more exacting than may be visible at first sight.

Some of the requirements for the envelope tracking power supply include:

Bandwidth: It is necessary that the envelope tracking power supply is able to accurately follow the modulation envelope. This means that it must be able to accurately follow the modulation envelope at the highest frequencies at which it runs. To accurately follow the modulation, the envelope tracking power supply must be able to typically achieve frequencies around two to three times that of the signal bandwidth. For current systems this can require power supply bandwidth levels of 50 MHz and more.
Efficiency: In order to reap the full benefits of the envelope tracking technology, the power supply must be highly efficient. There is not point improving the PA efficiency by 10% for example, if the highly efficient DC-DC converter used in a traditional approach is replaced by an envelope tracking supply that is 10% worse.
Noise: Although switch mode power supplies offer very high efficiency levels, one of the challenges of using them is the switch mode noise generated. The problem is compounded by the very wide power supply bandwidths needed to achieve envelope tracking. Therefore innovative design is required to ensure this parameter is fully met.

Noise on an envelope tracking supply . modulator is a very key issue in many instances. Any noise on the supply will appear as amplitude modulation on the signal. Noise will have a wide bandwidth and therefore it will superimpose wide band noise on the output signal. In turn this will mean that adjacent channels may suffer from interference. This issue is of particular importance for FDD - frequency division - duplex systems - where transmitted noise appearing in the receive channel masks out the received signal. For FDD systems to operate satisfactorily, transmitter spurious signals, including noise must be at exceedingly low levels in the receive bands, placing very stringent requirements on any envelope tracking power supply.
No decoupling capacitors: It is normal practice in traditional power supplies to add a decoupling capacitor to the output to ensure that noise and ripple, etc are reduced to the minimum. In view of the high bandwidth required for envelope tracking systems, the power supply cannot have any output decoupling. This means that design of supply / modulator is crucial to the overall performance.
Very low output impedance: In view of the fact that no decoupling capacitors are allowed, the ET supply or modulator must have a very low output impedance extending up to the maximum modulation frequency and beyond. This will enable it to absorb any of the noise that may appear on the line.
Supply capability: It is necessary that the envelope tracking power supply can fully supply the current and voltage requirements for the RF amplifier. Again this may not always be easy to achieve within the constraints of the overall design.

ET supply connection to PA

The envelope tracking modulator or power supply is not able to have a decoupling capacitor on the link to the power amplifier. This would be standard practice for traditional configurations, but in view of the high bandwidth modulation required by the envelope tracking supply, a decoupling capacitor cannot be used.

The connection between the DC modulator / envelope tracking supply and the RF amplifier must have a low resistance and inductance — Requirements for connection between envelope tracking supply and the RF amplifier

This obviously places some critical design requirements onto the envelope tracking modulator / supply, but it also place requirements on the connection between the supply and the PA:

Low resistance: In order that no voltage is developed across the link between the envelope tracking supply and the PA, it must have a resistance that is as low as possible. This often means that it should be as short as possible and also wide to ensure that as much copper is included on the circuit board as possible.
Low inductance: Similarly inductance is an issue as inductance will also cause voltages to develop across the inductive element of any reactance. This can often present problems when testing the circuit and trying to sense the current. Inductive sensing is not really feasible to achieve this.
Low capacitance: While it is tempting to make the link between the envelope tracking supply and the PA very wide, this may also increase the level of capacitance to ground and this could limit the tracking performance. The capacitance needs to be kept to a minimum. One way to achieve this could be by removing the ground plane underneath this track on the circuit board.

Achieving the full requirements for the envelope tracking power supply has been one of the major issues that has prevented the concept from being adopted earlier. Now that companies are able to implement the envelope tracking power supply successfully, this has enabled the technology to be incorporated into many systems for many applications from broadcast transmitters to cellphones where battery power must be conserved.

The considerable design challenges on the supply mean that there are many patents associated with these circuits and the technologies used, and as such companies are not willing to publish the exact approaches, circuits and technologies used.

Envelope tracking supply approaches

There are several approaches that can be used to design an envelope tracking power supply or modulator. The approach used in any given design depends upon a number of factors relating to the particular applications, performance required, costs and the like.

There are three main approaches that can be used:

Analogue envelope tracking supply A linear supply for an envelope tracking RF amplifier follows the same basic principles used by ordinary linear regulators. They could be in either a series or shunt configuration, but generally a series configuration is adopted as they are typically more efficient.

However as the regulated voltage of the linear supply / modulator must always be lower than that of the input voltage, the efficiency level is considerably limited. They become less efficient as the difference between the output voltage and input voltage increases.

The advantage of a linear ET supply is that it is able to provide a very clean voltage to the RF amplifier and this is of crucial importance as noise from the supply is added to the RF signal and will give rise to interference on adjacent channels.

Although many linear supplies use class A amplifier operation, it is possible to considerably improve the efficiency of an envelope tracking supply / modulator by using class AB / B and class G / H amplifiers in the circuit.

The first option: class AB / B can be implemented by effectively using an RF amplifier, although with a bandwidth that is lower than the RF amplifier itself. Often the RF amplifier for cellular applications would operate at frequencies of 800 MHz or higher in most cases whereas the modulation for an LTE signal may require an ET power supply bandwidth of 50MHz or so.

Typically class B amplifiers are arranged in some form of push-pull configuration so that they can both source and sink current to enable accurate following of the envelope. To implement this, a complementary pair may often be used.

The Class G / H concept has many merits for this form of application. This form of amplifier uses multiple supply rails which it utilises dependent upon the level of output voltage required. As high PAPR signals generally operate at lower power levels for longer than they do at the higher levels, this approach has many advantages.

There are differences between class G and class H amplifiers. The class G amplifiers typically have two or three discrete voltages that they utilise. A class H amplifier is more able to smoothly regulate the the output voltage according to the RF power amplifier requirements. It can create smooth transitions from the lowest supply voltages to any other, thereby providing a clean output.
Switching ET supply Switch mode power supplies, SMPS, provide a considerably more efficient voltage conversion than linear ones. In view of the fact that envelope tracking is all about improving efficiency, linear supplies are not normally viable and switch mode technology needs to be used in one form or another.

The drawback with switching mode technology is that it is not as easy to make very clean in terms of noise and spikes as a result of the switching of the series pass element. A further issue with switching technology when used with envelope tracking technology is that the switching frequency must be higher than the maximum bandwidth required for the supply - typically five times the signal bandwidth. This can lead to some very high switching frequencies being required. In turn these high switching frequencies can lead to lower efficiency levels than those normally expected by switching supplies.

One of the key elements for a switch mode envelope tracking supply is the output filtering. The low pass filter required on the output governs the ripple specification of the supply which is very important in reducing wideband noise that could fall outside the required bandwidth and cause interference to other users. However the filter also plays a major role in determining the way in which the supply can follow the required modulation envelope - it needs a high cut-off frequency to enable it to pass the high frequencies required to track the modulation envelope.

The type of filter used needs careful choice. There are several types that may be more commonly considered. For a given filter order, Butterworth filters provide a better level of stop-band attenuation than a Bessel filter, but not as good as a Legendre-Papoulis. However the Bessel provides better group delay performance. Whatever form of filter is adopted, it will be a compromise and the exact requirements for the given application need to be carefully considered.
Hybrid ET supply In order to provide the best overall performance a hybrid envelope tracking supply / modulator, using elements of both linear and switching technologies can be developed. This is the approach being used by a number of envelope tracking designs for cellular telecommunications and other applications. In this way it is possible to combine the benefits of the high efficiency of the switch mode supply with the low noise of linear technology.

There are three approaches that can be taken:
- Serial hybrid: Using this technique a multilevel switch mode supply / modulator provides a linear modulator with discrete voltage levels that are close to the required voltage for the RF amplifier. The linear modulator is then able to provide the final voltage that has the benefit of the low noise of the linear modulator. As the switch mode supply provides the majority of the voltage reduction, the efficiency is maintained.
- Parallel hybrid: The parallel approach for the envelope tracking modulator / supply uses a linear modulator to control the switching element of the supply using a sensed envelope signal with a linear modulator and switching supply in the loop. The feedback mechanism minimises the current supplied by the linear modulator. In this configuration, the switching supply is able to supply the lower frequency components of the RF amplifier, whereas the linear regulator supplies the higher frequency components and also nulls the switching noise.
- Combined hybrid: The combined hybrid approach enables further efficiencies over those that can be obtained using the serial or parallel techniques on their own. It utilises two switching elements and a single linear element. One switch mode supply / modulator is used with a linear modulator connected in a parallel configuration as described above for the parallel hybrid configuration. This provides broadband high efficiency capability. The second switching supply is inserted into the circuit in cascade to feed the linear modulator with a bandwidth reduced variable supply to further reduce the power loss in the case of the serial hybrid. In this way, both serial and parallel techniques are used to provide the optimum efficiency.

Each type of envelope tracking supply / DC modulator has its own advantages and disadvantages. The actual type used will depend upon the particular application.