Envelope Tracking Shaping Characteristics

One of the key areas of the operation of an RF envelope tracking amplifier is the determination and use of the shaping characteristic.

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

Utilising the shaping characteristics of an envelope tracking amplifier enables its operation to be matched to the exact needs of the application.

It is possible to set the operation of the amplifier up to maximise different features by altering the shaping characteristics of the amplifier using envelope tracking technology.

Effect of modulating the supply voltage

Varying the voltage on the RF power amplifier enables the optimum performance point to be selected for any given level of RF power output.

It is found that for any given supply voltage, there is an optimum level of efficiency that can be obtained. The aim of an envelope tracking system is generally to operate at the most efficient point on the curve for any given power output.

It can be seen by the graph below that the for any given supply voltage, i.e. drain voltage or collector voltage dependent upon the type of device, the power efficiency rises as the power output starts to rise, then reaches a peak before falling away.

If many plots are made for different supply voltages, it can be seen that there is curve for the maximum efficiency level. If the supply voltage follows this curve, then the RF amplifier can operate at its maximum efficiency level for any given power level.

Power added efficiency levels for different device voltages — Power efficiency levels for different device voltages

The diagram references power added efficiency. The power added efficiency, PAE is a metric that takes into consideration all the power that enters the amplifier. It is particularly useful when working with relative low gain amplifiers, such as RF power amplifiers.

Power added efficiency, PAE = \frac{P_{RFout} - P_{RFin}}{P_{DCin}}

It can be seen that if the amplifier requires a higher drive level, then the power added efficiency falls. It would have no effect on an efficiency that only considered the DC power in and the RF power out.

It is normal for the maximum possible power-added efficiency of a device to decrease with frequency. The reason for this is that there is a natural tendency for maximum gain of an active device to decrease with frequency.

Envelope tracking shaping

The obvious aim for an envelope tracking system is to optimise the efficiency of the system. Under these circumstances the envelope tracking signal should be such that the points of maximum efficiency are used for each voltage.

However it is also possible to adopt other envelope tracking profiles. One in particular follows a constant gain and is known as the Isogain contour and provides the maximum linearity.

Envelope shaping - maximum efficiency

In order to obtain the maximum efficiency a supply voltage shape profile that follows the points of maximum efficiency for the various output power levels is required.

This can be plotted on the various curves for gain and power output for the different supply voltage levels.

Maximum efficiency contour for envelope tracking amplifier — Maximum efficiency envelope tracking shape / contour

In order to obtain the required voltage, it is necessary to develop a shape look-up table. This ensures that the required voltage is provided to the amplifier for a given envelope amplitude. This can be developed from the curves for the amplifier.

Shape lookup curve stored in memory for the maximum efficiency case — Typical shape lookup curve for maximum efficiency

Envelope shaping - linear gain

Under some circumstances it may be necessary to optimise the envelope tracking system to provide linear gain rather than maximum efficiency. Under these circumstances it is necessary to alter the operating conditions slightly and this results in a different curve for the look-up table.

Rather than taking the points of maximum efficiency for the amplifier, it is necessary to ensure they operate in a linear fashion. As a result the points where the amplifier remains linear need to be taken.

Linear gain contour for envelope tracking amplifier — Linear gain envelope tracking shape / contour

The figures from these contours are taken and added into the shape look-up table to provide the required envelope tracking contour for linear gain. IT will be seen that a different curve is obtained. These graphs are only examples, but the principle can be seen, and also the different shapes can be seen, indicating the different operating conditions used.

Shape lookup curve stored in memory for the linear gain case — Typical shape lookup curve for linear gain

The figures in the look-up tables are those used in the envelope shaping look-up memory. They enable the required contour to be provided to match the required supply voltage for the given instantaneous amplitude of the RF amplifier.

By taking different characteristics for the amplifier and plotting the curves and then using these figures in the look-up table, it is possible to change the performance of the amplifier in a number of different ways, optimising for different aspects of performance.