Doherty Power Amplifier Tutorial

- the Doherty power amplifier technology is widely used in RF power amplifier design to improve the efficiency of the amplifier over a wide signal range.

Doherty Amplifier Tutorial Includes

The Doherty power amplifier is a form of class B amplifier that is used for RF power amplifiers in situations where RF signals need to be amplified in a linear fashion.

Doherty power amplifiers are often used in situations where a signal having a varying amplitude needs to be amplified in a linear fashion while maintaining a good level of efficiency.

The Doherty amplifier configuration achieves this by having two amplifier sections, one that caters for the lower amplitude signal situations, and second that is additionally brought in to provide the capability to meet the higher level conditions without running into compression.

Doherty power amplifier development

The Doherty power amplifier configuration was invented by William H. Doherty of Bell Telephone Laboratories in 1936.Although developed in the days of vacuum tube / thermionic valves, the Doherty amplifier met the need for transmitters running high power, while still needing to maintain reasonable power efficiency levels to reduce costs, heat dissipation and running costs.

While the original idea for the Doherty power amplifier was aimed at broadcast transmitters using amplitude modulation, the scheme is finding far more widespread use as many systems require high data rate digital signals to be transmitted.

The first Doherty amplifier used two vacuum tube amplifiers, both biased in Class B and able to deliver tens of kilowatts to the antenna.


Requirement for Doherty amplifier

One of the drivers for the increased use of the Doherty power amplifier is the need to maintaining amplifier efficiency while needing a linear amplifier operating mode and the increasing peak to average power ratios being found with the newer modulation formats being used.

One of the key elements of any power amplifier design is the efficiency level. This affects many issues including overall power consumption, heat sink requirements, battery life, and output device capability, etc.

In basic terms, the efficiency is defined as the output power divided by the input power, but this is affected by a number of issues including the peak to average power ratio.

To look at how the peak to average power ratio affects efficiency it is necessary to look at the operation of an amplifier.

When operating in a linear mode, the output device must always be in conduction, with the output voltage rising and falling between the two limits.

When operating in this mode, often called Class A, the maximum theoretical efficiency that can be achieved is 50%, but in real systems the levels that are achieved are always below this. Circuit losses are one reason, but another is that the signal may not be reaching the maximum levels for the amplifier.

The input output curve for an amplifier showing its linear curve and its maximum output before the onset of compression.
Linear operation of an amplifier

To achieve better efficiency levels, it is possible to drive the amplifier into compression. Much greater levels of efficiency can be achieved using this approach. Signals like frequency modulation, FM that have no amplitude constituent are not distorted by this. The only signal degradation is that additional harmonics of the fundamental carrier are generated, but these can be filtered out using RF filters.

Unfortunately when a signal modulated with an amplitude component is applied to an amplifier run in compression, amplitude distortion results.

For data transmission systems that are used today like UMTS, HSPA, and 4G LTE, etc, the RF waveforms that are used incorporate an amplitude component in addition to the phase elements and therefore they require a linear amplifier.

The situation becomes worse when the peak to average ratio increases because the amplifier has to be able to accommodate the peaks while still only running at a low average power level.

A waveform with a high peak to average ratio.
Waveform with high peak to average ratio


Doherty amplifier basics & theory

The Doherty power amplifier is able to accommodate signals with higher peak to average power ratios. The basic Doherty amplifier theory uses two amplifier circuits within the one overall amplifier to accommodate the different conditions experienced.

They are biased differently and provide different functions.

  • Carrier amplifier:   This section of the overall Doherty amplifier normally operates in class A or AB and provides gain at any power level. It is particularly aimed at carrying the average signal levels
  • Peaking amplifier:   This is normally biased in Class C so that it only handles the higher power levels. It provides the extra power capability that the carrier amplifier on its own cannot provide.

In addition to the amplifiers themselves, the Doherty amplifier configuration requires a splitter and a combiner. These items enable the power to be directed to the two amplifiers and then the output from them to be summed to provide the composite output.

Doherty amplifier concept showing the carrier amplifier and peaking amplifier together with the splitter and combiner
Basic Doherty amplifier concept

The basic Doherty amplifier theory requires that the signals between the two halves are matched in phase so that the combination occurs in a way that both signals add together to provide the required output.

Doherty amplifier types

There are two main types of Doherty power amplifier that can be designed:

  • Symmetric Doherty amplifier:   This is the more straightforward design of amplifier. It uses two identical amplifiers in the circuit, but does not offer quite the performance of the second type.
  • Asymmetric Doherty amplifier :   The asymmetric Doherty amplifier is the one that is more widely used. It has two different amplifiers within the overall block. In this approach the peaking amplifier has a higher power capability. This means that it can accommodate the signal peaks, leaving the lower power amplifier to cater more effectively for the lower signal levels. This approach enables much better performance levels to be achieved.

Doherty amplifier advantages & disadvantages

While the Doherty amplifier is widely used because it offers several advantages, RF designers will all say that it is not always perfect and there are disadvantages to its use.

Doherty Amplifier Advanatges & Disadvantages
Advantages Disadvantages

  • Enables higher efficiency levels to be achieved.
  • Technology is not as complicated as envelope tracking which also improves RF amplifier efficiency

  • Difficult to maintain phase shifts of splitters over a wide bandwidth and therefore Doherty amplifier can only be used over a limited bandwidth.
  • Higher cost than a single amplifier.
  • Difficult to get the correct design

Although there are several disadvantages to the use of this form of amplifier, it is nevertheless widely used for many applications where RF power amplifier efficiency is important.

By Ian Poole

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