Doherty Power Amplifier (DPA): dividi et impera.

Doherty Amplification
A contemporary telecommunication equipment must have features, in terms of signal linearity, high power and also top efficiency, such to meet energy saving requirements.

Transistor amplification with latest MOSFET generation allows valuable efficiency levels (above all considering their evolution in these last years), raised with the introduction of liquid cooling in the past 20 years.

Nowadays, with Doherty technology application in amplifier stages of television transmitters, a new challenge was launched.

William Doherty was an engineer of Bell Telephone Laboratories who, in 1936, designed a circuit using two class B tubes that in parallel were operating as a linear amplifier for AM radio transmitters.

The previous version was using class B tubes in “push-pull” configuration with a transformer combining the tubes’ output signal. The push-pull system was functional and powerful but not energy efficient.

A DPA uses two amplifiers: the polarized MAIN to operate, for instance, in class AB; and the polarized PEAK to operate, for instance, in class C. The input signal is divided in equal parts by means of a power hybrid with a 90° phase shift.

The signals, amplified separately, are then re-combined through a power combiner that re-alligns them in phase.

In presence of high level input signal, both amplifiers are active: the MAIN assigned for amplifying the low part of the frequency modulation and the PEAK entering into operation in case of high levels of frequency modulation, over preset thresholds.


Time trend of a multicarrier signal


In presence of low level signals, the PEAK amplifier switches off (our class C example): only the MAIN stays active (class AB). In this phase, the MAIN operates, alone, in high gain and efficiency condition.

The system is very efficient in presence of signals having a high PAR (Peak/Average Ratio); therefore characterized by a high ratio between reachable peak power and medium power of the signal to be spread.

The DPA introduction in AM equipment increased the conversion efficiency from 30 to ver 60%, representing the standard of the AM “tube” transmitters manufactured by Western Electric, Continental, Marconi and by RCA (until the 70s).

The first application of this technology was a 500kW AM transmitter manufactured by Western Electric Company, installed in 1938 for WHAS, a radio station nearby Louisville in Kentucky.

The AM signals have a low PAR (Peak to Average) ratio and, therefore, very little changed till the 2000s.

Only with the arrival of digital and mobile telephony, the William Doherty technology had a new youth.

In fact, a peculiarity of UMTS/CMDA telephone signals, and in general of OFDM (WiMAX, LTE) modulation systems, is the high Peak/Average ratio (going from 4dB in analog to over 8db in DVB-T).

Moreover, many thousands small mobile telephony stations made necessary the research of technologies allowing savings in terms of power consumption or of heat dissipation (with DPA, mobile telephony operators’ savings increase up to 15% on network management costs).

When Doherty designed his amplification system, technologies were obviously very different.

By examining the “pure” DPA, it can be seen that the output linearity is lower compared to the one of a class AB amplifier and, therefore, producing a higher distortion.

However, we know that operating the power amplifiers in non linear zone, these gain in efficiency. Here helps the modern progress in digital predistortion, in linearity techniques, in design and in new MOSFETS introduced on the market by the manufacturers of semiconductors.

 Nowadays, out of the most popular linearity techniques: mixed techniques, Feedback, Feedforward, Predistortion, only this lattest is largely used in broadcast.

Notwithstanding the modern evolutions in terms of design and technology, broadcasting approached Doherty since a couple of years only. This is due mainly to the difficulty in reaching 35dB MER without retreating high efficiency levels in precorrection and to the Doherty being a narrow band system (two or three channels). This was a growth limiting factor in a market where frequency agile amplifiers are requested.

Recently progresses were achieved in design of Doherty technology UHF pallets. It was achieved to split the UHF band in 3 parts, i.e. 3 different pallets design (generally naming them as “broadband”).

To reach the real broadband pallet in full-doherty technology a miracle is to occur…. Or maybe not.

In 2016 Itelco again moved ahead towards the very next future.

The REAL efficiency, measured at mains input/Pout up to 42% and over, became a fact and again will represent the comparison stone for the reference market.

Prof. Eugenio Fumi