This article gives a brief overview of the Doherty technique. The Idea of the Doherty is the most straightforward technique for providing efficiency at the back-off, where no additional digital signal processing (DSP) nor additional circuits are required to be attached with the design.
Classical Doherty Power Amplifier Operation
In 1936, W.H. Doherty invented a new combiner designed for broadcasting stations using high power tube amplifiers [
1]. A λ/4 transmission line was used as a combiner at the output of power amplifiers to achieve a linear output power. The classic DPA consists of two amplifiers known as the carrier (main) amplifier and the auxiliary (peaking) amplifier (). A class AB amplifier used for the carrier amplifier whereas a class C amplifier is used for the peaking amplifier. The RF input signal is split between the two amplifiers, where the carrier amplifier is working all the time and should almost reach saturation at the back-off input power due to seeing a high impedance which causes a change in the load-line as shown in . At the same power level, the auxiliary amplifier works only in the Doherty region and starts feeding current to the output till it becomes saturated at the peak region, where the two power amplifiers give their maximum designed output power.
Structure of the Doherty power amplifier [
2].
Figure 2. Main amplifier load line [
2].
The idea of the Doherty amplifier depends on the so-called active load-pull technique [
1]. Where the DPA operation can be divided into three regions:
The low RF input power region, where the signal level is not sufficient to turn the auxiliary amplifier on, in this case, the auxiliary amplifier can (theoretically) be represented as an open circuit as shown in a. At the same time, the main amplifier is amplifying the input signal as an ordinary power amplifier, however, the load is seen by the main amplifier through the impedance inverter (λ/4 transmission line) and is increased because the characteristic impedance of the λ/4 transmission line is higher than the load impedance. In this case, the main amplifier will be almost saturated because its load line has changed, as illustrated in . The impedance seen by the main amplifier depends on the following equation: