Question

The following amplifier is Doherty power amplifier. Please answer the following questions.

RFin RF liary XZ/V2 Zos Block diagram of a classical Doherty amplifier.

1. Please explain the operation principle of this amplifier in detail.

2. Please draw the efficiency (%) vs. input power in dBm and explain why this efficiency curve is needed.

Answer 1

ona Doherty amplifier morking principle divided into three The openting principle of Doherty complifier pont of operation : (1) low power level operation. (2) medium power level operation. (3) High power deres openation. The Basic block diag poherty amplifier. main amplifier KL load. cuxulary chem amplizie. 0) Low power Levele operation when the low power operation take plake turned off and all the signal recieved which acted as a controlled current the auxillary amplifier by the main amplifien. source. Main ampliften have high output impedance deads to premature, saturation of the main amplifier while the current has reached only half of its maximum value. Hence voltage reached at maximum value then systom start work with Maxissum efficiency though it does not deliver maximum power.

(1) Medium level operation. luchers the saturation will take place in main amplifies which biating enables the auxiliary amplifier to turn on resulting the flow of. Auxillary current will take place. - Now auxillary amplifien act as a controlled anrent source. and the main amplifier act as a voltage Controlled source. Due to increasing the auxitary carrrent increases the impedance Rout. The charcterstic's impedance of output anadres ware length. line is RTL = Rin -kout - Hence Result in increase, Rout causes decreases in Rin this cause main amplifian output voltage to remain constant getting into saturation and increasing output ament - maximum of the voltage devel The efficiency stay close to the Remain's closer to saturation. → The output impedance of auxillary keep's decreasing and that of the main amplifier with the quarter were transmission line keep increasing

(hi) High power level operation, - I The Pnput impodonce increases the load keep's increasing untill the auxillary amplifier sahurates once maxisumu lonel is reached bothe main and auxillary amplifies see an optimum Resistance.. - The main and hence current reaches maximum point at this level the output power delivered by the configuration. rain. The load during the peat's in order to prevent the amplifies from getting into saturation there by maintaining maximum efficiency. @B) Officiency curve of potenty configuration 7- I Lim VM Vi - point at The Dohenty amplifier has maximum PAE at transistion full pourer. ne efficung Pmax. Pa