Inductor‐less envelope modulated radio frequency power amplifier using stacked amplifiers and envelope shaping

Abstract

An envelope modulated radio frequency (RF) power amplifier (PA) is presented using a ‘stacked amplifiers’ modulator. The modulator combines the efficiency of class G with the convenience of a charge pump. Since it is a single band modulator, no large inductors or transformers are required. A two level prototype has been developed and tested with a 7.5 dB peak‐to‐average power ratio 1.4 MHz bandwidth long term evolution (LTE) signal. Envelope shaping was used to improve linearity. Individually the modulator achieved between 54 and 60% efficiency, dependant on the envelope shaping function. When combined with a suitable class E PA, 40% DC to RF efficiency was achieved at 1.38 W output power with the 1.4 MHz LTE signal.

An envelope tracking RF power amplifier with capacitive charge pump modulator

“…The curved function has better ACPR, as suggested by [15], although the clipped function was more efficient due to the improved “fit” between V PA and the output signal. There exists a trade-off in the ES function between efficiency and linearity.…”

“…It also used DPD to improve linearity. Similarly, [23] has a CMOS modulator, but a GaAs RF PA. Watkins [15] is also based on a charge pump structure, but is more complex, since it tracks the whole dynamic range of the envelope with two analog inputs. In [12] a number of different configurations are presented based on two, three, and four supply voltages.…”

“…An alternative approach to tracking between multiple power supplies is a linear capacitive charge pump [14] that doubles the V PA around the peaks [9, 15]. In [15] the whole envelope signal range is tracked, requiring two analog inputs, hence two digital-to-analog converters and associated baseband. The solution shown in Fig.…”

“…A conditional branch is used to only pass V env to the push-pull class B PA when it exceeds a threshold. Below this, V PA is fixed at V CC minus the voltage drop of D 1 , therefore not tracking the nulls: This results in a harsh transition – referred to as a “clipped” function – which can degrade the ACPR [15]. Watkins [15] also shows that ACPR can be improved with a “curved” function to smooth the transition between linear and tracking regions.…”

“…τ n is the relative time between 0 and t . Equation (6) was simulated to analyze how the value of C 1 impacts the peaks with the assumptions given above, and that R L is 50 Ω [15]. The voltage drop of V C 1 over 5 µs is shown in Fig.…”