>OUT-C is equal to the peak output power P_OUT-Pat the P_PEAK.

In the conventional symmetrical power amplifier circuit like the Doherty power amplifier circuit 10, the carrier impedance Z_Mseen by the carrier amplifier 16C is typically equal to the peak impedance Z_Pseen by the peak amplifier 16P. As such, the peak amplifier 16P would need to increase the modulated peak current I_Pat a faster rate than that of the modulated carrier current I_Msuch that the peak output power P_OUT-Pcan become equal to the carrier output power P_OUT-Cat the P_PEAK. The Doherty power amplifier circuit 10 is said to perform an ideal load modulation when a change rate of the modulated peak current ΔI_Pis twice a change rate of modulated carrier current ΔI_M(ΔI_P=2ΔI_M).

FIG. 1D shows a relationship between the modulated carrier voltage V_OUT-Cand the modulated peak voltage V_OUT-Pin the Doherty power amplifier circuit 10 of FIG. 1A. When the time-variant output power P_OUTis lower than the predefined power threshold P_TH, only the carrier amplifier 16C is active. In this regard, the modulated carrier voltage V_OUT-Cwill increase as the time-variant output power P_OUTincreases. When the time-variant output power P_OUTis higher than or equal to the predefined power threshold P_TH, the peak amplifier 16P becomes active. Accordingly, the modulated peak voltage V_OUT-Pwill increase as the time-variant output power P_OUTcontinues to increase. In the meantime, the carrier amplifier 16C will maintain the modulated carrier voltage V_OUT-Cconstantly at the V_CC-MAX. As shown in FIG. 1D, the modulated carrier voltage V_OUT-Cand the modulated peak voltage V_OUT-Pare both equal to the V_CC-MAXat the P_PEAK.

FIG. 1E shows the modulated voltage V_CCreceived by the carrier amplifier 16C and the peak amplifier 16D at the respective collector electrode C. When the time-variant output power P_OUTis lower than the predefined power threshold P_TH, only the carrier amplifier 16C is active. In this regard, the modulated voltage V_CCcan be so generated in accordance with the equation (Eq. 3) to be substantially close to the modulated carrier voltage V_OUT-C. As a result, the carrier amplifier 16C can operate in compression to achieve optimal efficiency.

When the time-variant output power P_OUTreaches the predefined power threshold P_TH, the modulated voltage V_CCreaches, and is maintained at, the V_CC-MAX. Thereafter, the peak amplifier 16P becomes active to amplify the peak signal 20P based on the V_CC-MAX. Given that the V_CC-MAXis always higher than the modulated peak voltage V_OUT-P, the peak amplifier 16P will not operate in compression until the time-variant output power P_OUTreaches the P_PEAKof 37 dBm. In other words, the peak amplifier 16P will not operate with optimal efficiency prior to the P_PEAK.

Although it may be possible to improve the efficiency of the peak amplifier 16P by providing another modulated voltage to keep track of the modulated peak voltage V_OUT-P, doing so may require an additional power management chip that can lead to an increase in footprint and cost. Hence, it is desirable to improve the efficiency of the carrier amplifier 16C and the peak amplifier 16P based on the same modulated voltage V_CC.

FIG. 2 is a schematic diagram of an exemplary asymmetrical power amplifier circuit 32 configured according to an embodiment of the present disclosure to achieve improved overall efficiency over the Doherty power amplifier circuit 10 of FIG. 1A. Like the Doherty power amplifier circuit 10, the asymmetrical power amplifier circuit 32 includes a carrier amplifier 34C (denoted as “PA-C”), a peak amplifier 34P (denoted as “PA-P”), and a splitter 36.

The splitter 36 is configured to split an RF signal 38 into a carrier signal 38C and a peak signal 38P, each having one-half of a time-variant input power P_INof the RF signal 38. The carrier amplifier 34C is always active to amplify the carrier signal 38C to a carrier output power P_OUT-Cbased on a modulated voltage V_CC. The peak amplifier 34P is only active when a time-variant output power P_OUTof the RF signal 38 is higher than or equal to a predefined power threshold P_TH. When active, the peak amplifier 34P amplifies the peak signal 38P to a peak output power P_OUT-Pbased on the modulated voltage V_CC.

Each of the carrier amplifier 34C and the peak amplifier 34P can be configured in a similar fashion as illustrated in FIG. 1B. As such, in accordance with the equation (Eq. 3), the carrier amplifier 34C and the peak amplifier 34P will each operate with optimal efficiency when the modulated voltage V_CCis substantially equal to a modulated carrier voltage V_OUT-Cand a modulated peak voltage V_OUT-P, respectively.

The asymmetrical power amplifier circuit 32 differs from the Doherty power amplifier circuit 10 in several aspects. In one aspect, the carrier output power P_OUT-Cwill not be equal to the peak output power P_OUT-Pat the P_PEAK. In a non-limiting example, the carrier output power P_OUT-Cis higher than the peak output power P_OUT-Pat the P_PEAK.

In another aspect, the predefined power threshold P_THin the asymmetrical power amplifier circuit 32 is higher than the predefined power threshold P_THin the Doherty power amplifier circuit 10. Recall that in the Doherty power amplifier circuit 10 of FIG. 1A, the predefined power threshold P_THis set at 31 dBm, which is 6 dB below the P_PEAK(37 dBm). In a non-limiting example, the predefined power threshold P_THin the asymmetrical power amplifier circuit 32 can be set at 33 dBm. As a result, the peak amplifier 34P will be activated later than the peak amplifier 16P in the Doherty power amplifier circuit 10.

In an embodiment, the asymmetrical power amplifier circuit 32 may include a control circuit 40, which can be a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC), as an example. The control circuit 40 may be configured to store the predefined power threshold P_TH. Accordingly, the control circuit 40 can activate or deactivate the peak amplifier 34P based on the time-variant output power P_OUTand the stored predefined power threshold P_TH. In one embodiment, the control circuit 40 may derive the time-variant output power P_OUTfrom the modulated voltage V_CCand/or the time-variant input power P