In this work, injection locking in switching power amplifiers (PAs) is studied. Traditionally, injection-locked PAs (ILPAs) have supported phase modulation, with injection locking used primarily to improve the power-added efficiency by reducing the power required to drive the ILPA. Since the output power in conventional ILPA architectures is mainly contributed by the locked oscillator in an ILPA, the amplitude modulation is difficult to achieve unless supply modulation is employed. In the ILPA presented in this work, it is shown that by injection locking a switching PA and a power oscillator, improvement in both power-added efficiency and drain efficiency can be achieved as compared to just a switching PA. Moreover, amplitude modulation at a fixed supply voltage is achieved using an RF-DAC approach to scale both the switching PA and the locked oscillator. This approach employs variable injection-strengths varying from ≤1 (weak injection locking) to >1 (strong injection locking) to achieve the required power back-off. Accordingly, new formulations are introduced to extend the existing injection locking theory for injection strengths >1 case in ILPAs. Benefits of a larger injection strength on lock-range, maximum allowable symbol rate, AM-PM distortion and phase noise performance for an ILPA is provided. An ILPA is designed to support 64-QAM and implemented in a standard 65-nm bulk CMOS process. A peak drain efficiency of 42.7% and power-added efficiency of 40% is measured at the highest output power of 23 dBm, operating from a 1.45 V PA power supply at 2.5 GHz. Modulation tests with 5/50 MSym/s 64-QAM signals achieve the measured RMS EVM of 1.9%/3.1% with the average output power, drain efficiency and power-added efficiency of 18.1 dBm, 27.9% and 25.8% at 2.5 GHz, respectively. © 2020 Institute of Electrical and Electronics Engineers Inc.. All rights reserved.