Envelope modulators often use a linear class AB or a class B amplifier to amplify high frequency AC signal components. Envelope modulators that use such an amplifier to amplify the entire bandwidth of a signal are inherently inefficient. Another type of envelope modulator splits the frequencies of the signals to be operated upon and applies only a higher signal frequency component to a linear amplifier and a low frequency signal component to a switched mode power supply, thereby increasing the efficiency to some degree. However, this configuration has two draw backs. Firstly the frequency response of the modulator is distorted by a null present in the amplitude response and a phase flip in the phase response. These effects degrade the signal fidelity which contributes to the error vector magnitude (EVM) and adjacent channel power ratio (ACPR). Secondly, splitting the envelope in the frequency domain requires a large inductor in a combining network, which takes up considerable board space making this architecture unsuitable for integrated circuit integration and expensive. A further type of envelope modulator includes two or more amplifiers cascaded or stacked with power supplies, where the output of one amplifier drives the power supply of the next amplifier in the amplifier stack for providing an amplified output. The presence of multiple amplifiers in this type of envelope modulator requires synchronisation of the amplifiers in gain and in the time domain, which can cause distortion.
USPTO Backgrounds
USPTO Backgrounds