Digitally controlled envelope modulator for a polar transmitter with low code-AM distortion

This paper presents a digitally controlled envelope modulator with low code-AM distortion that can be used as a building block in a digital to RF polar transmitter. The RF envelope signal modulation is performed by digitally controlling the number of activated unit amplifier cells. The phase modulated LO input signal drives a switching transistor gate of the each cell. The 8-bit cell array has 63 unary-weighted cells for MSB 6 bits and 3 segmented unary-weighted cells for LSB 2 bits. Low code-AM distortion is achieved by introducing a current source below the switch transistor in the unit cell. A Class E output matching network is used for the high drain efficiency. The proposed modulator delivers 4.91-dBm peak output power at 970MHz with the power consumption of 28.4mW. An EDGE signal is applied to demonstrate the dynamic operations. It is fabricated in 0.13μm CMOS process and the chip size without pad is 380um × 770um.