A multiband multimode transmitter without driver amplifier

Fourth-generation mobile devices use wide channel bandwidths and OFDM modulation to achieve high data rates. In particular, OFDM modulation in LTE20 creates up to 100 Resource Blocks (RB) each containing 12 subcarriers with 15kHz spacing, spread over the channel bandwidth. For an RB with frequency offset f
_s
from the LO, transmitter non-linearity causes counter-intermodulation (CIM) products at -3f
_s
, +5f
_s
, -7f
_s
, ... from the LO. These spurious signals directly, or through remixing due to PA non-linearity, may fall on other bands. The worst case is when one or just a few RBs are transmitted at the edge of the channel. The CIM effect, specific to 4G signals, makes the design of multimode transmitters more challenging since such transmitters also need to meet the substantially different 2G and 3G requirements. Previous works on LTE have mainly focused on the stringent RX-band noise and ACLR requirements for SAW-less transmitter design without discussing the CIM performance [1]. CIM terms are first generated by mixers. A drive amplifier (DA) placed after the mixers will cause these terms to be regenerated due to high-level harmonic sidebands. DA noise contribution can also be significant, especially at low output power. If current-mode mixers are employed, the use of a differential DA may be prohibitive due to large power consumption [2]. A transmitter without DA avoids these problems while it helps reduce die area and power consumption. This paper describes a multi-mode multiband DA-less transmitter which meets the counter-intermodulation and RX-band noise requirements for SAW-less operation for 2G, 3G and 4G standards while avoiding excessive power consumption, as reported in [3,4].